Steven W Leavitt
- Professor, Dendrochronology
- Associate Director, Tree Ring Laboratory
- Professor, Global Change - GIDP
- Ph.D. Geosciences
- University of Arizona, Tucson, Arizona
- Inference of past atmospheric d13C and PCO2 from 13C/12C measurements in tree rings
- M.S. Environmental Sciences
- University of Virginia, Charlottesville, Virginia
- Soil-plant relationships of nutrient and non-nutrient metals in Louisa Co., Virginia
- B.S. Geology
- University of Illinois, Champaign, Illinois
- The geology and genesis of magmatic iron deposits at Kiruna and Gellivare, Sweden, and Iron Mountain, Missouri
- Editor, Tree-Ring Research
- Tree-Ring Society, Spring 2018
Stable-isotope dendrochronology; light stable-isotope geochemistry applied to geological and environmental problems; the global carbon cycle in geologic time and the recent imbalance from anthropogenic effects; environmental and climate reconstructions with particular interest in the last 15,000 years.
Intro to Global ChangeGC 170A1 (Spring 2020)
Intro to Global ChangeGC 170A1 (Spring 2019)
Intro to Global ChangeGC 170A1 (Fall 2018)
Intro to Global ChangeGC 170A1 (Spring 2018)
Global Biogeochem CyclesGC 572 (Fall 2017)
Global Biogeochem CyclesGEOS 572 (Fall 2017)
Global Biogeochem CyclesHWRS 572 (Fall 2017)
Intro to Global ChangeGC 170A1 (Fall 2017)
Intro to Global ChangeGC 170A1 (Spring 2017)
Intro to Global ChangeGC 170A1 (Fall 2016)
Intro to Global ChangeGC 170A1 (Spring 2016)
- Kassie, B. T., Kimball, B. A., Jamieson, P. D., Bowden, J. W., Sayre, K. D., Groot, J. R., Pinter, P. P., LaMorte, R. L., Wall, G. H., Leavitt, S. W., White, J. W., & Asseng, S. (2018). Field experimental data for crop modeling of wheat growth response to nitrogen fertilizer, elevated CO2, water stress, and high temperature. Open Data Journal for Agricultural Research, 4, 9-15.
- Liu, S., Li, X., Rossi, S., Wang, L., Li, W., Liang, E., & Leavitt, S. W. (2018). Differences in xylogenesis between dominant and suppressed trees. American Journal of Botany, 105(5), 950-956. doi:doi.org/10.1002/ajb2.1089
- Liu, X., Liang, E., Wang, Y., Babst, F., Leavitt, S. W., & Camarero, J. J. (2018). Past the climate optimum: Recruitment is declining at the world’s highest juniper shrublines on the Tibetan Plateau. Ecology. doi:doi.org/10.1002/ecy.2557
- Sigdel, S. T., Dawadi, B., Camarero, J. J., Liang, E., & Leavitt, S. W. (2018). Moisture-limited tree growth for the subtropical Himalayan conifer forest, western Nepal. Forests, 9(6), 340. doi:doi.org/10.3390/f9060340
- Szejner, P., Wright, W. E., Belmecheri, S., Meko, D., Leavitt, S. W., Ehleringer, J. R., & Monson, R. K. (2018). Disentangling seasonal and interannual legacies from inferred patterns of forest water and carbon cycling using tree-ring stable isotopes. GLOBAL CHANGE BIOLOGY, 24(11), 5332-5347.
- Webber, H., White, J. W., Kimball, B. A., Ewert, F., Asseng, S., Rezaei, E. E., Pinter Jr., P. J., Hatfield, J. L., Reynolds, M. P., Ababaei, B., Bindi, M., Doltra, J., Ferrise, R., Kage, H., Kassie, B. T., Kersebaum, K., Luig, A., Olesen, J. E., Semenov, M. A., , Stratonovitch, P., et al. (2018). Physical robustness of canopy temperature models for crop heat stress simulation across environments and production conditions. FIELD CROPS RESEARCH, 216, 75-88.
- Johnson, J. E., Hamann, L., Dettman, D. L., Kim-Hak, D., Leavitt, S. W., Monson, R. K., & Papuga, S. A. (2017). Performance of induction module cavity ring-down spectroscopy (IM-CRDS) for measuring delta O-18 and delta H-2 values of soil, stem, and leaf waters. RAPID COMMUNICATIONS IN MASS SPECTROMETRY, 31(6), 547-560.
- Leavitt, S. W., Kimball, B. A., P.inter, Jr, P. J., LaMorte, R. L., Hunsaker, D. J., Wall, G. W., Wechsung, F., Wechsung, G., Bloom, A. J., & White, J. W. (2017). Data from the Arizona FACE (Free-Air CO2 Enrichment) experiments on wheat at ample and limiting levels of water and nitrogen.. Open Data Journal for Agricultural Research, 3, 29-38.
- Liu, Y., Cobb, K. M., Song, H., Li, Q., Li, C., Nakatsuka, T., An, Z., Zhou, W., Cai, Q., Li, J., Leavitt, S. W., Sun, C., Mei, R., Shen, C., Chan, M., Sun, J., Yan, L., Lei, Y., Ma, Y., , Li, X., et al. (2017). Recent enhancement of central Pacific El Nino variability relative to last eight centuries. NATURE COMMUNICATIONS, 8.
- Liu, Y., Wang, Y., Li, Q., Song, H., Linderholm, H. W., Leavitt, S. W., Wang, R., & An, Z. (2015). Corrigendum to "Tree-ring stable carbon isotope-based May-July temperature reconstruction over Nanwutai, China, for the past century and its record of 20th century warming" [Quat. Sci. Rev. 93 (2014) 67-76]. Quaternary Science Reviews, 117, 164.
- Panyushkina, I. P., Leavitt, S. W., & Mode, W. N. (2017). A 1400-year Bølling-Allerød tree-ring record from the U.S. Great Lakes. Tree-Ring Research, 73(2), 102-112.
- Gleadow, R. M., Ottman, M. J., Kimball, B. A., Wall, G. W., Pinter Jr., P. J., LaMorte, R. L., & Leavitt, S. W. (2016). Drought-induced changes in nitrogen partitioning between cyanide and nitrate in leaves and sterns of sorghum grown at elevated CO2 are age dependent. FIELD CROPS RESEARCH, 185, 97-102.
- Hatton, P., Chatterjee, S., Filley, T. R., Dastmalchi, K., Plante, A. F., Abiven, S., Gao, X., Masiello, C. A., Leavitt, S. W., Nadelhoffer, K. J., Stark, R. E., & Bird, J. A. (2016). Tree taxa and pyrolysis temperature interact to control the efficacy of pyrogenic organic matter formation. BIOGEOCHEMISTRY, 130(1-2), 103-116. doi:10.1007/s10533-016-0245-1
- Panyushkina, I. P., Shishov, V. V., Grachev, A. M., Knorre, A. A., Kirdyanov, A., Leavitt, S. W., Vaganov, E. A., Chebykin, E. P., Zhuchenko, N. A., & Hughes, M. K. (2016). Trends In Elemental Concentrations of Tree Rings From the Siberian Arctic. Tree-Ring Research, 72(2), 67-77. doi:DOI 10.3959/1536-1098-72.02.67
- Szejner, P., Wright, W., Babst, F., Belmecheri, S., Trouet, V. M., Leavitt, S. W., Ehleringer, J., & Monson, R. K. (2016). Latitudinal gradients in tree-ring stable carbon and oxygen isotopes reveal differential climate influences of the North American Monsoon system.. Journal of Geophysical Research - Biogeosciences, 121(7), 1978-1991. doi:10.1002/2016JG003460
- Szejner, P., Wright, W., Babst, F., Belmecheri, S., Trouet, V. M., Leavitt, S. W., Ehleringer, J., & Monson, R. K. (2016). Latitudinal gradients in tree-ring stable carbon and oxygen isotopes reveal differential climate influences of the North American Monsoon system.. Journal of Geophysical Research - Biogeosciences.
- Voelker, S. L., Brooks, J. R., Meinzer, F. C., Anderson, R., Bader, M., Battipaglia, G., Becklin, K. M., Beerling, D., Bert, D., Betancourt, J. L., Dawson, T. E., Domec, J., Guyette, R. P., Koerner, C., Leavitt, S. W., Linder, S., Marshall, J. D., Mildner, M., Ogee, J., , Panyushkina, I., et al. (2016). A dynamic leaf gas-exchange strategy is conserved in woody plants under changing ambient CO2: evidence from carbon isotope discrimination in paleo and CO2 enrichment studies. GLOBAL CHANGE BIOLOGY, 22(2), 889-902.
- Liu, Y. u., Wang, Y., Li, Q., Song, H., Linderholm, H. W., Leavitt, S. W., Wang, R., & An, Z. (2015). Corrigendum: Tree-ring stable carbon isotope-based May July temperature reconstruction over Nanwutai, China, for the past century and its record of 20th century warming (vol 93, pg 67, 2014). QUATERNARY SCIENCE REVIEWS, 117, 164-164. doi:10.1016/j.quascirev.2014.03.023
- Panyushkina, I. P., Leavitt, S. W., Domack, E. W., & Wiedenhoeft, A. C. (2015). TREE-RING INVESTIGATION OF HOLOCENE FLOOD-DEPOSITED WOOD FROM THE ONEIDA LAKE WATERSHED, NEW YORK STATE. TREE-RING RESEARCH, 71(2), 83-94.
- Voelker, S. L., Stambaugh, M. C., Guyette, R. P., Feng, X., Grimley, D. A., Leavitt, S. W., Panyushkina, I., Grimm, E. C., Marsicek, J. P., Shuman, B., & Curry, B. B. (2015). Deglacial hydroclimate of midcontinental North America. QUATERNARY RESEARCH, 83(2), 336-344.
- Jull, A. J., & Leavitt, S. W. (2014). Editors’ note. Radiocarbon, 56(4), V.
- Jull, A. J., Panyushkina, I. P., Lange, T. E., Kukarskih, V. V., Myglan, V. S., Clark, K. J., Salzer, M. W., Burr, G. S., & Leavitt, S. W. (2014). Excursions in the 14C record at AD 774-775 in tree rings from Russia and America. Geophysical Research Letters, doi: 10.1002/2014GL059874.
- Liu, X., Wenling, A., Leavitt, S. W., Wang, W., Guobao, X., Zeng, X., & Qin, D. (2014). Recent strengthening of correlations between tree-ring δ13C and δ18O in mesic western China: Implications to climatic reconstruction and physiological responses. Global and Planetary Change, 113, 23-33.More infoAbstract: We examined the temporal relationships between stable isotopes of carbon (δ13C) and oxygen (δ18O) from annual tree-ring cellulose of Abies georgei on the moist Batang-Litang plateau, western China. Climatic response analysis reveals that tree-ring δ13C contains a strong moisture signal, and that δ13C was especially influenced by relative humidity and precipitation in May through August during the period 1960-2005. In addition, tree-ring δ13C and temperature in May to August are positively correlated because of the high-elevation tree growth sites. Drought is also recorded in tree-ring δ13C, reflecting the integrated influences of temperature and moisture on stomatal regulation and photosynthesis. Generally, correlations between δ13C and δ18O remained positive, but not significant, in most of the investigated period, and this correlation switched to highly significant from 1969 onwards. Changes in the δ13C-δ18O relationship reflect changes in water stress on carbon isotopic discrimination, suggesting an increase in stomatal control of photosynthesis in recent decades, which is consistent with the overall drying trend in the region. Spatial correlation maps confirm that during the past century the response of tree-ring δ13C to temperature and precipitation differs before and after 1969. Using previously reported δ18O results and new δ13C results on the same samples, we determined that the climatic signal recorded in tree-ring δ18O is more stable than that of δ13C. Analysis of temporal changes in the δ13C-δ18O correlations indicates that dominant processes and the controlling factors on tree-ring δ13C potentially shift over the long term under temporal fluctuations of dry-wet spells, especially under recent global warming, and stomatal control of photosynthesis may adjust to region-wide changes in climate in this region where water stress on forest was formerly minor. © 2013.
- Liu, Y., Wang, Y., Li, Q., Song, H., Linderhlom, H. W., Leavitt, S. W., Wang, R., & An, Z. (2014). Tree-ring stable carbon isotope-based May-July temperature reconstruction over Nanwutai, China, for the past century and its record of 20th century warming. Quaternary Science Reviews, 93, 67-76.
- An, W., Liu, X., Leavitt, S. W., Guobao, X., Zeng, X., Wang, W., Qin, D., & Ren, J. (2013). Relative humidity history on the Batang-Litang Plateau of western China since 1755 reconstructed from tree-ring δ18O and δD. Climate Dynamics, 1-16. doi:DOI 10.1007/s00382-013-1937-zMore infoAbstract: We measured the annual variation in the stable isotopes of oxygen (δ18O) and hydrogen (δD) in tree rings of Abies georgei on the Batang-Litang Plateau of western China. Although correlations between tree-ring δ18O and δD are relatively weak in semi-arid regions, we found a strong correlation between the δ18O and δD time series from 1755 to 2009 under the wetter environment. Tree-ring δ18O and δD time series are both significantly and negatively correlated with moisture conditions from June to August, including relative humidity and total precipitation, respectively, from 1960 to 2009. Considering the difference in low-frequency domain between the two isotopes, the relative humidity histories from June to August, reconstructed separately from the tree-ring δ18O and δD data with instrumental climate data, reveal a persistent drying trend since 1850s, especially since the early 1970s. There is an obvious offset of reconstructed relative humidity from tree-ring δ18O and δD in the period 1755-1820, despite the strong similarity in their 21-year moving averages. The decreased relative humidity since the 1850s may be associated with the thermal contrast between the sea surface temperature of the Indian Ocean and the Qinghai-Tibetan Plateau, which determines the strength of moisture transfer via the Indian summer monsoon. © 2013 Springer-Verlag Berlin Heidelberg.
- Csank, A. Z., Fortier, D., & Leavitt, S. W. (2013). Annually resolved temperature reconstructions from a late Pliocene-early Pleistocene polar forest on Bylot Island, Canada. Palaeogeography, Palaeoclimatology, Palaeoecology, 369, 313-322.More infoAbstract: Here we use δ18O ratios measured in tree rings of crossdated sub-fossil wood to reconstruct an annually resolved record of temperature and δ18O of meteoric water for an interglacial late Pliocene-early Pleistocene fossil forest found on Bylot Island, Nunavut, Canada. Our record represents the first crossdated record of Pliocene wood. Mean annual temperatures determined in this study average -3.4±3.8°C, which is 11.4±4.4°C warmer than present-day Bylot Island (-14.8±2.2°C). June-July temperatures average 13.5±1.1°C, approximately 12.6±1.6°C warmer than present-day. Meteoric water δ18O values average -15.5±2.9‰, ~2-6‰ more enriched than present values of precipitation δ18O. Our temperatures are comparable to mid-Pliocene modeled temperatures for the Arctic (3-5°C warmer than present), suggesting that interglacial warm periods in the late Pliocene-early Pleistocene may have been as warm as the mid-Pliocene warm period. That both the Bylot Island forest deposit and the Kap København deposit represent the remains of northern tree-line vegetation that lived during warm interglacial periods within the overall cool Plio-Pleistocene suggests that forest deposits in the Arctic capture a snapshot of interglacial conditions during the Plio-Pleistocene rather than the average Pliocene climate and may not be suitable records to study Pliocene cooling. © 2012 Elsevier B.V.
- Grachev, A., Vaganov, E., Leavitt, S., Panyushkina, I., Chebykin, E., Shishov, V., Zhuchenko, N., Knorre, A., Hughes, M., & Naurzbaev, M. (2013). Methodology for development of a 600-year tree-ring multi-element record for larch from the Taymir Peninsula, Russia. Journal of Siberian Federal University Biology, 6(1), 61-72.
- Griffin, D., Woodhouse, C. A., Meko, D. M., Stahle, D. W., Faulstich, H. L., Carrillo, C., Touchan, R., Castro, C. L., & Leavitt, S. W. (2013). North American monsoon precipitation reconstructed from tree-ring latewood. Geophysical Research Letters, 40(5), 954-958.More infoAbstract: The North American monsoon is a major focus of modern and paleoclimate research, but relatively little is known about interannual- to decadal-scale monsoon moisture variability in the pre-instrumental era. This study draws from a new network of subannual tree-ring latewood width chronologies and presents a 470-year reconstruction of monsoon (June-August) standardized precipitation for southwestern North America. Comparison with an independent reconstruction of cool-season (October-April) standardized precipitation indicates that southwestern decadal droughts of the last five centuries were characterized not only by cool-season precipitation deficits but also by concurrent failure of the summer monsoon. Monsoon drought events identified in the past were more severe and persistent than any of the instrumental era. The relationship between winter and summer precipitation is weak, at best, and not time stable. Years with opposing-sign seasonal precipitation anomalies, as noted by other studies, were anomalously frequent during the mid to late 20th century. © 2013. American Geophysical Union. All Rights Reserved.
- Hultine, K. R., Dudley, T. L., & Leavitt, S. W. (2013). Herbivory-induced mortality increases with radial growth in an invasive riparian phreatophyte. Annals of Botany, 111(6), 1197-1206.More infoPMID: 23576688;PMCID: PMC3662518;Abstract: Background and Aims Under equal conditions, plants that allocate a larger proportion of resources to growth must do so at the expense of investing fewer resources to storage. The critical balance between growth and storage leads to the hypothesis that in high-resource environments, plants that express high growth rates are more susceptible to episodic disturbance than plants that express lower growth rates. Methods This hypothesis was tested by measuring the radial growth, basal area increment (BAI) and carbon isotope ratios (δ13C) in tree-ring cellulose of 62 mature tamarisk trees (Tamarix spp.) occurring at three sites in the western USA (n = 31 live and 31 killed trees across all sites, respectively). All of the trees had been subjected to periods of complete foliage loss by episodic herbivory over three or more consecutive growing seasons by the tamarisk leaf beetle (Diorhabda carinulata), resulting in approx. 50% mortality at each site. Key Results Mean annual BAI (measured from annual ring widths) in the 10 years prior to the onset of herbivory was on average 45% higher in killed trees compared with live trees (P < 0.0001). Killed trees that had higher growth rates also expressed higher (less negative) δ13C ratios compared with live trees. In fact, at one site near Moab, UT, the mean annual BAI was 100% higher in killed trees despite having about a 0.5 ‰ higher δ13C relative to live trees (P = 0.0008). Patterns of δ13C suggest that the intrinsic water-use efficiency was higher in killed than surviving trees, possibly as a consequence of lower whole-canopy stomatal conductance relative to live trees. Conclusions The results show that a likely trade-off occurs between radial growth and survival from foliage herbivory in Tamarix spp. that currently dominates riparian areas throughout the western USA and northern Mexico. Thus, herbivory by D. carinulata may reduce the overall net primary productivity of surviving Tamarix trees and may result in a reduction in genetic variability in this dominant invasive tree species if these allocation patterns are adaptive. © 2013 The Author. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved.
- Linderholm, H. W., Liu, Y., Leavitt, S. W., & Liang, E. (2013). Dendrochronology in Asia. Quaternary International, 283, 1-2.
- Liu, X., Zeng, X., Leavitt, S. W., Wang, W., Wenling, A. n., Guobao, X. u., Sun, W., Wang, Y., Qin, D., & Ren, J. (2013). A 400-year tree-ring δ18O chronology for the southeastern Tibetan Plateau: Implications for inferring variations of the regional hydroclimate. Global and Planetary Change, 104, 23-33.More infoAbstract: We developed a new tree-ring cellulose δ18O chronology for the southeastern Tibetan Plateau from Balfour spruce (Picea likiangensis var. balfouriana [Rehd. et Wils.]) that covered the period from 1600 to 2008, and compared the results with a previous study to explore climatic variations in the Nyingchi-Bomi area. Our tree-ring δ18O chronology correlated significantly with the previous study (Shi et al., 2012; Climate of the Past 8, 205-213) during the common period from 1781 to 2005, and provided new insights into long-term regional hydroclimatic variations. Besides the significant positive correlations between tree-ring δ18O and the temperature and sunshine duration during the growing season, tree-ring δ18O was strongly negatively correlated with regional cloud cover, relative humidity, and precipitation in July and August. The correlations with cloud cover data were stronger than in previous research, but the correlations with precipitation and relative humidity in July and August were weaker. When Indian summer monsoon conditions prevail, regional hydroclimate variations (and especially cloud cover) have the dominant influence on tree-ring δ18O in the study area. Based on the regional data, δ18O in tree rings can be an effective proxy to infer the temporal variations in regional hydroclimatic conditions and the strength of the Indian Summer Monsoon. Our results reveal that the Indian Summer Monsoon weakened from 1600 to 1650, followed by continuous strengthening until 1740 and a slight weakening from 1740 to present. The temporal variations in the cellulose δ18O chronology generally corresponded well to the δ18O and glacier snow accumulation records found in ice cores from the middle Himalaya. © 2013 Elsevier B.V.
- Meko, D. M., Touchan, R., Díaz, J. V., Griffin, D., Woodhouse, C. A., Castro, C. L., Carillo, C., & Leavitt, S. W. (2013). Sierra San Pedro Mártir, Baja California, cool-season precipitation reconstructed from earlywood width of Abies concolor tree rings. Journal of Geophysical Research G: Biogeosciences, 118(4), 1660-1673.More infoAbstract: Tree ring data are analyzed for a multicentury record of drought history in the Sierra San Pedro Mártir (SSPM) of Baja California, Mexico. Climatic variation in the study area is of particular interest because the SSPM is a rich biotic environment at the southern limit of the California floristic province and the southern limit of the planetary jet stream. Future shifts in the jet stream would be expected to have amplified effect on this marginal environment. The study applies linear regression to tree ring indices of earlywood-width of Abies concolor to estimate a 353 year (1658-2010 C.E.) record of cool-season (October-April) precipitation, P, in SSPM. Time-nested regression models account for more than half the variance of grid point P in calibration periods of length 50-65 years. Cross-spectral analysis indicates strong tracking of observed P by the reconstruction over a broad range of frequencies. Robustness of the reconstruction is supported by synchrony of reconstructed P with tree ring variations in other tree species from SSPM. The reconstruction emphasizes the severity of the 1950s drought in a long-term context and the single-year intensity of droughts in the last decade: 2007 stands out as the driest reconstructed year, with a high percentage of missing rings in A. concolor. The reconstruction identifies the early twentieth century pluvial as the wettest epoch in the last 353 years in the SSPM. High-elevation tree species in SSPM may be especially well-suited to sensing snowpack-related moisture variations associated with a southerly branched jet stream and the types of weather systems active in the pluvial. Key Points White fir tree rings closely track precipitation in Sierra San Pedro Mártir White fir in Sierra San Pedro Mártir is vulnerable to climate change The early 1900s in Sierra San Pedro Mártir is the wettest period in 353 years ©2013. American Geophysical Union. All Rights Reserved.
- Panyushkina, I. P., & Leavitt, S. W. (2013). Ancient boreal forests under the environmental instability of the glacial to postglacial transition in the great lakes region (14 000 -11 000 years BP). Canadian Journal of Forest Research, 43(11), 1032-1039.More infoAbstract: Retreat of the Laurentide Ice Sheet 20 000 years ago tremendously altered environmental conditions and opened territory to the boreal spruce forest expansion. However, the details of forest colonization during the rapid climate warming and the adaptation of the newly developed stands to short cooling episodes during the warming and degradation of the ice sheet are not known. Preservation of wood from the glacial to postglacial transition offers the opportunity for examination of high-frequency growth variability in response to hemispheric and local forcings on temperature and hydrology. Here we consider growth of spruce at three sites from the interior of Northern America developed at ca. 13 700,12100, and 11300 calibrated years before present (cal years BP), with well-replicated tree-ring chronologies spanning from 116 to 310 years. The data show at least two generations of trees established at each of the sites promoted by short, warm intervals. The tree mortality was variously affected by both cold conditions and the influence of rising water table and sediment burial. The history of these stands indicates breaks in forest colonization following a century (or two) of successful migrations. Interestingly, the thinning of the spruce forest did not seem to open pioneering opportunities for other tree species at those times.
- Vaganov, E. A., Grachev, A. M., Shishov, V. V., Panyushkina, I. P., Leavitt, S. W., Knorre, A. A., Chebykin, E. P., & Menyailo, O. V. (2013). Elemental composition of tree rings: A new perspective in biogeochemistry. Doklady Biological Sciences, 453(1), 375-379.
- An, W., Liu, X., Leavitt, S., Sun, W., Wang, W., Wang, Y., Xu, G., Chen, T., Ren, J., & Qin, D. (2012). Specific climatic signals recorded in earlywood and latewood ?18O of tree rings in southwestern China. Tellus B, 64, 18703.
- Creasman, P. P., Bannister, B., Towner, R. H., Dean, J. S., & Leavitt, S. W. (2012). Reflections on the foundation, persistence, and growth of the laboratory of tree-ring research, circa 1930-1960. Tree-Ring Research, 68(2), 81-89.More infoAbstract: On the occasion of the 75th anniversary of the Laboratory of Tree-Ring Research, it is appropriate to reflect on the origin of the LTRR and the oft overlooked early period of its history. The period from the "Bridging the gap" event in 1929 to the semi-retirement of A.E. Douglass in 1958 was a crucial time in the development of the LTRR. Although this paper focuses on the history of the LTRR between those events, at points the history of the LTRR is, essentially, the history of the field, making a holistic understanding all the more important. The information presented here is rooted in a series of transcribed historical lectures delivered in 1992 and 1993 by Director/Professor Emeritus Bryant Bannister, and several historical reports composed by him between 1963 and 1998. © The Tree-Ring Society.
- Liu, X., Wenling, A. n., Treydte, K., Shao, X., Leavitt, S., Hou, S., Chen, T., Sun, W., & Qin, D. (2012). Tree-ring δ 18O in southwestern China linked to variations in regional cloud cover and tropical sea surface temperature. Chemical Geology, 291, 104-115.More infoAbstract: In this paper, we present the first annual oxygen isotope (δ 18O) record (1902 to 2004) from the latewood cellulose of trees growing in a temperate-moist forest in southwestern China. Tree-ring δ 18O ranges from 12.8 to 18.6‰ and averages 15.2‰ δ 18O in the latewood negatively correlates with total cloud cover, relative humidity, and precipitation during the moist months (August to October) from 1951 to 2004 and, to a lesser extent, positively correlates with the temperature in June. Spatial correlation analysis revealed that tree-ring latewood δ 18O is more strongly related to variability in regional cloud cover and precipitation in autumn. Tree-ring δ 18O negatively correlates with the indices of the Indian summer monsoon (1948 to 2004), the western North Pacific monsoon (1948 to 2004), and with the East Asian summer monsoon (1902 to 2000), suggesting that these monsoons strongly influence the regional climate. Under global warming of last century, we found a strong inverse correlation between tree-ring δ 18O and the Southern Oscillation index in different periods, indicating a complex association between the El Niño-Southern Oscillation and the regional climate, which in turn affects tree-ring δ 18O. Correlation with the global sea surface temperature (SST) indicates that climatic conditions in the Indian Ocean might play a dominant role in modulating cloud cover and precipitation over the study region during the monsoon seasons. We also found a significant correlation between tree-ring δ 18O and SST over the western-north Pacific Ocean. Our results suggest strong links between tropical oceans and an ENSO-like climate with the oxygen-isotope sources and discrimination for trees in the study region. © 2011 Elsevier B.V.
- Liu, Y., Wang, R., Leavitt, S. W., Song, H., Linderholm, H. W., Qiang, L. i., & Zhisheng, A. n. (2012). Individual and pooled tree-ring stable-carbon isotope series in Chinese pine from the Nan Wutai region, China: Common signal and climate relationships. Chemical Geology, 330-331, 17-26.More infoAbstract: To investigate the differences in the climatic signals in stable-carbon isotopic composition captured by averaging series from individual trees versus raw wood of trees pooled prior to analysis, we analyzed two groups of Chinese pine (Pinus tabulaeformis Carr.) from the Nan Wutai region of the Qinling Mountains, China. One group included three trees that were analyzed separately, and the other group comprised four other trees that were pooled prior to preparation and analysis. All δ 13C series were positively correlated (r=0.50-0.58, p
- Bale, R. J., Robertson, I., Salzer, M. W., Loader, N. J., Leavitt, S. W., Gagen, M., Harlan, T. P., & McCarroll, D. (2011). An annually resolved bristlecone pine carbon isotope chronology for the last millennium. Quaternary Research, 76(1), 22-29.More infoAbstract: We present the first near millennium-length, annually resolved stable isotope record from bristlecone pines (Pinus longaeva, D.K Bailey). The carbon isotope ratios from the cellulose of seven trees from the White Mountains of California, corrected for anthropogenic changes in atmospheric chemistry, are used to reconstruct growing season (June through August) precipitation back to AD 1085. Extremely negative isotope results are strongly correlated with proposed severest El Niño events over the last 500yr, and similar values in the first half of the millennium are used to reconstruct a further 13 strong El Niño events, concentrated in the 12th Century and the mid 13th and 14th Centuries. Ring-width chronologies from adjacent sites in the White Mountains demonstrate a high degree of decadal covariance with the δ13C series, although there are several periods of notable divergence. © 2011 University of Washington.
- Griffin, D., Meko, D. M., Touchan, R., Leavitt, S. W., & Woodhouse, C. A. (2011). Latewood chronology development for summer-moisture reconstruction in the US Southwest. Tree-Ring Research, 67(2), 87-101.More infoAbstract: Tree-ring studies have demonstrated that conifer latewood measurements contain information on long-term North American monsoon (NAM) variability, a hydroclimatic feature of great importance to plants, animals, and human society in the US Southwest. This paper explores data-treatment options for developing latewood chronologies aimed at NAM reconstruction. Archived wood samples for five Douglas-fir (Pseudotsuga menziesii, Mirb. Franco) sites in southeastern Arizona are augmented with new collections. The combined dataset is analyzed along with time series of regionally averaged observed precipitation to quantify the strength of regional precipitation signal in latewood time series and to identify ways of increasing the signal strength. Analysis addresses the signal strength influences of including or excluding "false" latewood bands in the nominal "latewood" portion of the ring, the necessary adjustment of latewood width for statistical dependence on antecedent earlywood width, and tree age. Results suggest that adjusted latewood width chronologies from individual sites can explain around 30% of the variance of regional summer (July-August) precipitation - increasing to more than 50% with use of multiple chronologies. This assessment is fairly insensitive to the treatment of false latewood bands (in intra-annual width and δ 13C variables), and to whether latewood-width is adjusted for dependence on earlywood-width at the core or site level. Considerations for operational chronology development in future studies are (1) large tree-to-tree differences in moisture signal, (2) occasional nonlinearity in EW-LW dependence, and (3) extremely narrow and invariant latewood width in outer portions of some cores. A protocol for chronology development addressing these considerations is suggested. © 2011 The Tree-Ring Society.
- Leavitt, S. W., Woodhouse, C. A., Castro, C. L., Wright, W. E., Meko, D. M., Touchan, R., Griffin, D., & Ciancarelli, B. (2011). The North American monsoon in the U.S. Southwest: Potential for investigation with tree-ring carbon isotopes. Quaternary International, 235(1-2), 101-107.More infoAbstract: The North American Monsoon (NAM) contributes critical summer moisture to the U.S. Southwest from July through September, but instrumental records of monsoon precipitation are limited to 100 years or less. Tree-ring investigation offers a means of improving our understanding of its long-term spatial and temporal variability. Available evidence indicates the stable-carbon isotopic composition (δ13C) of tree rings in this region is strongly linked to moisture. In addition to latewood width as a precipitation proxy, the δ13C of latewood also appears to be a strong proxy, largely manifesting water stress effects on stomatal conductance and their consequence to isotopic discrimination against 13CO2. In one promising study, the δ13C of 11 years of latewood from 8 sites regressed against their corresponding precipitation exhibited a coefficient of -0.061‰ per cm of July + August + September precipitation (r2 = 0.41). Long latewood δ13C chronologies are currently being developed from tree rings of ponderosa pine and Douglas-fir at several sites in NAM core regions in the Southwest to evaluate its usefulness in supplementing precipitation reconstructions derived from latewood widths. Among planned outcomes, the improved monsoon precipitation records can be used to better evaluate natural variability of NAM precipitation and its linkage to winter precipitation, document the character of monsoon season droughts, and test accuracy of regional climate models. © 2010 Elsevier Ltd and INQUA.
- Maseyk, K., Hemming, D., Angert, A., Leavitt, S. W., & Yakir, D. (2011). Increase in water-use efficiency and underlying processes in pine forests across a precipitation gradient in the dry Mediterranean region over the past 30 years. Oecologia, 167(2), 573-585.More infoPMID: 21590331;Abstract: Motivated by persistent predictions of warming and drying in the entire Mediterranean and other regions, we have examined the interactions of intrinsic water-use efficiency (W i) with environmental conditions in Pinus halepensis. We used 30-year (1974-2003) tree-ring records of basal area increment (BAI) and cellulose 13C and 18O composition, complemented by short-term physiological measurements, from three sites across a precipitation (P) gradient (280-700 mm) in Israel. The results show a clear trend of increasing W i in both the earlywood (EW) and latewood (LW) that varied in magnitude depending on site and season, with the increase ranging from ca. 5 to 20% over the study period. These W i trends were better correlated with the increase in atmospheric CO 2 concentration, C a, than with the local increase in temperature (~0.04°C year -1), whereas age, height and density variations had minor effects on the long-term isotope record. There were no trends in P over time, but W i from EW and BAI were dependent on the interannual variations in P. From reconstructed C i values, we demonstrate that contrasting gas-exchange responses at opposing ends of the hydrologic gradient underlie the variation in W i sensitivity to C a between sites and seasons. Under the mild water limitations typical of the main seasonal growth period, regulation was directed at increasing C i/C a towards a homeostatic set-point observed at the most mesic site, with a decrease in the W i response to C i with increasing aridity. With more extreme drought stress, as seen in the late season at the drier sites, the response was W i driven, and there was an increase in the W i sensitivity to C a with aridity and a decreasing sensitivity of C i to C a. The apparent C a-driven increases in W i can help to identify the adjustments to drying conditions that forest ecosystems can make in the face of predicted atmospheric change. © 2011 Springer-Verlag.
- Wang, W., Liu, X., Shao, X., Leavitt, S., Guobao, X. u., Wenling, A. n., & Qin, D. (2011). A 200 year temperature record from tree ring δ 13C at the Qaidam Basin of the Tibetan Plateau after identifying the optimum method to correct for changing atmospheric CO 2 and δ 13C. Journal of Geophysical Research G: Biogeosciences, 116(4).More infoAbstract: Improved understanding of climate influences on tree ring stable carbon isotope (δ 13C) ratios for Qilian juniper (Sabina przewalskii Kom.) will improve prospects for long climate reconstructions in northwestern China's Qaidam Basin, where weather stations are widely scattered with relatively short records. Here, we developed an annual-resolution δ 13C series from 1800 to 2005 for trees in this extremely arid, high-elevation area. As expected, a significant decline in δ 13C (of about 3.5‰) occurred from 1850 to 2005 in response to increasing atmospheric CO 2 concentrations and decreasing atmospheric δ 13C. High-frequency correlation analysis based on comparison of the tree ring δ 13C chronology with recorded weather parameters revealed that mean temperature during the current growing season (April-August) most strongly influenced tree ring δ 13C discrimination from 1956 to 2005. To clarify the climatic implications of the long-term trend, we systematically compared four previously published approaches to remove the effects of decreasing atmospheric δ 13C from the climate signals. The optimal correction, which accounted for the decline in atmospheric δ 13C (δ 13C cor) and for a discrimination rate of about 0.016‰ ppmv -1 for the CO 2 partial pressure, captured the strongest temperature signal (r = 0.75, P < 0.001). The historical mean April-August temperatures inferred from the correlations of tree ring δ 13C with climate data revealed a persistent warming trend during the past two centuries, especially since the 1980s. Our results therefore reveal a high potential for reconstruction of growing season temperatures on a millennial scale in the northeastern Tibetan Plateau. Copyright 2011 by the American Geophysical Union.
- Zhang, Y., Tian, Q., Gou, X., Chen, F., Leavitt, S. W., & Wang, Y. (2011). Annual precipitation reconstruction since AD 775 based on tree rings from the Qilian Mountains, northwestern China. International Journal of Climatology, 31(3), 371-381.More infoAbstract: A millennium-long tree-ring-width chronology from the middle Qilian Mountains in northwestern China has been used to reconstruct annual precipitation variation (from the prior August to current July) since AD 775. The reconstruction explains 37.8% of variance of the observed data. Based on the mean and standard deviation of the reconstructed series, several prolonged severe dry and wet periods were indentified: drought spells in AD 1144-1154 (11 years) and 1925-1932 (8 years) and wet spells in AD 985-999 (15 years), 1089-1097 (9 years) and 1979-1991 (12 years). Both multi-taper spectral analysis (MTM) and wavelet analysis suggest that periods of the reconstructed precipitation are consistent with those associated with the Asian Summer Monsoon and perhaps solar activity. Copyright © 2010 Royal Meteorological Society.
- Bale, R. J., Robertson, I., Leavitt, S. W., Loader, N. J., Harlan, T. P., Gagen, M., Young, G. H., Csank, A. Z., Froyd, C. A., & McCarroll, D. (2010). Temporal stability in bristlecone pine tree-ring stable oxygen isotope chronologies over the last two centuries. Holocene, 20(1), 3-6.More infoAbstract: The absolutely dated bristlecone pine (Pinus longaeva) tree-ring chronology spans almost 9000 years, offering great potential for inferring past environmental change. Existing ring width chronologies have been widely used to produce some of the most influential millennial length temperature reconstructions for the Northern Hemisphere. A recently published δ 18O record from two bristlecone pine trees growing at Methuselah Walk in the White Mountains of California showed a dramatic decrease in δ 18O between AD 1850 and 1920 (c. 13‰), interpreted as indicating a major shift in Pacific storm tracks over the past 300 years. Here we present new bristlecone pine δ18O time series from 15 trees at three White Mountains sites, including two series from Methuselah Walk. Whilst occasional high interannual variability is observed in our δ 18O series, none of our chronologies exhibit an equivalent pronounced or sustained twentieth-century decrease, suggesting the earlier results are anomalous and may require palaeoclimatic re-interpretation. © The Author(s), 2010.
- Kagawa, A., & Leavitt, S. W. (2010). Stable carbon isotopes of tree rings as a tool to pinpoint the geographic origin of timber. Journal of Wood Science, 56(3), 175-183.More infoAbstract: Illegal logging is a major cause of worldwide deforestation, and demands for scientific methods to identify the geographic origin of timber are increasing. "Dendroprovenancing" is one such method, in which the origin of unknown wood is estimated by calculating correlations of the ring-width series of the unknown wood with reference trees of known geographic origins. We applied the dendroprovenancing method to carbon isotope network data of pinyon pines (Pinus edulis and Pinus monophylla) from the southwestern United States to test the efficacy of using a carbon isotope time series for provenancing wood. First, we calculated correlations (t values) between test trees temporarily assumed to be of unknown origin and reference trees from 13 surrounding sites. Then, we plotted the t values on a map. When provenancing was successful, the tested trees showed the strongest correlation with reference trees from sites close to the actual origins of the test trees, and the correlations decreased with the distance between the original sites of test and reference trees. This conical distribution of t values enabled provenancing of wood with precision of 114-304 km. Although isotope measurement is more expensive and laborious than ring-width measurement, our tests of provenancing pinyon pines in the southwestern United States showed a higher success rate with carbon isotopes. © 2009 The Japan Wood Research Society.
- Leavitt, S. W. (2010). Tree-ring C-H-O isotope variability and sampling. Science of the Total Environment, 408(22), 5244-5253.More infoPMID: 20719360;Abstract: In light of the proliferation of tree-ring isotope studies, the magnitude and cause of variability of tree-ring δ-3C, δ18O and δ2H within individual trees (circumferential) and among trees at a site is examined in reference to field and laboratory sampling requirements and strategies. Within this framework, this paper provides a state-of-knowledge summary of the influence of "juvenile" isotope effects, ageing effects, and genetic effects, as well as the interchangeability of species, choice of ring segment to analyze (whole ring, earlywood or latewood), and the option of sample pooling. The range of isotopic composition of the same ring among trees at a site is ca. 1-3‰ for δ-3C, 1-4‰ δ18O, and 5-30‰ for δ2H, whereas the circumferential variability within a tree is lower. A standard prescription for sampling and analysis does not exist because of differences in field environmental circumstances and mixed findings represented in relevant published literature. Decisions in this regard will usually be tightly constrained by goals of the study and project resources. Sampling 4-6 trees at a site while avoiding juvenile effects in rings near the pith seems to be the most commonly used methodology, and although there are some reasoned arguments for analyzing only latewood and developing separate isotope records from each tree, the existence of some contradictory findings together with efforts to reduce cost and effort have prompted alternate strategies (e.g., most years pooled with occasional analysis of rings in the sequence separately for each tree) that have produced useful results in many studies. © 2010 Elsevier B.V.
- Leavitt, S., & Leavitt, S. W. (2010). Tree-ring C-H-O isotope variability and sampling. The Science of the total environment, 408(22).More infoIn light of the proliferation of tree-ring isotope studies, the magnitude and cause of variability of tree-ring δ(13)C, δ(18)O and δ(2)H within individual trees (circumferential) and among trees at a site is examined in reference to field and laboratory sampling requirements and strategies. Within this framework, this paper provides a state-of-knowledge summary of the influence of "juvenile" isotope effects, ageing effects, and genetic effects, as well as the interchangeability of species, choice of ring segment to analyze (whole ring, earlywood or latewood), and the option of sample pooling. The range of isotopic composition of the same ring among trees at a site is ca. 1-3‰ for δ(13)C, 1-4‰ δ(18)O, and 5-30‰ for δ(2)H, whereas the circumferential variability within a tree is lower. A standard prescription for sampling and analysis does not exist because of differences in field environmental circumstances and mixed findings represented in relevant published literature. Decisions in this regard will usually be tightly constrained by goals of the study and project resources. Sampling 4-6 trees at a site while avoiding juvenile effects in rings near the pith seems to be the most commonly used methodology, and although there are some reasoned arguments for analyzing only latewood and developing separate isotope records from each tree, the existence of some contradictory findings together with efforts to reduce cost and effort have prompted alternate strategies (e.g., most years pooled with occasional analysis of rings in the sequence separately for each tree) that have produced useful results in many studies.
- Panyushkina, I. P., & Leavitt, S. W. (2010). Ancient tree ring archives in the U.S. great lakes region. Eos, 91(50), 489-490.More infoAbstract: Rather than being a seamless transition from Late Glacial Maximum to the start of the Holocene between 15,000 and 8000 years ago, the warming during this period was punctuated by abrupt climatic instabilities. These include the Younger Dryas cold event, the Preboreal Oscillation, and an isolated cooling event around 8200 years ago (see Figure 1, bottom right). In the Great Lakes area the terrestrial fingerprints of these events are often present in pollen records, yet even greater details of the environmental course of deglaciation during the Holocene transition could be gleaned with high-resolution tree ring proxies. A campaign is under way to locate, sample, and analyze tree rings of surprisingly abundant subfossil wood preserved in the Great Lakes area to improve understanding of past (and possibly future) abrupt climate change in a region where these events may have even been triggered (e.g., by glacial meltwater discharge [Murton et al., 2010]) and where early human and megafauna populations were likely profoundly affected.
- Williams, A. P., Allen, C. D., Millar, C. I., Swetnam, T. W., Michaelsen, J., Still, C. J., & Leavitt, S. W. (2010). Forest responses to increasing aridity and warmth in the southwestern United States. Proceedings of the National Academy of Sciences of the United States of America, 107(50), 21289-21294.More infoPMID: 21149715;PMCID: PMC3003095;Abstract: In recent decades, intense droughts, insect outbreaks, and wildfires have led to decreasing tree growth and increasingmortality inmany temperate forests. We compared annual tree-ring width data from 1,097 populations in the coterminous United States to climate data and evaluated site-specific tree responses to climate variations throughout the 20th century. For each population, we developed a climate-driven growth equation by using climate records to predict annual ring widths. Forests within the southwestern United States appear particularly sensitive to drought and warmth.We input 21st century climate projections to the equations to predict growth responses. Our results suggest that if temperature and aridity rise as they are projected to, southwestern trees will experience substantially reduced growth during this century. As tree growth declines, mortality rates may increase at many sites. Increases in wildfires and bark-beetle outbreaks in the most recent decade are likely related to extreme drought and high temperatures during this period. Using satellite imagery and aerial survey data, we conservatively calculate that ≈2.7% of southwestern forest and woodland area experienced substantialmortality due to wildfires from1984 to 2006, and≈7. 6%experiencedmortality associated with bark beetles from 1997 to 2008. We estimate that up to ≈18% of southwestern forest area (excluding woodlands) experienced mortality due to bark beetles or wildfire during this period. Expected climatic changes will alter future forest productivity, disturbance regimes, and species ranges throughout the Southwest. Emerging knowledge of these impending transitions informs efforts to adaptively manage southwestern forests.
- Williams, A. P., Michaelsen, J., Leavitt, S. W., & Still, C. J. (2010). Using tree rings to predict the response of tree growth to climate change in the continental United States during the twenty-first century. Earth Interactions, 14(19), 1-20.More infoAbstract: In the early 1900s, tree-ring scientists began analyzing the relative widths of annual growth rings preserved in the cross sections of trees to infer past climate variations. Now, many ring-width index (RWI) chronologies, each representing a specific site and species, are archived online within the International Tree-Ring Data Bank (ITRDB). Comparing annual tree-ringwidth data from 1097 sites in the continental United States to climate data, the authors quantitatively evaluated how trees at each site have historically responded to interannual climate variations. For each site, they developed a climate-driven statistical growth equation that uses regional climate variables to model RWI values. The authors applied these growth models to predict how tree growth will respond to twenty-first-century climate change, considering four climate projections. Although caution should be taken when extrapolating past relationships with climate into the future, the authors observed several clear and interesting patterns in the growth projections that seem likely if warming continues. Most notably, the models project that productivity of dominant tree species in the southwestern United States will decrease substantially during this century, especially in warmer and drier areas. In the northwest, nonlinear growth relationships with temperature may lead to warming-induced declines in growth for many trees that historically responded positively to warmer temperatures. This work takes advantage of the unmatched temporal length and spatial breath of annual growth data available within the ITRDB and exemplifies the potential of this ever-growing archive of tree-ring data to serve in meta-analyses of large-scale forest ecology. Copyright © 2010, Paper 14-019;.
- Leavitt, S. W. (2009). Editor's note. Tree-Ring Research, 65(1), 3-.
- Leavitt, S. W., & Bannister, B. (2009). Dendrochronology and radiocarbon dating: The laboratory of tree-ring research connection. Radiocarbon, 51(1), 373-384.More infoAbstract: The field of dendrochronology had a developmental "head start" of at least several decades relative to the inception of radiocarbon dating in the late 1940s, but that evolution was sufficiently advanced so that unique capabilities of tree-ring science could assure success of the 14C enterprise. The Laboratory of Tree-Ring Research (LTRR) at the University of Arizona played a central role in the cross-pollination of these disciplines by providing the first wood samples of exactly known age for the early testing and establishment of the "Curve of Knowns" by Willard Libby. From the 1950s into the early 1980s, LTRR continued to contribute dated wood samples (bristlecone pine and other wood species) to 14C research and development, including the discovery and characterization of de Vries/Suess "wiggles," calibration of the 14C timescale, and a variety of tests to understand the natural variability of 14C and to refine sample treatment for maximum accuracy. The long and varied relationship of LTRR with 14C initiatives has continued with LTRR contributions to high-resolution studies through the 1990s and systematic efforts now underway that may eventually extend the bristlecone pine chronology back beyond its beginning 8836 yr ago as of 2009. This relationship has been mutualistic such that a half-century ago the visibility and stature of LTRR and dendrochronology were also elevated through their association with 14C-allied "hard sciences.". © 2009 by the Arizona Board of Regents on behalf of the University of Arizona.
- Porteaus, F., Hill, J., Ball, A. S., Pinter, P. J., Kimball, B. A., Wall, G. W., Adamsen, F. J., Hunsaker, D. J., LaMorte, R. L., Leavitt, S. W., Thompson, T. L., Matthias, A. D., Brooks, T. J., & Morris, C. F. (2009). Effect of Free Air Carbon dioxide Enrichment (FACE) on the chemical composition and nutritive value of wheat grain and straw. Animal Feed Science and Technology, 149(3-4), 322-332.More infoAbstract: The global impact of an increased concentration of CO2 in the atmosphere on plants has been studied extensively, but little information has been published on the effect of enrichment of atmospheric CO2 on the nutritive value of grain and straw used as ruminant feeds. This paper reports the chemical composition and nutritive value of grain and straw harvested from the drought tolerant hard red spring wheat (Triticum aestivum L.) variety Yecora Rojo managed with two carbon dioxide regimes (ambient, 350 μl/l and elevated, 550 μl/l), two rates of nitrogen application (low N: 53 kg N/ha and high N: 393 kg N/ha) grown under a water-fed (i.e., no deficit) regime. Accumulation of carbon in straw did not differ among crops grown under elevated CO2 and low N supplementation and crops grown under ambient CO2 with low levels of N supplementation. Increased N application increased sequestration of C (P
- Péwé, T., Westgate, J. A., Preece, S. J., Brown, P. M., & Leavitt, S. W. (2009). Late pliocene dawson cut forest bed and new tephrochronological findings in the Gold Hill Loess, east-central Alaska. Bulletin of the Geological Society of America, 121(1-2), 294-320.More infoAbstract: Thick loess deposits in the Fairbanks region of interior Alaska are a rich source of information on past climates and environments during the late Cenozoic, and the numerous distal, silicic tephra beds preserved within them offer the potential for good chronological control. The Dawson Cut Forest Bed lies in the lower part of this loess cover. Plant macrofossils consist of Picea glauca, Picea mariana, Betula papyrifera, and Populus balsamifera but no Pinus. These fossils, together with the abundance and size of spruce remains, pollen, tree-ring characteristics, and δ13C values of spruce wood, demonstrate that the boreal forest represented by the Dawson Cut Forest Bed was similar to the modern boreal forest of central Alaska. Warming conditions during the early part of the Dawson Cut Interglaciation initiated thawing of permafrost and melting of ground ice, as evidenced in the presence of ice-wedge casts and major erosion of the lower Gold Hill Loess. Tephrochronological, magnetostratigraphic, and glass fission-track dating studies in the Fairbanks area and at the Palisades site on the Yukon River in central Alaska suggest an age for the Dawson Cut Forest Bed of ca. 2 Ma. Hence, the northern boreal forest of northwestern North America, as we know it today, has a long history that probably extends back to at least 2 Ma. Copyright © 2009 by Geological Society of America.
- Schneider, A. F., Leavitt, S. W., & Lange, T. (2009). Stratigraphy, age, and flora of the Southport forest bed, Southeastern Wisconsin. Journal of Great Lakes Research, 35(4), 538-547.More infoAbstract: A mid-Holocene buried organic layer 10 to 60 cm thick is present along the Lake Michigan shoreline in southeastern Wisconsin. Named the Southport forest bed for its location in Kenosha County, the unit has yielded abundant wood specimens, including large logs, branches, twigs, stumps, and root material. Roots of truncated in situ oak (Quercus) and elm (Ulmus) trees extend into the underlying till of the late Wisconsin Oak Creek Formation. Nearshore lacustrine sand above the organic layer contains abundant driftwood, overlain by 2 to 3 m of cross-bedded dune sand. More than 50 wood samples have been identified; the assemblage is that of a mixed hardwood forest dominated by oak (Quercus) and hickory (Carya), which account for about 60% of the assemblage. Although the site has been mostly concealed for many years, it was beautifully exposed in the 1960s and early 1970s, when it was initially studied by Phil Sander, who discovered the site and documented it with field notes and numerous photographs. Here we report several new (unpublished) radiocarbon dates and details of the stratigraphy and arboreal flora. Radiocarbon dates suggest that the Southport forest probably lived for 900-1000 14C years, possibly longer, reaching its climax at about 5300 B.P. The site is of particular importance because of its proximity to the Nipissing shoreline and provides a significant point on the Nipissing transgression time curve. The unusually large number of identified specimens affords an accurate evaluation of the warm and relatively dry climate that characterized the southern Great Lakes region during the mid-Holocene. © 2009 Elsevier Inc. All rights reserved.
- Anchukaitis, K. J., Evans, M. N., Lange, T., Smith, D. R., Leavitt, S. W., & Schrag, D. P. (2008). Consequences of a rapid cellulose extraction technique for oxygen isotope and radiocarbon analyses. Analytical Chemistry, 80(6), 2035-2041.More infoPMID: 18293945;Abstract: We use infrared, radiocarbon, and stable isotope analyses to investigate the purity of cellulose extracted from wood using a rapid processing technique. Replicate laboratory standards processed using the standard Brendel method are not significantly different with respect to δ18O from those prepared using traditional techniques, although the process does result in a slight acetylation of the wood samples. Radiocarbon comparisons, however, show significant differences. We conclude mat the standard Brendel method is appropriate for developing stable isotope time series for high-resolution isotope dendroclimatology but must be used with caution for precision radiocarbon measurements. © 2008 American Chemical Society.
- Haavik, L. J., Stephen, F. M., Fierke, M. K., Salisbury, V. B., Leavitt, S. W., & Billings, S. A. (2008). Dendrochronological parameters of northern red oak (Quercus rubra L. (Fagaceae)) infested with red oak borer (Enaphalodes rufulus (Haldeman) (Coleoptera: Cerambycidae)). Forest Ecology and Management, 255(5-6), 1501-1509.More infoAbstract: Oak-dominated forests in northwestern Arkansas have recently experienced an oak mortality event associated with an unprecedented outbreak of a native insect, the red oak borer, Enaphalodes rufulus (Haldeman). To determine whether prior drought was associated with increased E. rufulus infestation level of Quercus rubra L. trees, we employed a suite of dendrochronological measurements from Q. rubra in affected forest stands. We examined patterns of tree growth and physiological indicators of stress including α-cellulose δ13C, intrinsic water-use efficiency (Wi) and photosynthetic discrimination (Δ) in Q. rubra from 1954 to 2002. Basal area increment measurements revealed that Q. rubra with lower levels of E. rufulus infestation experienced approximately 20% greater stem growth than Q. rubra more severely infested for ∼17 years prior to the onset of high rates of Q. rubra mortality. This trend emerged immediately following a severe drought in 1979-1981 and continued through 2002, suggesting that drought influenced E. rufulus infestation levels of some Q. rubra trees. Indicators of tree carbon and water relations (δ13C, Wi, and Δ) revealed no relationship with degree of E. rufulus infestation, except for Δ in 2000, when physiology likely was responding to the advanced stage of decline in heavily infested trees. δ13C, Wi, and Δ responded to drought periods as predicted early in the study period, but lost their sensitivity to drought after differences emerged in stem growth rate with degree of infestation. Though reduced stem growth in heavily infested trees suggests these trees were negatively influenced by E. rufulus infestation, the lack of infestation response of δ13C, Wi, and, for much of the study period, Δ, suggests that infestation had negligible influence on C-water relations. The study highlights how δ13C and δ13C-based parameters may not indicate variation in tree C-water relations when forest disturbances are associated with reductions in tree growth rates. © 2007 Elsevier B.V. All rights reserved.
- Knapp, A. K., Beier, C., Briske, D. D., Classen, A. T., Yiqi, L., Reichstein, M., Smith, M. D., Smith, S. D., Bell, J. E., Fay, P. A., Heisler, J. L., Leavitt, S. W., Sherry, R., Smith, B., & Weng, E. (2008). Consequences of more extreme precipitation regimes for terrestrial ecosystems. BioScience, 58(9), 811-821.More infoAbstract: Amplification of the hydrological cycle as a consequence of global warming is forecast to lead to more extreme intra-annual precipitation regimes characterized by larger rainfall events and longer intervals between events. We present a conceptual framework, based on past investigations and ecological theory, for predicting the consequences of this underappreciated aspect of climate change. We consider a broad range of terrestrial ecosystems that vary in their overall water balance. More extreme rainfall regimes are expected to increase the duration and severity of soil water stress in mesic ecosystems as intervals between rainfall events increase. In contrast, xeric ecosystems may exhibit the opposite response to extreme events. Larger but less frequent rainfall events may result in proportional reductions in evaporative losses in xeric systems, and thus may lead to greater soil water availability. Hydric (wetland) ecosystems are predicted to experience reduced periods of anoxia in response to prolonged intervals between rainfall events. Understanding these contingent effects of ecosystem water balance is necessary for predicting how more extreme precipitation regimes will modify ecosystem processes and alter interactions with related global change drivers. © 2008 American Institute of Biological Sciences.
- Leavitt, S. W. (2008). Tree-ring isotopic pooling without regard to mass: No difference from averaging δ13C values of each tree. Chemical Geology, 252(1-2), 52-55.More infoAbstract: Pooling of growth rings from several trees before subsequent isotopic analysis of the mixture may offer the advantage of obtaining "representative" isotopic values fairly rapidly. In principle, however, unless equal masses are taken from the individuals to make the composite sample, the measured isotope value will be biased by the relative mass contribution of the trees, each of which likely has a different isotopic composition. Although biasing associated with direct pooling without regard to mass may happen, this study provides evidence (at least for stable-carbon isotope composition) from three sites in the U.S. Midwest and Southwest that the "error" (probably more appropriately called "difference") is negligible with respect to error of preparation and analysis, and variability of isotopic composition within and among trees at a site. © 2008 Elsevier B.V. All rights reserved.
- Leavitt, S. W., Chase, T. N., Rajagopalan, B., Lee, E., & Lawrence, P. J. (2008). Southwestern U.S. tree-ring carbon isotope indices as a possible proxy for reconstruction of greenness of vegetation. Geophysical Research Letters, 35(12).More infoAbstract: Southwestern U.S. pinyon tree-ring carbon-isotope indices showed promise two decades ago as an indicator of moisture and drought. However, because those isotopic indices were developed on 5-year ring groups rather than annual rings, the full extent of their effectiveness as environmental proxies was not established. Recent resampling of the sites and subsequent availability of annual isotopic indices for 1985 through 1999 has allowed us to more definitively characterize environmental relationships across 14 sites in six southwestern states. Principal component (PC) analysis reveals a significant positive relationship (p ≤ 0.02) between isotope indices and summer-season Normalized Difference Vegetation Index (NDVI), a satellite-derived "greenness index". Correlation and PC analysis also show a negative relationship of isotope indices with cumulative monthly precipitation (p ≤ 0.05) and confirm a strong positive relationship with Palmer Drought Index (especially spring and summer, p ≤ 0.0002). Although the drought and NDVI relationships are not as simply interconnected as hypothesized, the NDVI link reveals the potential of the existing multi-century record of isotope indices for long-term assessment of southwestern ecology and carbon cycling. Copyright 2008 by the American Geophysical Union.
- Panyushkina, I. P., Leavitt, S. W., Thompson, T. A., Schneider, A. F., & Lange, T. (2008). Environment and paleoecology of a 12 ka mid-North American Younger Dryas forest chronicled in tree rings. Quaternary Research, 70(3), 433-441.More infoAbstract: Until now, availability of wood from the Younger Dryas abrupt cooling event (YDE) in N. America ca. 12.9 to 11.6 ka has been insufficient to develop high-resolution chronologies for refining our understanding of YDE conditions. Here we present a multi-proxy tree-ring chronology (ring widths, "events" evidenced by microanatomy and macro features, stable isotopes) from a buried black spruce forest in the Great Lakes area (Liverpool East site), spanning 116 yr at ca. 12,000 cal yr BP. During this largely cold and wet period, the proxies convey a coherent and precise forest history including frost events, tilting, drowning and burial in estuarine sands as the Laurentide Ice Sheet deteriorated. In the middle of the period, a short mild interval appears to have launched the final and largest episode of tree recruitment. Ultimately the tops of the trees were sheared off after death, perhaps by wind-driven ice floes, culminating an interval of rising water and sediment deposition around the base of the trees. Although relative influences of the continental ice sheet and local effects from ancestral Lake Michigan are indeterminate, the tree-ring proxies provide important insight into environment and ecology of a N. American YDE boreal forest stand. © 2008 University of Washington.
- Tardif, J. C., Conciatori, F., & Leavitt, S. W. (2008). Tree rings, δ13C and climate in Picea glauca growing near Churchill, subarctic Manitoba, Canada. Chemical Geology, 252(1-2), 88-101.More infoAbstract: White spruce (Picea glauca (Moench) Voss) trees were sampled to investigate the relationship between ring width and δ13C as well as between these descriptors and climate. Cross-sections from eight white spruce trees growing in the Hudson Bay Lowlands, subartic Manitoba, with an establishment date prior to 1775 were selected from our archive. For each cross-section, two radii were measured to produce ring-width chronologies. For stable-carbon isotope measurements, every fifth ring (2000, 1995, ... 1750) was analyzed separately from each tree, whereas approximately equal masses from the eight trees were pooled into composite samples for each of the intermediate years mixing masses from each tree equal to that from the tree with the least amount. Alpha-cellulose was extracted and δ13C was determined using continuous-flow mass spectrometry. The ring-width residual chronology derived from eight trees was highly correlated to that previously developed for the area. A small but significant correlation was also found between the ring-width and the δ13C residual chronologies. The climate signal contained in both ring-width and the δ13C residual chronologies suggests that warm early summers were conducive to larger rings whereas the δ13C best reflects the overall growing season temperature. Precipitation, relative humidity and/or drought index have little to no association with either chronology. Significant correlations were observed between δ13C and the historical ice records from the Hudson Bay Co. archive data suggesting that during long (warm) growing season the 13C content of alpha-cellulose increases. Our results indicate that both ring-width and δ13C provide complementary information. Future work will involve looking at long-term trends in carbon isotope ratios and the potential use of δ13C for dendroclimatic reconstruction. © 2008 Elsevier B.V. All rights reserved.
- Williams, A. P., Still, C. J., Fischer, D. T., & Leavitt, S. W. (2008). The influence of summertime fog and overcast clouds on the growth of a coastal Californian pine: A tree-ring study. Oecologia, 156(3), 601-611.More infoPMID: 18368424;Abstract: The coast of California is home to numerous rare, endemic conifers and other plants that are limited in distribution by drought sensitivity and the summer-dry climate that prevails across most of the state. Ecologists have long assumed that some coastal plant populations survived the early Pleistocene transition to a warmer and drier environment because they benefit from frequent fog and stratus clouds that provide water and shade during the rainless summer. One such population is that of Torrey pine (Pinus torreyana ssp. Insularis) on Santa Rosa Island in Channel Islands National Park. Here we report that the tree-ring width record from this population indicates strong growth sensitivities to summer fog drip and cloud shading. We quantified the effects of summer cloud cover by comparing ring-width indices to coastal airport cloud-frequency records (1944-2004). For the first time observed, summertime cloud frequency correlated positively with ring-width indices, regardless of whether the effect of rainfall was first removed from the ring-width record. The effect of ground-level fog was strongest in July early mornings (03:00 PST, R 2 = 0.262, P < 0.0002). The effect of clouds high enough to provide shade but not fog water was also strongest in July, but climbed steadily throughout the day before becoming strongest in late afternoon (16:00-18:00 PST, R 2 = 0.148, P < 0.004). Correlations were substantially stronger in years with higher soil moisture, suggesting that growth response to summer clouds is strongly affected by pre-summer rainfall. A change in the height and/or timing of coastal cloud formation with climate change would likely affect this and other populations of California's coastal vegetation. © 2008 Springer-Verlag.
- Cheng, L., Leavitt, S. W., Kimball, B. A., Pinter Jr., P. J., Ottman, M. J., Matthias, A., Wall, G. W., Brooks, T., Williams, D. G., & Thompson, T. L. (2007). Dynamics of labile and recalcitrant soil carbon pools in a sorghum free-air CO2 enrichment (FACE) agroecosystem. Soil Biology and Biochemistry, 39(9), 2250-2263.More infoAbstract: Experimentation with dynamics of soil carbon pools as affected by elevated CO2 can better define the ability of terrestrial ecosystems to sequester global carbon. In the present study, 6 N HCl hydrolysis and stable-carbon isotopic analysis (δ13C) were used to investigate labile and recalcitrant soil carbon pools and the translocation among these pools of sorghum residues isotopically labeled in the 1998-1999 Arizona Maricopa free air CO2 enrichment (FACE) experiment, in which elevated CO2 (FACE: 560 μmol mol-1) and ambient CO2 (Control: 360 μmol mol-1) interact with water-adequate (wet) and water-deficient (dry) treatments. We found that on average 53% of the final soil organic carbon (SOC) in the FACE plot was in the recalcitrant carbon pool and 47% in the labile pool, whereas in the Control plot 46% and 54% of carbon were in recalcitrant and labile pools, respectively, indicating that elevated CO2 transferred more SOC into the slow-decay carbon pool. Also, isotopic mixing models revealed that increased new sorghum residue input to the recalcitrant pool mainly accounts for this change, especially for the upper soil horizon (0-30 cm) where new carbon in recalcitrant soil pools of FACE wet and dry treatments was 1.7 and 2.8 times as large as that in respective Control recalcitrant pools. Similarly, old C in the recalcitrant pool under elevated CO2 was higher than that under ambient CO2, indicating that elevated CO2 reduces the decay of the old C in recalcitrant pool. Mean residence time (MRT) of bulk soil carbon at the depth of 0-30 cm was significantly longer in FACE plot than Control plot by the averages of 12 and 13 yr under the dry and wet conditions, respectively. The MRT was positively correlated to the ratio of carbon content in the recalcitrant pool to total SOC and negatively correlated to the ratio of carbon content in the labile pool to total SOC. Influence of water alone on the bulk SOC or the labile and recalcitrant pools was not significant. However, water stress interacting with CO2 enhanced the shift of the carbon from labile pool to recalcitrant pool. Our results imply that terrestrial agroecosystems may play a critical role in sequestrating atmospheric CO2 and mitigating harmful CO2 under future atmospheric conditions.
- Leavitt, S. W. (2007). Regional expression of the 1988 U.S. Midwest drought in seasonal δ13C of tree rings. Journal of Geophysical Research D: Atmospheres, 112(6).More infoAbstract: This study analyzes δ13C of subdivisions of the 1988 and 1989 growth rings from three conifer species (Pinus resinosa, Pinus strobus, and Picea glauca) across a network of nine sites over four states in the upper U.S. Midwest to infer seasonal and interannual environmental moisture conditions. Both years were characterized by different spatial patterns of drought (expressed through rainfall and Palmer Drought Indices), with the area of 1989 drought being smaller and shifting westward relative to 1988. Rings were subdivided into four equal segments from which holocellulose was separated. The range of δ 13C variation within each ring was typically 1-2‰ for both years, and the single site where all three species co-occurred revealed coherent seasonal δ13C patterns for the pines but no correlation of the spruce δ13C patterns with those of the pines. Various averaged combinations of monthly Palmer Drought Severity Index (PDSI) and precipitation assigned to each subdivision versus both δ13C and discrimination (Δ ≈ δ13 Cair - δ 13C plant) yielded significant correlations (generally higher with PDSI) and even stronger correlations when the spruce data were removed. The fraction of average monthly cumulative precipitation (relative to 1900-2001 means) for these 2 years was more strongly related to δ13C than either monthly precipitation or monthly fraction of average precipitation, but was not as strongly related to δ13 C as was PDSI. Furthermore, the pattern of increasing δ13C in 1988 observed at several sites is an indicator of the increasing moisture stress of that year, and the difference between the 1988 and 1989 average isotopic compositions of the third and fourth subdivisions matched the regional moisture shift well. Copyright 2007 by the American Geophysical Union.
- Leavitt, S. W., Follett, R. F., Kimble, J. M., & Pruessner, E. G. (2007). Radiocarbon and δ13C depth profiles of soil organic carbon in the U.S. Great Plains: A possible spatial record of paleoenvironment and paleovegetation. Quaternary International, 162-163, 21-34.More infoAbstract: Soil profiles from undisturbed grassland sites around the Great Plains of the USA were sampled for analysis of soil organic carbon (SOC) content (%), radiocarbon age and stable-carbon isotope composition (δ13C). With the exception of a few pronounced dating anomalies, SOC radiocarbon age generally increases steadily with depth back to 10-15,000 cal yr BP, the deepest soil intervals in 9 of the 12 sites. The radiocarbon ages were used to establish the chronology of changes in past plant distribution over time and space, based on SOC δ13C as an indicator of C3 and C4 plant abundance. Changes were referenced to an SOC δ13C value of -20‰, which is the approximate mid-point between C3 and C4 carbon isotope composition, i.e., an equal mixture of C3 and C4 carbon. Prior to 10,000 cal yr BP, the region was dominated by C3 plants with the exception of the southernmost Texas sites. From 10,000 to 2000 cal yr BP, C4 plants expanded their range, initially as a peninsula of C4-predominant grasses extending northeastward and ultimately dominating all but the northernmost border of the region. Finally, the C3-predominant region re-expanded after 1000 cal yr BP, perhaps as a response to the Little Ice Age cooling. Despite uncertainties associated with using radiocarbon-dated SOC-depth profiles as a proxy, the past C3 and C4 plant distribution inferred from SOC δ13C conforms well to results from other paleoclimate proxies, and differences may be helpful in targeting future research. © 2006 Elsevier Ltd and INQUA.
- Leavitt, S. W., Panyushkina, I. P., Lange, T., Cheng, L., Schneider, A. F., & Hughes, J. (2007). Radiocarbon "wiggles" in great lakes wood at about 10,000 to 12,000 bp. Radiocarbon, 49(2), 855-864.More infoAbstract: High-resolution radiocarbon calibration for the last 14,000 cal yr has been developed in large part using Euro- pean oaks and pines. Recent subfossil wood collections from the Great Lakes region provide an opportunity to measure 14C activity in decadal series of rings in North America prior to the White Mountains bristlecone record. We developed decadal 14C series from wood at the classic Two Creeks site (~11,850 BP) in east-central Wisconsin, the Liverpool East site (~10,250 BP) in northwestern Indiana, and the Gribben Basin site (~10,000 BP) in the Upper Peninsula of Michigan. Initial AMS dates on holocellulose produced younger-than-expected ages for most Two Creeks subsamples and for a few samples from the other sites, prompting a systematic comparison of chemical pretreatment using 2 samples from each site, and employing holocellulose, AAA-treated holocellulose, alpha-cellulose, and AAA-treated whole wood. The testing could not definitively reveal the source of error in the original analyses, but the "best" original ages together with new AAA-treated holocellulose and α-cellulose ages were visually fitted to the IntCal04 calibration curve at ages of 13,760-13,530 cal BP for the Two Creeks wood, 12,100-12,020 cal BP for Liverpool East, and 11,300-11,170 cal BP for Gribben Basin. The Liverpool East age falls squarely within the Younger Dryas (YD) period, whereas the Gribben Basin age appears to postdate the YD by ~300 yr, although high scatter in the decadal Gribben Basin results could accommodate an older age nearer the end of the YD. © 2007 by the Arizona Board of Regents on behalf of the University of Arizona.
- Brigham-Grete, J., Anderson, S., Clague, J., Cole, J., Doran, P., Gillespie, A., Grimm, E., Guccione, P., Hijghen, K., Jackson, S., Jull, T., Leavitt, S., Mandel, R., Ortiz, J., Rodbell, D., Schweger, C., Smith, A., & Styles, B. (2006). Petroleum Geologist's award to novelist Crichton is inappropriate. Eos, 87(36), 364-.
- Hunter, R. D., Panyushkina, I. P., Leavitt, S. W., Wiedenhoeft, A. C., & Zawiskie, J. (2006). A multiproxy environmental investigation of Holocene wood from a submerged conifer forest in Lake Huron, USA. Quaternary Research, 66(1), 67-77.More infoAbstract: Remains of a Holocene drowned forest in southern Lake Huron discovered in 12.5 m of water (164 m above sea level), 4.5 km east of Lexington, Michigan USA (Sanilac site), provided wood to investigate environment and lake history using several proxies. Macrofossil evidence indicates a forest comprised primarily of conifers equivalent to the modern "rich conifer swamp" community, despite generally low regional abundance of these species in pollen records. Ages range from 7095 ± 50 to 6420 ± 70 14C yr BP, but the clustering of stump dates and the development of 2 floating tree-ring chronologies suggest a briefer forest interval of no more than c. 400 years. Dendrochronological analysis indicates an environment with high inter-annual climate variability. Stable-carbon isotope composition falls within the range of modern trees from this region, but the stable-oxygen composition is consistent with warmer conditions than today. Both our tree-ring and isotope data provide support for a warmer environment in this region, consistent with a mid-Holocene thermal maximum. This drowned forest also provides a dated elevation in the Nipissing transgression at about 6420 14C yr BP (7350 cal yr BP) in the southern Lake Huron basin, a few hundred years before reopening of the St. Clair River drainage. © 2006 University of Washington.
- Leavitt, S. W., Panyushkina, I. P., Lange, T., Wiedenhoeft, A., Cheng, L., Hunter, R. D., Hughes, J., Pranschke, F., Schneider, A. F., Moran, J., & Stieglitz, R. (2006). Climate in the Great Lakes region between 14,000 and 4000 years ago from isotopic composition of conifer wood. Radiocarbon, 48(2), 205-217.More infoAbstract: The isotopic composition of ancient wood has the potential to provide information about past environments. We analyzed the Δ13C, Δ18O, and Δ2H of cellulose of conifer trees from several cross-sections at each of 9 sites around the Great Lakes region ranging from ~4000 to 14,000 cal BP. Isotopic values of Picea, Pinus, and Thuja species seem interchangeable for Δ18O and Δ2H comparisons, but Thuja appears distinctly different from the other 2 in its Δ13C composition. Isotopic results suggest that the 2 sites of near-Younger Dryas age experienced the coldest conditions, although the Gribben Basin site near the Laurentide ice sheet was relatively dry, whereas the Liverpool site 500 km south was moister. The spatial isotopic variability of 3 of the 4 sites of Two Creeks age shows evidence of an elevation effect, perhaps related to sites farther inland from the Lake Michigan shoreline experiencing warmer daytime growing season temperatures. Thus, despite floristic similarity across sites (wood samples at 7 of the sites being Picea), the isotopes appear to reflect environmental differences that might not be readily evident from a purely floristic interpretation of macrofossil or pollen identification. © 2006 by the Arizona Board of Regents on behalf of the University of Arizona.
- Wall, G. W., Garcia, R. L., Kimball, B. A., Hunsaker, D. J., Pinter Jr., P. J., Long, S. P., Osborne, C. P., Hendrix, D. L., Wechsung, F., Wechsung, G., Leavitt, S. W., LaMorte, R. L., & Idso, S. B. (2006). Interactive effects of elevated carbon dioxide and drought on wheat. Agronomy Journal, 98(2), 354-381.More infoAbstract: Atmospheric CO2 concentration (Ca) continues to rise. An imperative exists, therefore, to elucidate the interactive effects of elevated Ca and drought on plant water relations of wheat (Triticum aestivum L.). A spring wheat (cv. Yecora Rojo) crop was exposed to ambient (Control: 370 μmol mol-1) and free-air CO2 enrichment (FACE: ambient + 180 μmol mol-1) under ample (Wet), and reduced (Dry), water supplies (100 and 50% replacement of evapotranspiration, respectively) over a 2-yr study. Our objective was to characterize and quantify the responses of 26 edaphic, gas exchange, water relations, carbohydrate pool dynamics, growth, and development parameters to rising Ca and drought. Increasing Ca minimized the deleterious effects of soil-water depletion by increasing drought avoidance (i.e., lower stomatal conductance and transpiration rate, and growth and development of a more robust root system) and drought tolerance (i.e., enhanced osmoregulation and adaptation of tissue) mechanisms, resulting in a 30% reduction in water stress-induced midafternoon depressions in net assimilation rate. An elevated C a-based increase in daily and seasonal carbon gain resulted in a positive feedback between source capacity (shoots) and sink demand (roots). Devoid of a concomitant rise in global temperature resulting from the rise in Ca, improved water relations for a herbaceous, cool-season, annual, C3 cereal monocot grass (i.e., wheat) are anticipated in a future high-CO2 world. These findings are applicable to other graminaceous species of a similar function-type as wheat common to temperate zone grassland prairies and savannas, especially under dryland conditions. © American Society of Agronomy.
- Wright, W. E., & Leavitt, S. W. (2006). Boundary layer humidity reconstruction for a semiarid location from tree ring cellulose δ18O. Journal of Geophysical Research D: Atmospheres, 111(18).More infoAbstract: July-September boundary layer humidity has been reconstructed from the δ18O of cellulose in tree rings of Pinus arizonica growing at an elevation of 2300 m in the Santa Catalina Mountains north of Tucson, Arizona. The annual occurrence of morphological features in the tree rings, often called false rings, allowed accurate subdivision of the tree rings into premonsoon and monsoon growth. Highly significant correlations were found among the stable oxygen isotope time series of the wood produced during the North American Monsoon and 1953-2000 July-September average specific humidity (P < 0.0001) and relative humidity (P < 0.0001) derived from radiosonde data. The correlation coefficients were significant against data from both the Tucson surface (788 m; approximately 922 hPa) and 850 hPa pressure levels, suggesting that the δ18O time series can be interpreted as a proxy for mean seasonal boundary layer humidity. Twentieth century July-September reconstructions of specific humidity and relative humidity are presented. There are no long-term trends in the twentieth century reconstructions of boundary layer humidity at this site. Copyright 2006 by the American Geophysical Union.
- Wright, W. E., & Leavitt, S. W. (2006). Needle cell elongation and maturation timing derived from pine needle cellulose δ18O. Plant, Cell and Environment, 29(1), 1-14.More infoPMID: 17086748;Abstract: Estimates of the timing of Pinus arizonica Engelm. needle development in 1998 and 1999 were derived from the leaf-cellulose δ18O of weekly growth increments. Significant correlations were noted between time series of local humidity and leaf-cellulose δ18O for needles growing near Tucson, Arizona. Correlations with temperature were also significant, but much lower, suggesting these variations in cellulose δ18O were determined mostly by changes in humidity. The timing of all significant correlations lags the timing of the appearance of the new needle growth, and is interpreted as indicating 16-23 d were required for cell enlargement in 1998 and 13-17 d in 1999. Similarly, properties of the environmental time series, when significantly correlated, are interpreted as indicating the duration of cellulose deposition (7-27 d in 1998, 13-21 d in 1999). Variations in stable-isotope back diffusion (the Péclet effect) and the synthesis of cellulose using stored photosynthate are discussed as explanations for departures from a Craig and Gordon-type model of leaf water δ18O. The Péclet effect, use of stored photosynthate, and variations in the growing-season source-water δ18O, probably confound the development of a high-resolution paleohumidity proxy from subfossil needle cellulose δ18O in this region. © 2006 Blackwell Publishing Ltd.
- Adamsen, F. J., Wechsung, G., Wechsung, F., Wall, G. W., Kimball, B. A., Pinter Jr., P. J., LaMorte, R. L., Garcia, R. L., Hunsaker, D. J., & Leavitt, S. W. (2005). Temporal changes in soil and biomass nitrogen for irrigated wheat grown under free-air carbon dioxide enrichment (FACE). Agronomy Journal, 97(1), 160-168.More infoAbstract: Increasing atmospheric CO2 concentrations are expected to increase plant production and demand for N and other nutrients. The objectives of this investigation were to characterize and quantify the temporal trends in soil mineral N and aboveground biomass N during the growing season of wheat (Triticum aestivum L.) with adequate N, ambient and elevated CO2, and two levels of water stress. The free-air CO2 enrichment (FACE) technique was used to enrich the air from 370 to 550 μmol mol-1 CO2. Spring wheat was planted in late December of 1992 and 1993 and harvested at the end of May. Each main plot (CO2 level) was split into two irrigation treatments to replace 100 and 50% of the potential evapotranspiration. Soil and plant samples were taken for N analysis six times each year. Elevated CO2 lowered soil mineral N concentrations in the top 0.3 m of soil as much as 40% and increased aboveground biomass N by as much as 16% compared with the ambient treatment. Before anthesis, irrigation level had little effect on either soil mineral N or aboveground biomass N, but at harvest in 1992-1993 and at dough stage in 1993-1994 deficit-irrigated plots had higher soil mineral N (p < 0.05) and lower aboveground biomass N than plots that received adequate irrigation. There was little variation in the concentrations of N in the aboveground biomass among treatments within a sampling date. The data suggest elevated CO2 may lead to rapid N uptake, which could result in increased early vegetative growth.
- Garfin, G. M., Hughes, M. K., Liu, Y. u., Burns, J. M., Touchan, R., Leavitt, S. W., & Zhisheng, A. (2005). Exploratory temperature and precipitation reconstructions from the Qinling Mountains, North-Central China. Tree-Ring Research, 61(2), 59-72.More infoAbstract: February-April (FMA) temperature at Foping (1879-1989) and July-August (JA) precipitation at Xian (1895-1988) have been reconstructed using total ring width (TRW) and maximum latewood density (MXD) from trees in the Qinling Mountains, at the northern limit of the East Asian monsoon, in central China. The Xian JA precipitation reconstruction, albeit short, represents the first well-replicated, crossdated dendroclimatic reconstruction of summer monsoon precipitation for this region. Reconstructed Xian precipitation shows significant positive relationships with historical evidence from the region. The key feature of the precipitation reconstruction is prolonged summer drought during the late 1920s and early 1930s. The Foping reconstruction displays warmer-than-average FMA temperatures during this time period. These exploratory reconstructions, along with a previous reconstruction from Huashan, demonstrate the complexity of attempting dendroclimatic reconstructions from this region. Our results indicate that further attempts to locate long-lived conifers from here can result in an extended well-calibrated and verified reconstruction of summer monsoon precipitation. Copyright © 2005 by the Tree-Ring Society.
- Li, Z., Leavitt, S. W., Mora, C. I., & Liu, R. (2005). Influence of earlywood-latewood size and isotope differences on long-term tree-ring δ13C trends. Chemical Geology, 216(3-4), 191-201.More infoAbstract: Inter-annual δ13C values of α-cellulose in two Chinese pine trees (Pinus tabulaeformis Carr.) exhibit a long-term decline of more than 2‰ for the period of 1896 to 1990 AD. This decline is much larger than the decrease in atmospheric δ13C caused by industrialization since 1850 AD. Here we present evidence that long-term trends of 13C/12C in tree rings also reflect ecophysiological effects superimposed on the anthropogenic trend. These effects are associated with the decreasing/ increasing productivity of earlywood or latewood associated with the changing growth history of the tree. Theoretical calculations show that long-term decreases in the 13C/12C ratios in tree rings may be enhanced or reduced by these ecophysiological effects, depending on the pattern of seasonal differences in tree-ring δ13C values. Based on seasonal δ13C analyses, the influence of these effects on δ13C values for one of these trees (HL02) has been removed. The corrected data are in agreement with the observed size and timing of the δ13C decline in atmospheric CO2. This study suggests that long-term tree-ring δ13C data used to infer the anthropogenic impact on atmospheric CO2 carbon isotope compositions may be improved by correcting for possible ecophysiological effects using measurement of latewood and earlywood ring widths and seasonally resolved isotopic analysis. Removal of growth effects may also benefit the assessment of long-term changes in the water use efficiency in response to increasing atmospheric CO2 concentrations. © 2004 Elsevier B.V. All rights reserved.
- Pendall, E., Williams, D. G., & Leavitt, S. W. (2005). Comparison of measured and modeled variations in piñon pine leaf water isotopic enrichment across a summer moisture gradient. Oecologia, 145(4), 605-618.More infoPMID: 16012819;Abstract: Stable hydrogen and oxygen isotopic composition of bulk leaf water (δDlw and δ18Olw) in piñon pine (Pinus edulis and P. monophylla) and gas exchange parameters were measured under field conditions to examine the effects of seasonal moisture stress on leaf water isotopic enrichment. Study sites were located near the lower elevation limit for piñon in the southwestern USA. Leaf-level transpiration measurements were made four times daily in spring, summer and early autumn; simultaneously, leaf samples were collected for water extraction and stable isotope analysis. Diurnal variations in δDlw and δ18Olw values were small, especially when leaf water residence times (molar leaf water content divided by transpiration rate) were high. Stomatal conductance explained most of the variance (60%) in leaf water enrichment across the dataset. Observed leaf water enrichment was compared with predictions of steady-state and nonsteady-state models. Nonsteady-state predictions fit observations the best, although D enrichment was often lower than predicted by any model. Hydrogen isotope ratios of leaf water and cellulose nitrate were strongly correlated, demonstrating preservation of a leaf water signal in wood and leaf cellulose. © Springer-Verlag 2005.
- Follett, R. F., Kimble, J., Leavitt, S. W., & Pruessner, E. (2004). Potential use of soil C isotope analyses to evaluate paleoclimate. Soil Science, 169(7), 471-488.More infoAbstract: The objective of this study was to evaluate potential use of the naturally occurring isotopic carbon (C) in soil organic matter to help evaluate paleoclimate in the Central United States. Sampling sites were selected on major geomorphic units with similar slope and aspect in three soil temperature regimes and three soil moisture regimes within the historic grasslands in US Great Plains and Western Corn Belt. Soil samples were collected from approximate 0- to 5- and 5- to 10-cm (A1 and A2 profile) depths and by genetic soil horizon thereafter in pits dug by backhoe or by hand to ∼2 m depth. Bulk density and soil texture were determined. Sieved, handpicked, and delimed subsamples of soil from each horizon were analyzed for total soil organic carbon (SOC) and δ13C and 14C dated (as mean residence time of SOC in calendar yr B.P.). Data from some sites support that major aeolian movement of soil occurred during the Holocene, often coincidental with literature reports of long periods of drought. However, soil profiles provide a low-resolution record of drought and/or wetter climatic conditions and thus time frames are general. Measurement of SOC age and δ13C signature in soil horizons was related to preboreal warming during entry into the Holocene. An important question raised by our data but requiring further confirmation by other studies is the timing of the peak of the Medieval Warm Period that is reported to have occurred prior to commencement of climate cooling that preceded the Little Ice Age. Based on δ13C, our data indicate a change from C 4 plants to increasing C3 plant dominance (as a surrogate of cooler temperature) at ∼1,500 yr B.P., whereas the reported start of the Little Ice Age is ∼600 yr B.P. In summary, the approach we used holds promise as an additional proxy for evaluating prior climate back thousands of years, but additional measurements besides those from the isotopes of C will be needed.
- Grant, R. F., Kimball, B. A., Wall, G. W., Triggs, J. M., Brooks, T. J., Pinter Jr., P. J., Conley, M. M., Ottman, M. J., Lamorte, R. L., Leavitt, S. W., Thompson, T. L., & Matthias, A. D. (2004). Modeling elevated carbon dioxide effects on water relations, water use, and growth of irrigated sorghum. Agronomy Journal, 96(6), 1693-1705.More infoAbstract: Elevated concentrations of atmospheric CO2 (Ca) are believed to raise sorghum [Sorghum bicolor (L.) Moench] productivity by improving water relations. In ecosys, water relations are simulated by solving for the canopy water potential (ψc) at which water uptake from a model of soil-root-canopy water transfer equilibrates with transpiration from the canopy energy balance. Simulated water relations were tested with ψc, water uptake, and energy exchange measured under ambient (363 μmol mol-1) and elevated (566 μmol mol-1) C a and high vs. low irrigation in a free air CO2 enrichment experiment during 1998 and 1999. Model results, corroborated by field measurements, showed that elevated Ca raised ψc and lowered latent heat fluxes under high irrigation and delayed water stress under low irrigation. Changes in ψc modeled under ambient vs. elevated Ca varied diurnally, with lower ψc causing earlier midafternoon stomatal closure under ambient Ca. Modeled changes in sorghum water status caused elevated Ca to raise seasonal water efficiency under high and low irrigation by 20 and 26% (vs. 20 and 13% measured) in 1998 and by 9 and 27% (vs. 6 and 26% measured) in 1999. Ecosys was used to generate an irrigation response function for sorghum yield, which indicated that yields would rise by ≈13% for a range of irrigation rates if air temperatures were to rise by 3°C and Ca by 50%. Current high sorghum yields could be achieved with ≈120 mm or ≈20% less irrigation water if these rises in temperature and Ca were to occur.
- Liu, Y., Limin, M. a., Leavitt, S. W., Cai, Q., & Liu, W. (2004). A preliminary seasonal precipitation reconstruction from tree-ring stable carbon isotopes at Mt. Helan, China, since AD 1804. Global and Planetary Change, 41(3-4), 229-239.More infoAbstract: The relationship between the stable carbon isotope composition of tree rings from Mt. Helan, China, and precipitation was investigated for the period AD 1804-1997. The results reveal that in the Mt. Helan region, δ13C of tree rings negatively correlates with the total precipitation from February to July. We calculated discrimination (Δ) to remove the effects of δ13C of atmospheric CO2 on the tree-ring δ13C series, and reconstructed February to July precipitation back to 1804 using linear regression (R2=0.415, R 2adj=0.394, F=19.88, P
- Panyushkina, I. P., Leavitt, S. W., Wiedenhoeft, A., Noggle, S., Curry, B., & Grimm, E. (2004). Tree-ring records of near-Younger Dryas time in central North America - Preliminary results from the Lincoln quarry site, central Illinois, USA. Radiocarbon, 46(2), 933-941.More infoAbstract: The abrupt millennial-scale changes associated with the Younger Dryas (YD) event ("chronozone") near the dawn of the Holocene are at least hemispheric, if not global, in extent. Evidence for the YD cold excursion is abundant in Europe but fairly meager in central North America. We are engaged in an investigation of high-resolution environmental changes in mid-North America over several millennia (about 10,000 to 14,000 BP) during the Late Glacial-Early Holocene transition, including the YD interval. Several sites containing logs or stumps have been identified and we are in the process of initial sampling or re-sampling them for this project. Here, we report on a site in central Illinois containing a deposit of logs initially thought to be of YD age preserved in alluvial sands. The assemblage of wood represents hardwood (angiosperm) trees, and the ring-width characteristics are favorable to developing formal tree-ring chronologies. However, 4 new radiocarbon dates indicate deposition of wood may have taken place over at least 8000 14C yr (6000-14,000 BP). This complicates the effort to develop a single floating chronology of several hundred years at this site, but it may provide wood from a restricted region over a long period of time from which to develop a sequence of floating chronologies, the timing of deposition and preservation of which could be related to paleoclimatic events and conditions.
- Robertson, I., Loader, N. J., McCarroll, D., Carter, A. H., Cheng, L., & Leavitt, S. W. (2004). δ13C of tree-ring lignin as an indirect measure of climate change. Water, Air, and Soil Pollution: Focus, 4(2-3), 531-544.More infoAbstract: High-resolution paleoclimatic data are an essential requirement for testing numerical models of climate change and the global carbon cycle. If the long tree-ring chronologies, originally established for the purpose of dendrochronology, are to be fully exploited as an indirect measure of past climatic variability, additional techniques are required to obtain this information. The determination of the δ 13C value of tree-ring cellulose has been used successfully to reconstruct past climates. However, under both aerobic and anaerobic conditions, the polysaccharide components of vascular plants (mainly cellulose and hemicelluloses) are more prone to rapid degradation than lignin. This has serious implications for the use of carbon isotope values of tree-ring cellulose as an indirect measure of past climates. An absolutely dated ring-width chronology was established for oaks (Quercus robur L.) growing at Sandringham Park in eastern England. Carbon isotope values were determined on α-cellulose and 'Klason' lignin isolated from annual latewood samples over the period AD 1895-1999. The carbon isotope values of earlywood lignin are correlated with the latewood carbon isotope values of the previous year, supporting the theory that some of the carbon utilised in earlywood synthesis is assimilated in the previous year. The high-frequency variance in the carbon isotope indices of latewood lignin and cellulose is highly correlated with combined July and August environmental variables, indicating that they were formed at similar times. There was no evidence of secondary lignification. These results demonstrate that the determination of carbon isotope values of latewood lignin offers the potential to obtain unambiguous proxy climatic data covering several millennia. © 2004 Kluwer Academic Publishers.
- Tarhule, A., & Leavitt, S. W. (2004). Comparison of stable-carbon isotope composition in the growth rings of Isorberlinia doka, Daniella oliveri, and Tamarindus indica and West African climate. Dendrochronologia, 22(1), 61-70.More infoAbstract: Inter-ring variations in stable-carbon isotope composition of cellulose in Daniella oliveri, Isoberlinia doka, and Tamarindus indica stumps were analyzed for nine trees in the Soudan-Sahel zone of West Africa. This pioneer isotopic study is a contribution to on-going efforts to evaluate the feasibility of extracting proxy climatic records from tree-rings in this area. Our aim is to investigate trends in individual floating δ13C time series to assess if patterns in variability within and between rings are consistent with those in other climatically sensitive tree-ring δ13C series. Samples were collected primarily from tree stumps but kill dates were unknown. Tree age was estimated from stump condition and shoot regrowth. This approach, combined with low sample replication, hindered the establishment of absolute dates. Nevertheless, δ13C and discrimination (Δ13C), absolute values and variability in the time series are similar to those often seen in tree-rings. Average δ13C values for individual series range from -25.90‰ to -23.72‰ and average Δ13C values from 16.56‰ to 18.99‰. It is well known that seasonal rainfall is inversely related to the δ13C in growth-season-dry environments and positively covaries with Δ. We investigate the possibility of using this relationship to temporally anchor the time series and to identify missing and false rings in wood segments with indistinct growth. The variability in these time series is consistent with expected patterns and shows promise for facilitating the development of stable carbon isotope tree-ring chronologies for West Africa. © 2004 Elsevier GmbH. All rights reserved.
- Triggs, J. M., Kimball, B. A., Pinter Jr., P. J., Wall, G. W., Conley, M. M., Brooks, T. J., LaMorte, R. L., Adam, N. R., Ottman, M. J., Matthias, A. D., Leavitt, S. W., & Cerveny, R. S. (2004). Free-air CO2 enrichment effects on the energy balance and evapotranspiration of sorghum. Agricultural and Forest Meteorology, 124(1-2), 63-79.More infoAbstract: Increasing atmospheric carbon dioxide (CO2) likely will affect future water requirements of most plants, including agricultural crops. This research quantifies such effects on the energy balance and evapotranspiration (ET) of sorghum (Sorghum bicolor (L.) Möench, a C4 grain crop) using a residual energy balance approach. During the summer and autumn of 1998 and 1999, sorghum was grown under free-air CO2 enrichment (FACE) conditions near Maricopa, Arizona. Latent heat flux (λET) was determined by subtracting soil heat flux (G0) and sensible heat flux (H) from net radiation (Rn) values in both Control CO2 plots (about 370 μmol mol-1) and FACE plots (Control + 200 μmol mol -1). Rn was observed using net radiometers. G0 was measured with soil heat flux plates at a depth of 10 mm, then corrected for heat storage above the plates. H was determined using measurements of air temperature from aspirated psychrometers, leaf temperature from infrared thermometers, and wind data from a three-cup anemometer. Both FACE and Control plots were divided into semicircular halves to allow a well-watered (Wet) treatment and a drought-stressed (Dry) treatment. This allowed comparisons of the FACE effect on ET in normal and water-stressed conditions. Under Wet conditions, FACE decreased λET by 13.8±1.8% in 1998, and 11.8±1.9% in 1999. Drought-stress resulted in a reduction in λET of 8.5±3.7% for the FACE treatments in 1998, but an increase in λET of 10.5±5.1% in 1999. When soil water was readily available, midday canopy temperatures in the FACE plots were increased by 1.47±0.09°C in 1998, and 1.85±0.20°C in 1999, indicative of increased stomatal resistance due to CO2 enrichment. These data suggest that soil water availability is a determining factor for the FACE effect. Water use efficiency (WUE) increased about 28% due to elevated CO 2 under Wet conditions due to a savings of water for about the same growth, whereas under Dry conditions it increased about 16% due to much greater relative growth on only a slightly higher amount of water. © 2004 Elsevier B.V. All rights reserved.
- Cousins, A. B., Adam, N. R., Wall, G. W., Kimball, B. A., Pinter Jr., P. J., Ottman, M. J., Leavitt, S. W., & Webber, A. N. (2003). Development of C4 photosynthesis in sorghum leaves grown under free-air CO2 enrichment (FACE). Journal of Experimental Botany, 54(389), 1969-1975.More infoPMID: 12837815;Abstract: The developmental pattern of C4 expression has been well characterized in maize and other C4 plants. However, few reports have explored the possibility that the development of this pathway may be sensitive to changes in atmospheric CO2 concentrations. Therefore, both the structural and biochemical development of leaf tissue in the fifth leaf of Sorghum bicolor plants grown at elevated CO2 have been characterized. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenol-pyruvate carboxylase (PEPC) activities accumulate rapidly as the leaf tissue differentiates and emerges from the surrounding whorl. Rubisco was not expressed in a cell-specific manner in the youngest tissue at the base of the leaf, but did accumulate before PEPC was detected. This suggests that the youngest leaf tissue utilizes a C3-like pathway for carbon fixation. However, this tissue was in a region of the leaf receiving very low light and so significant rates of photosynthesis were not likely. Older leaf tissue that had emerged from the surrounding whorl into full sunlight showed the normal C4 syndrome. Elevated CO2 had no effect on the cell-specific localization of Rubisco or PEPC at any stage of leaf development, and the relative ratios of Rubisco to PEPC remained constant during leaf development. However, in the oldest tissue at the tip of the leaf, the total activities of Rubisco and PEPC were decreased under elevated CO2 implying that C4 photosynthetic tissue may acclimate to growth under elevated CO2.
- Derner, J. D., Johnson, H. B., Kimball, B. A., Pinter Jr., P. J., Polley, H. W., Tischler, C. R., Boutton, T. W., Lamorte, R. L., Wall, G. W., Adam, N. R., Leavitt, S. W., Ottman, M. J., Matthias, A. D., & Brooks, T. J. (2003). Above- and below-ground responses of C3-C4 species mixtures to elevated CO2 and soil water availability. Global Change Biology, 9(3), 452-460.More infoAbstract: We evaluated the influences of CO2 [Control, ∼ 370 μmol mol-1; 200 μmol mol-1 above ambient applied by free-air CO2 enrichment (FACE)] and soil water (Wet, Dry) on above- and below-ground responses of C3 (cotton, Gossypium hirsutum) and C4 (sorghum, Sorghum bicolor) plants in monocultures and two density mixtures. In monocultures, CO2 enrichment increased height, leaf area, above-ground biomass and reproductive output of cotton, but not sorghum, and was independent of soil water treatment. In mixtures, cotton, but not sorghum, above-ground biomass and height were generally reduced compared to monocultures, across both CO2 and soil water treatments. Density did not affect individual plant responses of either cotton or sorghum across the other treatments. Total (cotton + sorghum) leaf area and above-ground biomass in low-density mixtures were similar between CO2 treatments, but increased by 17-21% with FACE in high-density mixtures, due to a 121% enhancement of cotton leaf area and a 276% increase in biomass under the FACE treatment. Total root biomass in the upper 1.2 m of the soil was not influenced by CO2 or by soil water in monoculture or mixtures; however, under dry conditions we observed significantly more roots at lower soil depths (> 45 cm). Sorghum roots comprised 81-85% of the total roots in the low-density mixture and 58-73% in the high-density mixture. CO2-enrichment partly offset negative effects of interspecific competition on cotton in both low- and high-density mixtures by increasing above-ground biomass, with a greater relative increase in the high-density mixture. As a consequence, CO2-enrichment increased total above-ground yield of the mixture at high density. Individual plant responses to CO2 enrichment in global change models that evaluate mixed plant communities should be adjusted to incorporate feedbacks for interspecific competition. Future field studies in natural ecosystems should address the role that a CO2-mediated increase in C3 growth may have on subsequent vegetation change.
- Leavitt, S. W., Idso, S. B., Kimball, B. A., Burns, J. M., Sinha, A., & Stott, L. (2003). The effect of long-term atmospheric CO2 enrichment on the intrinsic water-use efficiency of sour orange trees. Chemosphere, 50(2), 217-222.More infoPMID: 12653293;Abstract: Every two months of 1992, as well as on three occasions in 1994-1995, we obtained leaf samples together with samples of surrounding air from eight well-watered and fertilized sour orange (Citrus aurantium L.) trees that were growing out-of-doors at Phoenix, Arizona, USA. These trees had been planted in the ground as small seedlings in July of 1987 and enclosed in pairs by four clear-plastic-wall open-top chambers of which two have been continuously maintained since November of that year at a CO2 concentration of 400 μmol mol-1 and two have been maintained at 700 μmol mol-1. In September 2000, we also extracted north-south and east-west oriented wood cores that passed through the center of each tree's trunk at a height of 45 cm above the ground. Stable-carbon isotope ratios (13C/12C) derived from these leaf, wood and air samples were used to evaluate each tree's intrinsic water-use efficiency (iWUE). The grand-average result was an 80% increase in this important plant parameter in response to the 300 μmol mol-1 increase in atmospheric CO2 concentration employed in the study. This increase in sour orange tree iWUE is identical to the long-term CO2-induced increase in the trees' production of wood and fruit biomass, which suggests there could be little to no change in total water-use per unit land area for this species as the air's CO2 content continues to rise. It is also identical to the increase in the mean iWUE reported for 23 groups of naturally occurring trees scattered across western North America that was caused by the historical rise in the air's CO2 content that occurred between 1800 and 1985. Published by Elsevier Science Ltd.
- Pataki, D. E., Ellsworth, D. S., Evans, R. D., Gonzalez-Meler, M., King, J., Leavitt, S. W., Lin, G., Matamala, R., Pendall, E., Siegwolf, R., Kessel, C. V., & Ehleringer, J. R. (2003). Tracing changes in ecosystem function under elevated carbon dioxide conditions. BioScience, 53(9), 805-818.More infoAbstract: Responses of ecosystems to elevated levels of atmospheric carbon dioxide (CO2) remain a critical uncertainty in global change research. Two key unknown factors are the fate of carbon newly incorporated by photosynthesis into various pools within the ecosystem and the extent to which elevated CO 2 is transferred to and sequestered in pools with long turnover times. The CO2 used for enrichment in many experiments incorporates a dual isotopic tracer, in the sense that ratios of both the stable carbon-13 (13C) and the radioactive carbon-14 (14C) isotopes with respect to carbon-12 are different from the corresponding ratios in atmospheric CO2. Here we review techniques for using 13C and 14C abundances to follow the fate of newly fixed carbon and to further our understanding of the turnover times of ecosystem carbon pools. We also discuss the application of nitrogen, oxygen, and hydrogen isotope analyses for tracing changes in the linkages between carbon, nitrogen, and water cycles under conditions of elevated CO2.
- Cousins, A. B., Adam, N. R., Wall, G. W., Kimball, B. A., Pinter Jr., P. J., Ottman, M. J., Leavitt, S. W., & Webber, A. N. (2002). Photosystem II energy use, non-photochemical quenching and the xanthophyll cycle in Sorghum bicolor grown under drought and free-air CO2 enrichment (FACE) conditions. Plant, Cell and Environment, 25(11), 1551-1559.More infoAbstract: The present study was carried out to test the hypothesis that elevated atmospheric CO2 (Ca) will alleviate over-excitation of the C4 photosynthetic apparatus and decrease non-photochemical quenching (NPQ) during periods of limited water availability. Chlorophyll a fluorescence was monitored in Sorghum bicolor plants grown under a free-air carbon-dioxide enrichment (FACE) by water-stress (Dry) experiment. Under Dry conditions elevated Ca increased the quantum yield of photosystem II (øPSII) throughout the day through increases in both photochemical quenching coefficient (qp) and the efficiency with which absorbed quanta are transferred to open PSII reaction centres (Fv′/Fm′). However, in the well-watered plants (Wets) FACE enhanced øPSII only at midday and was entirely attributed to changes in Fv′/Fm′. Under field conditions, decreases in øPSII under Dry treatments and ambient Ca corresponded to increases in NPQ but the de-epoxidation state of the xanthophyll pool (DPS) showed no effects. Water-stress did not lead to long-term damage to the photosynthetic apparatus as indicated by øPSII and carbon assimilation measured after removal of stress conditions. We conclude that elevated Ca enhances photochemical light energy usage in C4 photosynthesis during drought and/or midday conditions. Additionally, NPQ protects against photo-inhibition and photodamage. However, NPQ and the xanthophyll cycle were affected differently by elevated Ca and water-stress.
- Leavitt, S. W. (2002). Prospects for reconstruction of seasonal environment from tree-ring δ13C: Baseline findings from the Great Lakes area, USA. Chemical Geology, 192(1-2), 47-58.More infoAbstract: Plant δ13C and discrimination (Δ ≈ rates of carbon fixation and stomatal conductance. This study scrutinizes the seasonal variation of δ13C and Δ in growth rings developed in 1990, as represented in trees of 11 species at three sites along a 5° latitude transect in Illinois and Wisconsin, USA. Radial stem xylem growth of the trees was monitored with monthly punch cores, and the resulting growth curves precisely identified the growth period represented in the five segments into which each growth ring was subdivided. Seasonal δ13C variation of cellulose was usually 1-1.5‰, and the seasonal patterns were similar among angiosperm and conifer trees at a site, with angiosperms generally 13C-depleted by 0.5-1.5‰. Seasonal δ13C (and Δ) patterns was different among sites, however, and regression with several environmental parameters revealed most frequent significant correlation with soil moisture (Palmer Drought Severity Index [PDSI]). The site-averaged seasonal Δ patterns track each of their divergent local seasonal drought index chronologies very well, even when isotopic curves of the different trees are simply averaged without adjustment for different growth rates. Seasonal tree-ring δ13C analysis is affirmed as a valuable method for assessing seasonal moisture status. Furthermore, results favor successful extension to ancient tree rings where different species and slope aspects may be represented in wood samples at a collection site, and where rates of ring development and seasonal δ13Cair are not likely to be known. © 2002 Elsevier Science B.V. All rights reserved.
- Leavitt, S. W., Wright, W. E., & Long, A. (2002). Spatial expression of ENSO, drought, and summer monsoon in seasonal δ13C of ponderosa pine tree rings in southern Arizona and New Mexico. Journal of Geophysical Research D: Atmospheres, 107(18), 3-1-3-10.More infoAbstract: Major climatic impacts from ENSO and the North American Monsoon in the U.S. Southwest provide fortuitous circumstances for exploring the suitability of stable-carbon isotope (δ13 C) composition of tree rings to represent and reconstruct interannual climate variability. We sampled ponderosa pine tree rings at eight sites in seven mountain ranges over a 500-km transect in southern Arizona and New Mexico. We developed δ13C chronologies for 1985-1995 by subdividing rings into pre- and post-monsoon-onset segments based on the presence of a false-latewood band in each ring induced by the arid period immediately preceding the summer monsoon, pooling these segments from multiple trees at each site, and analyzing the cellulose component. Chronologies from two subsets of trees from the same site and separate chronologies from two different sites in the same mountain range indicate strong coherence and signal reproducibility. Additionally, the seasonal δ13C patterns showed remarkable coherence across the transect, with the spatially disjunct easternmost chronology tending to be most frequently dissimilar. The δ13 C values of ring subdivisions before the false-latewood band are generally positively correlated with winter Southern Oscillation Indices (SOI), resulting from the ENSO teleconnexion with southwestern winter/early spring precipitation. The 1313C from these sites correlates with monthly Palmer Drought Severity Indices (PDSI), with the strongest correlations occurring in progressively later months from first-formed to last-formed subdivisions. The post-false latewood subdivision 1313C was particularly strongly correlated with summer precipitation amount. These results confirm the strong influence of moisture on tree ring δ13C and support their use in both temporal and spatial modes to infer past climate variability. Copyright 2002 by the American Geophysical Union.
- Pedicino, L. C., Leavitt, S. W., Betancourt, J. L., & Van de Water, P. K. (2002). Historical variations in δ13Cleaf of herbarium specimens in the southwestern U.S.. Western North American Naturalist, 62(3), 348-359.More infoAbstract: The uncontrolled, global increase in atmospheric CO2 concentration (ca 80 ppmv) and decline in δ13Cair (ca 1.5%) since industrialization provide experimental boundary conditions by which to assess physiological response of vegetation. To examine consequences of these global atmospheric changes in the southwestern U.S., 350 specimens of Atriplex confertifolia, A. canescens, Ephedra viridis, Pinus edulis, P. flexilis, Juniperus scopulorum, and Quercus turbinella of precisely known age spanning the last 150 years were acquired from 9 herbaria. Cellulose analysis of δ13Cplant and estimation of isotopic discrimination (Δ) permitted calculation of water-use efficiency (A/g). The δ13Cplant chronologies of C4 Atriplex spp. show some promise as a reliable proxy for δ13Cair because their mean trends approximate the known δ13Cair chronology. However, the high variability would necessitate multiple samples at any time period to accurately represent the mean. The generally increasing A/g trends of the 5 C3 species are particularly pronounced for P. edulis and, after the 1950s, for J. scopulorum, but less evident for P. flexilis, E. viridis, and Q. turbinella, evidencing possible differences in species response to rising CO2 concentrations. The trends are statistically noisy, however, possibly due to complex microclimates, extreme seasonality, and great interannual variability typical of the southwestern U.S. Herbarium specimens, at least in the Southwest, may be less useful for precise detection of direct CO2 effects on plant physiology than tree rings, where the variability can be constrained to a single individual over time.
- Terwilliger, V. J., Betancourt, J. L., Leavitt, S. W., & Van de Water, P. K. (2002). Leaf cellulose δD and δ18O trends with elevation differ in direction among co-occurring, semiarid plant species. Geochimica et Cosmochimica Acta, 66(22), 3887-3900.More infoAbstract: The potential to reconstruct paleoclimate from analyses of stable isotopes in fossil leaf cellulose could be enhanced by adequate calibration. This potential is likely to be particularly great in mid-latitude deserts, where a rich store of fossil leaves is available from rodent middens. Trends in δD and δ18O of leaf cellulose were examined for three species growing across climatic gradients caused by elevation and slope aspect in southeastern Utah, USA. The species differed in morphology (Pinus edulis vs. Yucca glauca), photosynthetic pathway (C3 Y. glauca vs. CAM Yucca baccata) or both (P. edulis vs. Y. baccata). The δDLCN (leaf cellulose nitrate) and δ18OLC (leaf cellulose) values of P. edulis decreased with elevation. Stem water δD values either increased (in spring) or did not change with elevation (in summer). Needle water δD values usually decreased with elevation and differed greatly with leaf age. These results suggest that δ cellulose values of P. edulis record the effects of climate on the isotopic composition of leaf water but not climate effects on meteoric water. In contrast to P. edulis, δDLCN values of Y. glauca increased with elevation. The δ18O LC values ofc Y. glauca also increased with elevation but less significantly and only on south-facing slopes. The δ cellulose values in both P. edulis and Y. glauca were most significantly related to changes in temperature, although temperature and precipitation were negatively correlated in the study area. Where all three species co-occurred, their δDLCN values differed but their δ18O LC values were the same. The disparity in δDLCN between Y. baccata and the other species corresponds to differences in biochemical fractionations associated with photosynthetic pathway. Biochemical fractionations may also contribute to differences between the two C3 species. Knowledge of factors affecting responses of individual plant species to environment may be required to infer climate from δDLCN and δ18OLC. © 2002 Elsevier Science Ltd.
- Van de Water, P. K., Leavitt, S. W., & Betancourt, J. L. (2002). Leaf δ13C variability with elevation, slope aspect, and precipitation in the southwest United States. Oecologia, 132(3), 332-343.More infoAbstract: Leaves from several desert and woodland species, including gymnosperms and angiosperms with both C3 and C4 physiology, were analyzed to detect trends in δ13Cleaf with elevation and slope aspect along two transects in southeastern Utah and south-central New Mexico, USA. The main difference between the two transects is the steeper elevational gradient for mean annual and summer precipitation in the southern transect. For any given species, we found that isotopic differences between individual plants growing at the same site commonly equal differences measured for plants along the entire altitudinal gradient. In C3 plants, δ13Cleaf values become slightly enriched at the lowest elevations, the opposite of trends identified in more humid regions. Apparently, increasing water-use efficiency with drought stress offsets the influence of other biotic and abiotic factors that operate to decrease isotopic discrimination with elevation. For some species shared by the two transects (e.g., Pinus edulis and Cercocarpus montanus), δ13Cleaf values are dramatically depleted at sites that receive more than 550 mm mean annual precipitation, roughly the boundary (pedalfer-pedocal) at which soils commonly fill to field capacity in summer and carbonates are leached. We hypothesize that, in summer-wet areas, this may represent the boundary at which drought stress overtakes other factors in determining the sign of δ13Cleaf with elevation. The opposition of isotopic trends with elevation in arid versus humid regions cautions against standard correction for elevation in comparative studies of δ13Cleaf.
- Conley, M. M., Kimball, B. A., Brooks, T. J., Pinter Jr., P. J., Hunsaker, D. J., Wall, G. W., Adam, N. R., LaMorte, R. L., Matthias, A. D., Thompson, T. L., Leavitt, S. W., Ottman, M. J., Cousins, A. B., & Triggs, J. M. (2001). CO2 enrichment increases water-use efficiency in sorghum. New Phytologist, 151(2), 407-412.More infoAbstract: Sorghum (Sorghum bicolor) was grown for two consecutive seasons at Maricopa, AZ, USA, using the free-air CO2 enrichment (FACE) approach to investigate evapotranspiration of this C4 plant at ample and limited water supplies. Crop evapotranspiration (ET) was measured using two CO2 concentrations (control, c. 370 μmol mol-1; FACE, ambient +200 μmol mol-1) and two irrigation treatments (well watered and water-limited). Volumetric soil water content was measured before and after each irrigation using neutron scattering techniques. Averaged over both years, elevated CO2 reduced cumulative ET by 10% when plants were given ample water and by 4% under severe drought stress. Water-use efficiency based on grain yield (WUE-G) increased, due to CO2 enrichment, by 9% and 19% in wet and dry plots, respectively; based on total biomass, water-use efficiency (WUE-B) increased by 16% and 17% in wet and dry plots, respectively. These data suggest that in the future high-CO2 environment, water requirements for irrigated sorghum will be lower than at present, while dry-land productivity will increase, provided global warming is minimal. © New Phytologist (2001).
- Cousins, A. B., Adam, N. R., Wall, G. W., Kimball, B. A., Pinter Jr., P. J., Leavitt, S. W., LaMorte, R. L., Matthias, A. D., Ottman, M. J., Thompson, T. L., & Webber, A. N. (2001). Reduced photorespiration and increased energy-use efficiency in young CO2-enriched sorghum leaves. New Phytologist, 150(2), 275-284.More infoAbstract: • To determine the response of C4 plants to elevated CO2 it is necessary to establish whether young leaves have a fully developed C4 photosynthetic apparatus, and whether photosynthesis in these leaves is responsive to elevated CO2. • The effect of free-air CO2 enrichment (FACE) on the photosynthetic development of the C4 crop Sorghum bicolor was monitored. Simultaneous measurements of chlorophyll a fluorescence and carbon assimilation were made to determine energy utilization, quantum yields of carbon fixation (φCO2) and photosystem II (φPSII), as well as photorespiration. • Assimilation in the second leaf of FACE plants was 37% higher than in control plants and lower apparent rates of photorespiration at growth CO2 concentrations were exhibited. In these leaves, φPSll: φCO2 was high at low atmospheric CO2 concentration (Ca) due to overcycling of the C4 pump and increased leakiness. As Ca increased, φPSII: φCO2 decreased as a greater proportion of energy derived from linear electron transfer was used by the C3 cycle. • The stimulation of C4 photosynthesis at elevated Ca in young leaves was partially due to suppressed photorespiration. Additionally, elevated Ca enhanced energy-use efficiency in young leaves, possibly by decreasing CO2 leakage from bundle sheath cells, and by decreasing overcycling of the C4 pump.
- Grant, R. F., Kimball, B. A., Brooks, T. J., Wall, G. W., P.J., J. P., Hunsaker, D. J., Adamsen, F. J., Lamorte, R. L., Leavitt, S. W., Thompson, T. L., & Matthias, A. D. (2001). Modeling interactions among carbon dioxide, nitrogen, and climate on energy exchange of wheat in a free air carbon dioxide experiment. Agronomy Journal, 93(3), 638-649.More infoAbstract: Changes in mass and energy exchange by crops under rising atmospheric CO2 concentration (Ca) may be affected by N and weather; Ca interacts with weather on mass and energy exchange through limitations on latent heat flux imposed by stomatal conductance, which is affected by Ca, and aerodynamic conductance, which is affected by weather. We examined the bases for these interactions with the ecosystem model ecosys. Simulation results were tested with energy flux data from a Free Air CO2 Enrichment (FACE) experiment in which wheat (Triticum aestivum L.) was grown under 548 vs. 363 μmol mol-1 Ca and fertilized with 7 vs. 35 g N m-2. Both model and experimental results indicated that raising Ca from 363 to 548 μmol mol-1 reduced midday latent heat fluxes by ca. 50 W m-2 for wheat fertilized with 35 g N m-2, and by ca. 100 W m-2 for wheat fertilized with only 7 g N m-2 when N deficits developed later in the growing season. These reductions were smaller under low wind speeds (
- Kimball, B. A., Morris, C. F., Pinter Jr., P. J., Wall, G. W., Hunsaker, D. J., Adamsen, F. J., LaMorte, R. L., Leavitt, S. W., Thompson, T. L., Matthias, A. D., & Brooks, T. J. (2001). Elevated CO2, drought and soil nitrogen effects on wheat grain quality. New Phytologist, 150(2), 295-303.More infoAbstract: • The likely consequences of future high levels of atmospheric CO2 concentration on wheat (Triticum aestivum L.) grain nutritional and baking quality were determined. • Two free-air CO2 enrichment (FACE; 550 mmol mol-1) experiments were conducted at ample (Wet) and limiting (Dry) levels of irrigation, and a further two experiments at ample (High-N) and limiting (Low-N) nitrogen concentrations. Harvested grain samples were subjected to a battery of nutritional and bread-making quality tests. • The Dry treatment improved grain quality slightly (protein +2%; bread loaf volume +3%). By contrast, Low-N decreased quality drastically (protein -36%; loaf volume-26%). At ample water and N, FACE decreased quality slightly (protein-5%; loaf volume -2%) in the irrigation experiments and there was no change in the nitrogen experiments. At Low-N, FACE tended to make the deleterious effects of Low-N worse (protein -33% and -39%, at ambient CO2 and FACE, respectively; loaf volume -22% and -29% at ambient CO2 and FACE, respectively). • The data suggest that future elevated CO2 concentrations will exacerbate the deleterious effects of low soil nitrogen on grain quality, but with ample nitrogen fertilizer, the effects will be minor.
- Leavitt, S. W., Pendall, E., Paul, E. A., Brooks, T., Kimball, B. A., Pinter Jr., P. J., Johnson, H. B., Matthias, A., Wall, G. W., & LaMorte, R. L. (2001). Stable-carbon isotopes and soil organic carbon in wheat under CO2 enrichment. New Phytologist, 150(2), 305-314.More infoAbstract: • Stable-carbon isotopic tracers were enlisted in 1996 and 1997 wheat (Triticum aestivum) free-air CO2 enrichment (FACE) experiments to detect entry of new C into soil organic carbon (SOC) pools. Any enhanced soil inputs might mitigate rising atmospheric CO2. • The CO2 used to enrich FACE plots (to ambient +190 μmol-1 resulted in 13C-depleted wheat relative to ambient plants and the native SOC. To trace new C in control plots C4-plant-derived exotic soils were placed into subplots in high-N FACE and control treatments, and a 13CO2 gas tracer was pulsed to subplots in high-N control replicates. • Under high-N, isotopic mass balance showed 6% (P = 0.003) and 5% (P = 0.04) new C in 0-15-cm and 15-30-cm FACE SOC, respectively, after 2 yr. Results from the C4-soil subplots were ambiguous, but the 13CO2 tracer induced a sac δ13C increase (P = 0.08) at 15-30 cm in control-high N consistent with 6% new C. • We infer c, 3% year-1 (30-40 g C m-2 yr-1) SOC turnover in surface soils at high-N under both ambient and elevated CO2. The 13CO2-tracer result, however, is less reliable because of lower significance, fewer replicates and heterogeneous isotopic distribution within plants.
- Ottman, M. J., Kimball, B. A., Pinter, P. J., Wall, G. W., Vanderlip, R. L., Leavitt, S. W., Lamorte, R. L., Matthias, A. D., & Brooks, T. J. (2001). Elevated CO2 increases sorghum biomass under drought conditions. New Phytologist, 150(2), 261-273.More infoAbstract: • Atmospheric CO2 concentration is expected to increase by 50% near the middle of this century. The effects the free air CO2 enrichment (FACE) is presented here on growth and development of field-grown grain sorghum (Sorghum bicolor) at ample (wet) and limiting (dry) levels of irrigation water at Maricopa, AZ, USA. • Daytime CO2 mole fractions were 561 and 368 μmol mol-1 for the FACE and control treatments, respectively. Irrigation plus precipitation averaged 1132 mm for the wet plots and 396 mm in the dry plots. • During the growing season, FACE increased biomass accumulation in the dry plots but the effects in the wet plots were inconsistent. At final harvest, FACE increased total yield from 999 to 1151 g m-2 in the dry plots and had no effect in the wet plots. • If atmospheric CO2 continues to increase, total sorghum yield is likely to be higher in the future in areas where water is limited.
- Paul, E. A., Collins, H. P., & Leavitt, S. W. (2001). Dynamics of resistant soil carbon of midwestern agricultural soils measured by naturally occurring 14C abundance. Geoderma, 104(3-4), 239-256.More infoAbstract: Information on the mean residence time (MRT) of soil organic carbon (SOC) on different soil types and management regimes is required for pedo-geological, agronomic, ecological and global change interpretations. This is best determined by carbon dating the total soil together with acid hydrolysis and carbon dating of the non-hydrolyzable residue (NHC). Midwestern US soils in a transect from Lamberton, MN to Kutztown, PA were found to contain from 33% to 65% of their SOC in the non-hydrolyzable fraction. Soils on lacustrine deposits had the most NHC; glacial till and shale soils, the least. The MRTs of the SOC of surface horizons of soil ranged from modern to 1100 years with an average of 560 years. The MRT increased to an average of 1700 years in the 25-50-cm depth increment and 2757 years at 50-100 cm. The NHC was 1340 years greater at the surface and 5584 years at depth. The MRTs of the total SOC were inversely correlated to sand and directly related to clay content. Silt did not have a significant effect on the MRT of total SOC, but was significantly correlated with the MRT of the NHC. A four-parameter model described the relationship between the SOC content and MRT. The complexity of this equation reflected the strong effect of depth, which greatly decreased SOC while increasing the MRT. The MRT of these soils, as determined with carbon dating of the naturally occurring 14C, was compared to that measured with the 13C signal produced by approximately 30 years of continuous corn (Zea mays L.) (C4) on soils with a known plant history of C3-C4 cropping. The equation of 14C MRT = 176(13CMRT)0.54 with an R2 of 0.70 showed that although short-term 13C studies correlate well with the total MRT, they reflect the dynamics of the active and slow pools, not the total SOC. © 2001 Elsevier Science B.V. All rights reserved.
- Pendall, E., Leavitt, S. W., Brooks, T., Kimball, B. A., Pinter Jr., P. J., Wall, G. W., LaMorte, R. L., Wechsung, G., Wechsung, F., Adamsen, F., Matthias, A. D., & Thompson, T. L. (2001). Elevated CO2 stimulates soil respiration in a FACE wheat field. Basic and Applied Ecology, 2(3), 193-201.More infoAbstract: Understanding the response of soil carbon (C) dynamics to higher atmospheric CO2 concentrations is critical for evaluating the potential for soil C sequestration on time scales of decades to centuries. Here, we report on changes in soil respiration under Free-Air CO2 Enrichment (FACE) where spring wheat was grown in an open field at two CO2 concentrations (ambient and ambient+200 μmol mol-1), under natural meteorological conditions. FACE increased soil respiration rates by 40-70% during the peak of wheat growth. On the FACE plots, stable C isotopic composition of soil CO2 was used to partition the soil CO2 flux into C from rhizosphere respiration and decomposition of pre-existing C. Decomposition contributed 100% of the soil CO2 flux before crop growth commenced, and only 35-45% of the flux at the peak of the growing season. Decomposition rates were not correlated with soil temperature, but rhizosphere respiration rates were strongly correlated with green leaf area index.
- Rillig, M. C., Wright, S. F., Kimball, B. A., Pinter, P. J., Wall, G. W., Ottman, M. J., & Leavitt, S. W. (2001). Elevated carbon dioxide and irrigation effects on water stable aggregates in a Sorghum field: A possible role for arbuscular mycorrhizal fungi. Global Change Biology, 7(3), 333-337.More infoAbstract: While soil biota and processes are becoming increasingly appreciated as important parameters for consideration in global change studies, the fundamental characteristic of soil structure is a neglected area of research. In a sorghum [Sorghum bicolor (L.) Moench] field experiment in which CO2 [supplied using free-air CO2 enrichment (FACE) technology] was crossed factorially with an irrigation treatment, soil aggregate (1-2mm) water stability increased in response to elevated CO2. Aggregate water stability was increased by 40% and 20% in response to CO2, at ample and limited water supply treatments, respectively. Soil hyphal lengths of arbuscular mycorrhizal fungi (AMF) increased strongly (with a threefold increase in the dry treatment) in response to CO2, and the concentrations of one fraction (easily extractable glomalin, EEG) of the AMF-produced protein glomalin were also increased. Two fractions of glomalin, and AMF hyphal lengths were all positively correlated with soil aggregate water stability. The present results further support the hypothesis that AMF can become important in global change scenarios. Although in this field study a causal relationship between hyphal length, glomalin and aggregate stability cannot be demonstrated, the present data do suggest that AMF could mediate changes in soil structure under elevated CO2. This could be of great importance in agricultural systems threatened by erosional soil loss.
- Wall, G. W., Brooks, T. J., Adam, N. R., Cousins, A. B., Kimball, B. A., Pinter Jr., P. J., LaMorte, R. L., Triggs, J., Ottman, M. J., Leavitt, S. W., Matthias, A. D., Williams, D. G., & Webber, A. N. (2001). Elevated atmospheric CO2 improved Sorghum plant water status by ameliorating the adverse effects of drought. New Phytologist, 152(2), 231-248.More infoAbstract: The interactive effects of atmospheric CO2 concentration and soil-water content on grain sorghum (Sorghum bicolor) are reported here. Sorghum plants were exposed to ambient (control) and free-air CO2 enrichment (FACE; ambient + 200 μmol mol-1), under ample (wet, 100% replacement of evapo-transpiration) and reduced (dry, postplanting and mid-season irrigations) water supply over two growing seasons. FACE reduced seasonal average stomatal conductance (g5) by 0.17 mol (H2O) m-2 s-1 (32% and 37% for dry and wet, respectively) compared with control; this was similar to the difference between dry and wet treatments. FACE increased net assimilation rate (A) by 4.77 μmol (CO2) m-2s-1 (23% and 9% for dry and wet, respectively), whereas dry decreased A by 10.50 μmol (CO2) m-2s-1 (26%) compared with wet. Total plant water potential (Ψw) was 0.16 MPa (9%) and 0.04 MPa (3%) less negative in FACE than in the control treatment for dry and wet, respectively. Under dry, FACE stimulated final shoot biomass by 15%. By ameliorating the adverse effects of drought, elevated atmospheric CO2 improved plant water status, which indirectly caused an increase in carbon gain.
- Williams, D. G., Gempko, V., Fravolini, A., Leavitt, S. W., Wall, G. W., Kimball, B. A., Pinter Jr., P. J., LaMorte, R., & Ottman, M. (2001). Carbon isotope discrimination by Sorghum bicolor under CO2 enrichment and drought. New Phytologist, 150(2), 285-293.More infoAbstract: • Sorghum bicolor was exposed to free-air CO2 enrichment (FACE) and drought at the Maricopa Agricultural Center, AZ, USA, in summer 1998. We predicted that bundle sheath leakiness (Φ) would be insensitive to FACE under well-irrigated (wet) conditions, but would be lower in FACE compared with control-CO2 treatments when irrigation was withheld (dry). • Leaf and air δ13C values and leaf pi/pa from gas exchange were measured to estimate carbon isotope discrimination (Δ) and Φ. Midday leaf water potential (Ψ) and photosynthetic rate were simultaneously measured to evaluate the influence of plant water status on Φ and the association between Φ and carbon gain. • Irrigation treatments affected Ψ, pi/pa, Δ and Φ in control CO2 and FACE rings. Differences in leaf Δ between wet- and dry-treatment plots resulted from changes in Φ and to stomatal influences on pi/pa. FACE had very little effect on Ψ, Δ and Φ in wet-treatment plots. However, Φ and Δ in dry plots were higher in control than in FACE rings. • FACE ameliorated the effects of drought on bundle sheath leakiness and Δ by reducing transpiration, prolonging soil water availability and enhancing plant water status. Direct effects of CO2 enrichment on C4 photosynthetic metabolism in Sorghum apparently are minimal and indirect effects depend on soil water supply.
- Wright, W. E., Long, A., Comrie, A. C., Leavitt, S. W., Cavazos, T., & Eastoe, C. (2001). Monsoonal moisture sources revealed using temperature, precipitation, and precipitation stable isotope timeseries. Geophysical Research Letters, 28(5), 787-790.More infoAbstract: Results of analyses using timeseries of mean temperature, precipitation amount, and stable isotopes from precipitation from July-August in Tucson, Arizona, have revealed atmospheric circulation patterns related to the North American Monsoon in the U.S. Southwest. The isotope timeseries and Tucson air temperatures and precipitation amount are significantly correlated. The temperature and isotope timeseries also correlate significantly with regional and extra-regional specific humidity, and with Eastern Pacific SSTs near the Mexican coast, evidence for a dominantly Pacific/Gulf of California summer moisture source for the period 1983-1999. Separation of extra-regional wind vector datasets into groups of years matching relative isotopic depletion or enrichment of the Tucson July-August precipitation seasonal means for the stable isotope timeseries (usually the extreme years in the Tucson seasonal temperature means) suggest circulation patterns entraining more tropical moisture in cooler/isotopically depleted years, and entraining less tropical moisture in hotter/isotopically enriched years.
- Adam, N. R., Wall, G. W., Kimball, B. A., Pinter Jr, P. J., Lamorte, R. L., Hunsaker, D. J., Adamsen, F. J., Thompson, T., Matthias, A. D., Leavitt, S. W., & Webber, A. N. (2000). Acclimation response of spring wheat in a free-air CO2 enrichment (FACE) atmosphere with variable soil nitrogen regimes. 1. Leaf position and phenology determine acclimation response. Photosynthesis Research, 66(1-2), 65-77.More infoAbstract: We have examined the photosynthetic acclimation of wheat leaves grown at an elevated CO2 concentration, and ample and limiting N supplies, within a field experiment using free-air CO2 enrichment (FACE). To understand how leaf age and developmental stage affected any acclimation response, measurements were made on a vertical profile of leaves every week from tillering until maturity. The response of assimilation (A) to internal CO2 concentration (Ci) was used to estimate the in vivo carboxylation capacity (Vcmax) and maximum rate of ribulose-1,5-bisphosphate limited photosynthesis (Asat). The total activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), and leaf content of Rubisco and the Light Harvesting Chlorophyll a/b protein associated with Photosystem II (LHC II), were determined. Elevated CO2 did not alter Vcmax in the flag leaf at either low or high N. In the older shaded leaves lower in the canopy, acclimatory decline in Vcmax and Asatwas observed, and was found to correlate with reduced Rubisco activity and Content. The dependency of acclimation on N supply was different at each developmental stage. With adequate N supply, acclimation to elevated CO2 was also accompanied by an increased LHC II/Rubisco ratio. At low N supply, contents of Rubisco and LHC II were reduced in all leaves, although an increased LHC II/Rubisco ratio under elevated CO2 was still observed. These results underscore the importance of leaf position, leaf age and crop developmental stage in understanding the acclimation of photosynthesis to elevated CO2 and nutrient stress.
- Brooks, T. J., Wall, G. W., Pinter Jr, P. J., Kimball, B. A., Lamorte, R. L., Leavitt, S. W., Matthias, A. D., Adamsen, F. J., Hunsaker, D. J., & Webber, A. N. (2000). Acclimation response of spring wheat in a free-air CO2 enrichment (FACE) atmosphere with variable soil nitrogen regimes. 3. Canopy architecture and gas exchange. Photosynthesis Research, 66(1-2), 97-108.More infoAbstract: The response of whole-canopy net CO2 exchange rate (CER) and canopy architecture to CO2 enrichment and N stress during 1996 and 1997 for open-field-grown wheat ecosystem (Triticum aestivum L. cv. Yecora Rojo) are described. Every Control (C) and FACE (F) CO2 treatment (defined as ambient and ambient +200 μmol mol-1, respectively) contained a Low- and High-N treatment. Low-N treatments constituted initial soil content amended with supplemental nitrogen applied at a rate of 70 kg N ha-1 (1996) and 15 kg N ha-1 (1997), whereas High-N treatments were supplemented with 350 kg N ha-1 (1996 and 1997). Elevated CO2 enhanced season-long carbon accumulation by 8% and 16% under Low-N and High-N, respectively. N-stress reduced season-long carbon accumulation 14% under ambient CO2, but by as much as 22% under CO2 enrichment. Averaging both years, green plant area index (GPAI) peaked approximately 76 days after planting at 7.13 for FH, 6.00 for CH, 3.89 for FL, and 3.89 for CL treatments. Leaf tip angle distribution (LTA) indicated that Low-N canopies were more erectophile than those of High-N canopies: 48° for FH, 52° for CH, and 58° for both FL and CL treatments. Temporal trends in canopy greenness indicated a decrease in leaf Chlorophyll content from the flag to flag-2 leaves of 25% for FH, 28% for CH, 17% for CL, and 33% for FL during 1997. These results indicate that significant modifications of canopy architecture occurs in response to both CO2 and N-stress. Optimization of canopy architecture may serve as a mechanism to diminish CO2 and N-stress effects on CER.
- Hunsaker, D. J., Kimball, B. A., Pinter Jr., P. J., Wall, G. W., Lamorte, R. L., Adamsen, F. J., Leavitt, S. W., Thompson, T. L., Matthias, A. D., & Brooks, T. J. (2000). CO2 enrichment and soil nitrogen effects on wheat evapotranspiration and water use efficiency. Agricultural and Forest Meteorology, 104(2), 85-105.More infoAbstract: Evapotranspiration (ET) and water use efficiency (WUE) were evaluated for two spring wheat crops, grown in a well-watered, subsurface drip-irrigated field under ambient (about 370 μmol mol-1 during daytime) and enriched (200 μmol mol-1 above ambient) CO2 concentrations during 1995-1996 and 1996-1997 in Free-Air CO2 Enrichment (FACE) experiments in central Arizona. The enriched (FACE) and ambient (Control) CO2 treatments were replicated in four, circular plots, each 25 m in diameter. Two soil nitrogen (N) treatments, ample (High N) and limited (Low N), were imposed on one-half of each circular plot. Wheat ET, determined using soil water balance procedures, was significantly greater in High N than Low N treatments starting in late-March (anthesis) during both years. Differences in ET between CO2 treatments during the seasons were generally small and not statistically significant, however, there was a tendency for the ET to be lower for FACE than Control under the High N treatment. The reduction in the cumulative seasonal ET due to FACE averaged 3.7 and 4.0% under High N and 0.7 and 1.2% under Low N in the first and second years, respectively. However, WUE (grain yield per unit seasonal ET) was significantly increased for the FACE treatment under both soil N treatments. For the High N treatment, the WUE was 19 and 23% greater for FACE than Control and for the Low N treatment the WUE was 12 and 7% greater for FACE than Control in the 2 years, respectively.
- Pinter Jr., P. J., Kimball, B. A., Wall, G. W., LaMorte, R. L., Hunsaker, D. J., Adamsen, F. J., Frumau, K. F., Vugts, H. F., Hendrey, G. R., Lewin, K. F., Nagy, J., Johnson, H. B., Wechsung, F., Leavitt, S. W., Thompson, T. L., Matthias, A. D., & Brooks, T. J. (2000). Free-air CO2 enrichment (FACE): Blower effects on wheat canopy microclimate and plant development. Agricultural and Forest Meteorology, 103(4), 319-333.More infoAbstract: Free-air carbon dioxide enrichment (FACE) provides a realistic, cost-effective method for evaluating the effects of supra-ambient CO2 concentrations on growth, development, yield, and water use of agricultural crops and natural ecosystems with very few of the problems normally associated with glasshouse or chamber type research. There are no walls interfering with incident radiation and no artificial constraints on rooting depth. With current FACE technology, CO2 enriched air is injected around the perimeter of circular plots and natural wind disperses the CO2 across the experimental area. Under stable, nighttime wind conditions found in FACE wheat experiments at Maricopa, Arizona, the blowers used to inject CO2 exerted subtle effects on the microclimate in a manner analogous to wind machines used for orchard frost protection. Plots equipped with blowers had nighttime foliage and air temperatures that averaged 0.6-1.0°C warmer than controls without blowers. A secondary effect of these elevated temperatures was that plots equipped with blowers displayed differences in dew duration (time that leaves were wet was reduced 30%), plant development (anthesis occurred 4 days earlier), and senescence [as measured with the normalized difference vegetation index (NDVI)]. Natural wind and turbulence appear to overcome the blower effect during daytime treatments and on some nights. Aerial thermal imagery (8-12 μm) acquired during the 1998 FACE experiment with grain sorghum provided additional evidence of the blower effect on canopy temperatures. Since increased plant tissue temperatures also occur when elevated CO2 induces partial stomatal closure and reduces transpiration, not all instances of canopy temperature elevation in CO2 enriched plots can be ascribed solely to the presence of blowers. It is concluded that proper controls for FACE facilities should have similar air flows to those used in the FACE plots. Advantages and disadvantages to nighttime CO2 enrichment are discussed. (C) 2000 Elsevier Science B.V.
- Sinclair, T. R., Pinter Jr., P. J., Kimball, B. A., Adamsen, F. J., LaMorte, R. L., Wall, G. W., Hunsaker, D. J., Adam, N., Brooks, T. J., Garcia, R. L., Thompson, T., Leavitt, S., & Matthias, A. (2000). Leaf nitrogen concentration of wheat subjected to elevated [CO2] and either water or N deficits. Agriculture, Ecosystems and Environment, 79(1), 53-60.More infoAbstract: Leaf N concentration is important because it is associated with the CO2 assimilatory capacity of crops, and in grasslands, it is an important determinant of forage nutritive value. Consequently, the productivity of both domestic and native animals in future global environments may be closely linked to possible changes in leaf N concentration of grasses. Since grasslands are frequently subjected to water-deficit or N-deficit conditions, it is important to investigate the interactive responses between elevated [CO2] and these stress conditions. Therefore, this 4-year research program was undertaken with wheat (Triticum aestivum L.) as a model system for forage grasses, to document the potential changes in leaf N concentration in response to global environment changes. Wheat crops grown under field conditions near Phoenix, AZ, USA, were subjected to elevated [CO2] and either water-deficit or N-deficit treatments using large Free Air Carbon dioxide Enrichment (FACE) arrays. Surprisingly, the elevated [CO2] treatment under optimum conditions resulted in little change in leaf N concentration. Therefore, no change in the nutritive value of forage from highly managed pastures would be expected. Further, water-deficit treatment had little influence on leaf N concentration. To some extent, the lack of response to the water-deficit treatment resulted because severe deficits did not develop until late in the growing seasons. Only on one date late in the season was the water-deficit treatment found to result in decreased leaf N concentration. The low N treatment in combination with elevated [CO2], however, had a large influence on leaf N concentration. Low levels of applied N resulted in decreased leaf N concentration under both [CO2] treatments, but the lowest levels of leaf N concentration were obtained under elevated [CO2] through much of the growing season. These results point to a potential problem with grasslands in that the nutritive value of the forage consumed by animals will be decreased under future global environment changes. (C) Published by Elsevier Science B. V.
- Wall, G. W., Adam, N. R., Brooks, T. J., Kimball, B. A., Pinter Jr, P. J., Lamorte, R. L., Adamsen, F. J., Hunsaker, D. J., Wechsung, G., Wechsung, F., Grossman-Clarke, S., Leavitt, S. W., Matthias, A. D., & Webber, A. N. (2000). Acclimation response of spring wheat in a free-air CO2 enrichment (FACE) atmosphere with variable soil nitrogen regimes. 2. Net assimilation and stomatal conductance of leaves. Photosynthesis Research, 66(1-2), 79-95.More infoAbstract: Atmospheric CO2 concentration continues to rise. It is important, therefore, to determine what acclimatory changes will occur within the photosynthetic apparatus of wheat (Triticum aestivum L. cv. Yecora Rojo) grown in a future high-CO2 world at ample and limited soil N contents. Wheat was grown in an open field exposed to the CO2 concentration of ambient air [370 μmol (CO2) mol-1; Control] and air enriched to ∼200 μmol (CO2) mol-1 above ambient using a Free-Air CO2 Enrichment (FACE) apparatus (main plot). A High (35 g m-2) or Low (7 and 1.5 g m-2 for 1996 and 1997, respectfully) level of N was applied to each half of the main CO2 treatment plots (split-plot). Under High-N, FACE reduced stomatal conductance (gs) by 30% at mid-morning (2 h prior to solar noon), 36% at midday (solar noon) and 27% at mid-afternoon (2.5 h after solar noon), whereas under Low-N, gs was reduced by as much as 31% at mid-morning, 44% at midday and 28% at mid-afternoon compared with Control. But, no significant CO2 × N interaction effects occurred. Across seasons and growth stages, daily accumulation of carbon (A′) was 27% greater in FACE than Control. High-N increased A′ by 18% compared with Low-N. In contrast to results for gs, however, significant CO2 × N interaction effects occurred because FACE increased A′ by 30% at High-N, but by only 23% at Low-N. FACE enhanced the seasonal accumulation of carbon (A″) by 29% during 1996 (moderate N-stress), but by only 21% during 1997 (severe N-stress). These results support the premise that in a future high-CO2 world an acclimatory (down-regulation) response in the photosynthetic apparatus of field-grown wheat is anticipated. They also demonstrate, however, that the stimulatory effect of a rise in atmospheric CO2 on carbon gain in wheat can be maintained if nutrients such as nitrogen are in ample supply.
- Wechsung, F., Garcia, R. L., Wall, G. W., Kartschall, T., Kimball, B. A., Michaelis, P., Pinter Jr., P. J., Wechsung, G., Grossman-Clarke, S., Lamorte, R. L., Adamsen, F. J., Leavitt, S. W., Thompson, T. L., Matthias, A. D., & Brooks, T. J. (2000). Photosynthesis and conductance of spring wheat ears: Field response to free-air CO2 enrichment and limitations in water and nitrogen supply. Plant, Cell and Environment, 23(9), 917-929.More infoAbstract: The mid-day responses of wheat ear CO2 and water vapour exchange to full-season CO2 enrichment were investigated using a Free-Air CO2 Enrichment (FACE) apparatus. Spring wheat [Triticum aestivum (L). cv. Yecora Rojo] was grown in two experiments under ambient and elevated atmospheric CO2 (C(a)) concentrations (approximately 370 μmol mol-1 and 550 μmol mol-1, respectively) combined first with two irrigation (Irr) schemes (Wet: 100% and Dry: 50% replacement of evapotranspiration) and then with two levels of nitrogen (N) fertilization (High: 350, Low: 70 kg ha-1 N). Blowers were used for C(a) enrichment. Ambient C(a) plots were exposed to blower induced winds as well the C(a) x N but not in the C(a) x Irr experiment. The net photosynthesis for the ears was increased by 58% and stomatal conductance (g(s)) was decreased by 26% due to elevated C(a) under ample water and N supply when blowers were applied to both C(a) treatments. The use of blowers in the C(a)-enriched plots only during the C(a) x Irr experiment (blower effect) and Low N supply restricted the enhancement of net photosynthesis of the ear due to higher C(a). In the latter case, the increase of net photosynthesis of the ear amounted to 26%. The decrease in g(s) caused by higher C(a) was not affected by the blower effect and N treatment. The mid-day enhancement of net photosynthesis due to elevated C(a) was higher for ears than for flag leaves and this effect was most pronounced under ample water and N supply. The contribution of ears to grain filling is therefore likely to increase in higher C(a) environments in the future. In the comparison between Wet and Dry, the higher C(a) did not alter the response of net photosynthesis of the ear and g(s) to Irr. However, C(a) enrichment increased the drought tolerance of net photosynthesis of the glume and delayed the increase of the awn portion of net photosynthesis of the ear during drought. Therefore, the role of awns for maintaining high net photosynthesis of the ear under drought may decrease when C(a) increases.
- Adamsen, F. J., Pinter Jr., P. J., Barnes, E. M., LaMorte, R. L., Wall, G. W., Leavitt, S. W., & Kimball, B. A. (1999). Measuring wheat senescence with a digital camera. Crop Science, 39(3), 719-724.More infoAbstract: Documenting crop senescence rates is often difficult because of the need for frequent sampling during periods of rapid change and the subjective nature of human visual observations. The purpose of this study was to determine the feasibility of using images produced by a digital camera to measure the senescence rate of wheat and to compare the results with changes in greenness determined by two established methods. Measurements were made as part of an experiment to determine the effects of elevated CO2 and limited soil nitrogen on spring wheat (Triticum aestivum L.) at the University of Arizona's Maricopa Agricultural Center, near Phoenix, AZ. 'Greenness' measurements were made during senescence of the crop with a color digital camera, a hand-held radiometer, and a SPAD chlorophyll meter. The green to red (G/R) for each pixel in an image was calculated and the average G/R computed for cropped images from a digital camera representing 1 m2 for each treatment and sample date. The normalized difference vegetation index (NDVI) was calculated from the red and near-infrared canopy reflectances measured with a hand held radiometer. A SPAD reading was obtained from randomly selected flag leaves. All three methods of measuring plant greenness showed similar temporal trends. The relationships between G/R with NDVI and SPAD were linear over most of the range of G/R. However, NDVI was more sensitive at low values than G/R. G/R was more sensitive above G/R values of 1.2 than SPAD because the upper limits of SPAD measurements were constrained by the amount of chlorophyll in the leaf, while G/R responded to both chlorophyll concentration in the leaves as well as the number of leaves present. Color digital imaging appears useful for quantifying the senescence of crop canopies. The cost of color digital cameras is expected to decrease and the quality and convenience of use to improve.
- Kimball, B. A., LaMorte, R. L., Pinter Jr., P. J., Wall, G. W., Hunsaker, D. J., Adamsen, F. J., Leavitt, S. W., Thompson, T. L., Matthias, A. D., & Brooks, T. J. (1999). Free-air CO2 enrichment and soil nitrogen effects on energy balance and evapotranspiration of wheat. Water Resources Research, 35(4), 1179-1190.More infoAbstract: In order to determine the likely effects of the increasing atmospheric CO2 concentration on future evapotranspiration, ET, plots of field-grown wheat were exposed to concentrations of 550 μmol/mol CO2 (or 200 μmol/mol above current ambient levels of about 360 μmol/mol) using a free-air CO2 enrichment (FACE) facility. Data were collected for four growing seasons at ample water and fertilizer (high N) and for two seasons when soil nitrogen was limited (low N). Measurements were made of net radiation, R(n); soil heat flux; air and soil temperatures; Canopy temperature, T(s); and wind speed. Sensible heat flux was calculated from the wind and temperature measurements. ET, that is, latent heat flux, was determined as a residual in the energy balance. The FACE treatment increased daytime T(s) about 0.6°and 1.1°C at high and low N, respectively. Daily total R(n) was reduced by 1.3% at both levels of N. Daily ET was consistently lower in the FACE plots, by about 6.7% and 19.5% for high and low N, respectively.
- Pendall, E., Betancourt, J. L., & Leavitt, S. W. (1999). Paleoclimatic significance of δD and δ13C values in pinon pine needles from packrat middens spanning the last 40,000 years. Palaeogeography, Palaeoclimatology, Palaeoecology, 147(1-2), 53-72.More infoAbstract: We compared two approaches to interpreting δD of cellulose nitrate in pinon pine needles (Pinus edulis) preserved in packrat middens from central New Mexico, USA. One approach was based on linear regression between modem δD values and climate parameters, and the other on a deterministic isotope model, modified from Craig and Gordon's terminal lake evaporation model that assumes steady-state conditions and constant isotope effects. One such effect, the net biochemical fractionation factor, was determined for a new species, pinon pine. Regressions showed that δD values in cellulose nitrate from annual cohorts of needles (1989-1996) were strongly correlated with growing season (May-August) precipitation amount, and δ13C values in the same samples were correlated with June relative humidity. The deterministic model reconstructed δD values of meteoric water used by plants after constraining relative humidity effects with δ13C values; growing season temperatures were estimated via modem correlations with δD values of meteoric water. Variations of this modeling approach have been applied to tree-ring cellulose before, but not to macrofossil cellulose, and comparisons to empirical relationships have not been provided. Results from fossil pinon needles spanning the last ~40,000 years showed no significant trend in δD values of cellulose nitrate, suggesting either no change in the amount of summer precipitation (based on the transfer function) or δD values of meteoric water or temperature (based on the deterministic model). However, there were significant differences in δ13C values, and therefore relative humidity, between Pleistocene and Holocene.
- Barnes, E. M., Pinter Jr., P. J., Kimball, B. A., Wall, G. W., LaMorte, R. L., Hunsaker, D. J., Adamsen, F., Leavitt, S., Thompson, T., & Mathius, J. (1997). Modification of CERES-Wheat to accept leaf area index as an input variable. Paper - American Society of Agricultural Engineers, 1.More infoAbstract: CERES-Wheat is one in a family of crop simulation models used by the Decision Support System for Agrotechnology Transfer (DSSAT). Efforts have been devoted to integrating the DSSAT models with Geographic Information Systems (GIS) to account for spatial variation in soils and climate; however, validation and fine tuning of the models in a GIS environment is a difficult task. The relationship between multispectral data and plant characteristics such as green leaf area index (LAI) has been demonstrated; therefore, multispectral images have the potential to enhance the spatial predictive capability of existing growth models. CERES-Wheat was modified to accept observed LAI at defined times during the season. Initial results indicate that the proposed procedures are only effective when the growth stages of the model are properly predicted and leaf area index observations are available soon after the completion of leaf development. Future efforts will focus on an iterative approach to adjusting the model's parameters.
- Bert, D., Leavitt, S. W., & Dupouey, J. (1997). Variations of wood δ13C and water-use efficiency of Abies alba during the last century. Ecology, 78(5), 1588-1596.More infoAbstract: Variations of intrinsic water-use efficiency during the last century were investigated based on analysis of δ13C in tree rings of Abies alba from the Jura Mountains (eastern France). To separate the effects related to the age of the tree at the time the tree ring was formed from effects due to environmental changes, analyzed wood samples were extracted from a very large sample set including different tree ages and calendar dates of wood formation. For the first 75 yr of the life of Abies alba, δ13C of wood holocellulose increases with the age of the tree from -24.4‰ at age 15 to approximately -22.5‰ at age 75. Between the ages of 75 and 150 values remain constant at -22.5‰. Consequently, the effect of the tree age on isotopic discrimination has to be taken into account in studies on the long-term environmental effects on δ13C in tree rings. Divergent trends of δ13C during the last century were observed between tree rings formed at age 40 and bulk air data. The isotopic discrimination A varied insignificantly around a mean of 17.3‰ between the 1860s and the 1930s. It then decreased to 15.8‰ from the 1930s to the 1980s. Using these results and classical models of carbon discrimination, we calculated that the intrinsic water-use efficiency (A/g(w), the ratio of CO2 assimilation rate to stomatal conductance for water vapor), integrated over the year, has increased by 30% between the 1930s and the 1980s. These results, obtained at the level of mature trees, are consistent with the physiological effects of increasing CO2 concentrations as observed in controlled experiments on young seedlings. They are consistent with the strong increases in radial growth observed for Abies alba in western Europe over the past decades. However, other long-term environmental changes such as increasing nitrogen deposition could cause similar effects.
- Follett, R. F., Paul, E. A., Leavitt, S. W., Halvorson, A. D., Lyon, D., & Peterson, G. A. (1997). Carbon isotope ratios of Great Plains soils and wheat-fallow systems. Soil Science Society of America Journal, 61(4), 1068-1077.More infoAbstract: The purposes of this study were to improve knowledge of regional vegetation patterns of C3 and C4 plants in the North American Great Plains and to use δ13C methodology and long-term research sites to determine contributions of small-grain crops to total soil organic carbon (SOC) now present. Archived and recent soil samples were used. Detailed soil sampling was in 1993 at long-term sites near Akron, CO, and Sidney, NE. After soil sieving, drying, and deliming, SOC and δ13C were determined using an automated C/N analyzer interfaced to an isotope-ratio mass spectrometer. Yield records from long-term experimental sites were used to estimate the amount of C3 plant residue C returned to the soil. Results from δ13 analyses of soils from near Waldheim, Saskatchewan, to Big Springs, TX, showed a strong north to south decrease in SOC derived from C3 plants and a corresponding increase from C4 plants. The δ13 analyses gave evidence that C3 plant residue C (possibly from shrubs) is increasing at the Big Springs, TX, and Lawton, OK, sites. Also, δ13C analyses of subsoil and topsoil layers shows evidence of a regional shift to more C3 species, possibly because of a cooler climate during the past few hundreds to thousands of years. Data from long-term research sites indicate that the efficiency of incorporation of small-grain crop residue C was about 5.4% during 84 yr at Akron, CO, and about 10.5% during 20 yr at Sidney, NE. The 14C age of the SOC at 0- to 10-cm depth was 193 yr and at 30 to 45 cm was 4000 yr; 14C age of nonhydrolyzable C was 2000 and 7000 yr for these same two respective depths. Natural partitioning of the 13C isotope by the photosynthetic pathways of C3 and C4 plants provides a potentially powerful tool to study SOC dynamics at both regional and local scales.
- Leavitt, S. W., Paul, E. A., Pendall, E., Pinter Jr., P. J., & Kimball, B. A. (1997). Field variability of carbon isotopes in soil organic carbon. Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms, 123(1-4), 451-454.More infoAbstract: Free-air CO2 enrichment (FACE) plant-growth experiments conducted in Arizona have allowed a spinoff isotope tracer experiment (tank CO2 is 14C- and 13C-depleted) to follow the input of carbon into soil organic matter pools. Accurate assessment of the 14C and 13C inputs to soils by this pathway requires detailed knowledge of the isotopic composition of the soils before the experiments. We have examined the variability of δ13C in soil organic matter in 8 experimental plots prior to the beginning of the 1995-96 FACE experiments with wheat. δ13C variability was higher immediately after harvest of a previous crop in June, but the plots were much more homogeneous five months later immediately before planting. Intervening field management, including disking, plowing and installation of irrigation drip tape likely contributed to mixing the soils.
- Paul, E. A., Follett, R. F., Leavitt, S. W., Halvorson, A., Peterson, G. A., & Lyon, D. J. (1997). Radiocarbon dating for determination of soil organic matter pool sizes and dynamics. Soil Science Society of America Journal, 61(4), 1058-1067.More infoAbstract: The size and turnover rate of the resistant soil organic matter (SUM) fractions were measured by 14C dating and 13C/12C measurements. This involved soils archived in 1948, and recent samples, from a series of long-term sites in the North American Great Plains. A reevaluation of C dates obtained in the 1960s expanded the study scope. The 14C ages of surface soils were modern in some native sites and near modern in the low, moist areas of the landscape. They were much older at the catena summits. The 14C ages were not related to latitude although this strongly influenced the total SOM content. Cultivation resulted in lower C contents and increased the 14C age by an average of 900 yr. The 10- to 20-cm depths from both cultivated and native sites were 1200 yr older than the 0- to 10.cm depth. The 90- to 120-cm depth of a cultivated site at 7015 yr before present (BP) was 6000 yr older than the surface. The nonhydrolyzable C of this depth dated 9035 yr BP. The residue of 6 M HCl hydrolysis comprised 23 to 70% of the total soil C and was, on the average, 1500 yr older. The percentage of nonhydrolyzable C and its 14C age analytically identify the amount and turnover rate of the old resistant soil C.
- Péwé, T. L., Berger, G. W., Westgate, J. A., Brown, P. M., & Leavitt, S. W. (1997). Eva Interglaciation forest bed, unglaciated East-Central Alaska: Global warming 125,000 years ago. Special Paper of the Geological Society of America, 319, 1-54.More infoAbstract: The Eva Interglaciation Forest Bed represents a frozen, buried, ancient boreal forest in the Yukon-Tanana Upland of east-central Alaska. It consists of excellently preserved peat lenses, sticks, roots, and logs as well as rooted and unrooted stumps of trees, mainly spruce and birch. Consistent with the modern boreal forest, the largest and most common tree in the fossil forest is spruce, mainly white spruce (Picea glauca). Remains of birch trees are common, mostly Betula papyrifera. The forest remains were buried by loess that became frozen and so are well preserved. None of the wood is mineralized. Many of the fragments are black from buring, suggesting forest fires were widespread in the Yukon-Tanana Upland during the interglaciation. Also, evidence is presented for the first time of the existence of spruce bark beetles (Scolytidae) during the last interglaciation in Alaska. Efforts to determine the age of the Eva Forest Bed in this study have covered the past 50 years. Methods applied have varied from the use of stratigraphic interpretation of sedimentological events and preserved evidence of climatic changes to the use of modern geochronometry. Several methods of dating have come to fruition in the 1990s. New radiocarbon dating by liquid scintillation (LS) detectors indicates the forest wood to be older than 70,000 years. Perhaps the greatest breakthrough is the development of the isothermal-plateau fission-track method of dating geologically young volcanic glass shards. The Old Crow tephra closely underlying the Eva Forest Bed has been dated at 140 ± 10 ka and strongly supports the original interpretation of the forest bed as of last interglaciation. In the early 1990s, highly improved thermoluminescence (TL) sediment dating techniques were utilized for dating loess above and below the forest bed indicating the age of the Eva Forest Bed is probably 125,000 years with a duration of the Eva Interglaciation of probably only a few thousand years (Sangamon, Oxygen Isotope Substage 5e). Stratigraphically, the Eva Forest Bed lies at the prominent unconformity between the underlying massive, green Gold Hill Loess (pre-Sangamon) and the overlying blackish, ice-wedge-rich retransported loess of the Goldstream Formation (Wisconsin). Studies of the frozen Gold Hill Loess indicate that the warm interglacial interval was characterized by deep and rapid thawing of permafrost and erosion of loess accompanied by gullying and block slumping of frozen loess. After extensive slumping, the topography became smooth and the forest became extensive. Tilting of enclosed tephra layers outline the slump blocks. Evidence for deep permafrost thawing is supported by the absence today of ice wedges, buried pingos, and mammal carcasses in the presently refrozen loess of pre-Wisconsin age. Deep thawing is also indicated by reduction of iron on loess grains from ferric to ferrous turning the traditional tan color of loess to greenish in the buried Gold Hill Loess. It is the unique sequence of refreezing in Wisconsin time that has preserved the remarkable evidence for deep thawing in earlier Sangamontime - the green color. The forest bed formed after much of the thawing, erosion, and slumping activity had ceased, and it overlies the angular unconformity. More than halfa-dozen distinct tephra layers have been identified, characterized, and correlated in the upper part of the Gold Hill Loess, aiding in the reconstruction of the sequence of events leading to the erosion, thawing, and emplacement of the Eva Forest Bed. Dendrochronology studies of trees and 13C/12C isotopic ratios of wood from the Eva Forest Bed, and comparisons with wood from the modern boreal forest, strongly suggest environmental conditions at least similar to those of today. Some plant remains and ground beetle taxa of Eva Forest time in Canada represent species that extended farther north than they do today. Also, buried spruce macrofossils suggest that the boreal forest may have extended north of the Brooks Range in Alaska. These botanical and physical data indicate an environmentt warmer than the present interglaciation with the mean annual air temperature warmer than 0 °C, perhaps +1 or +2 °C or warmer to permit the ice to melt and permafrost to thaw from the surface downward. Supporting this concept are astronomical inferences that during the last interglacial (Oxygen Isotope Substage 5e) the July insolation anomaly at 65°N. latitude reached values of almost 50% higher than 10,000 years ago, the beginning of the Holocene Interglaciation.
- Leavitt, S. W., Follett, R. F., & Paul, E. A. (1996). Estimation of slow- and fast-cycling soil organic carbon pools from 6N HCl hydrolysis. Radiocarbon, 38(2), 231-239.More infoAbstract: Acid hydrolysis is used to fractionate the soil organic carbon pool into relatively slow- and fast-cycling compartments on soils from Arizona, the Great Plains states and Michigan collected for carbon isotope tracer studies related to soil carbon sequestration, for studies of shifts in C3/C4 vegetation, and for "pre-bomb" soil-carbon inventories. Prior to hydrolysis, soil samples are first treated with cold 0.5-1N HCl to remove soil carbonates if necessary. Samples are then dispersed in a concentrated NaCI solution (ρ-1.2 g cm-3) and floated plant fragments are skimmed off the surface. After rinsing and drying, all remaining recognizable plant fragments are picked from the soil under 20× magnification. Plant-free soils, and hot, 6N HCl acid-hydrolysis residue and hydrolyzate fractions are analyzed for carbon content, δ13C and 14C age, and the carbon distribution is verified within 1-2% by stable-carbon isotope mass balance. On average, the recalcitrant residue fraction is 1800 yr older and 2.6‰ more 13C-depleted than total soil organic carbon. A test of hydrolysis with fresh plant fragments produced as much as 71-76% in the acid-hydrolysis residue pool. Thus, if plant fragments are not largely removed prior to hydrolysis, the residue fraction may date much younger than it actually is.
- Leavitt, S. W., Paul, E. A., Galadima, A., Nakayama, F. S., Danzer, S. R., Johnson, H., & Kimball, B. A. (1996). Carbon isotopes and carbon turnover in cotton and wheat FACE experiments. Plant and Soil, 187(2), 147-155.More infoAbstract: The Maricopa cotton and wheat FACE (free-air CO2 enrichment) experiments offer propitious opportunity to quantify carbon turnover. The commercial CO2 (δ 13C ≃ -37‰) used to elevate CO2 concentration in field plots provided a strongly 13C-depleted tracer. Soil CO2 and δ13C of soil organic carbon (SOC) in CO2 -enriched and Control plots were measured between the final cotton FACE project (October 1991) and the end of the second wheat experiment (June 1994). The initial 13C-depletion in SOC of cotton FACE plots (measured by the difference in δ13C between FACE and Control plots) persisted at the same level (1.9‰) 1.5 years after the experiment ended. A similar depletion was observed in soil CO2 evolved in the same plots, indicating ongoing decomposition of the new SOC. The SOC δ13C of wheat plots before and after two growing seasons showed increasing 13C-depletion in FACE relative to Control. Isotopic mass balance was consistent with 5-6% new carbon input from the two wheat crops. This is lower than the 12-13% calculated for FACE cotton and perhaps a consequence of the larger root system of cotton or the 3-year duration of the cotton experiments versus 2 years for the wheat.
- Liu, Y., Xiangding, W. u., Leavitt, S. W., & Hughes, M. K. (1996). Stable carbon isotope in tree rings from Huangling, China and climatic variation. Science in China, Series D: Earth Sciences, 39(2), 152-161.More infoAbstract: By using a single-year discrimination chronology detrended from a δ13C chronology from Chinese pine (Pinus tabulaeformis) tree rings and meteorological data, the δ13C-climatic response is analyzed. The results show that high-frequency δ13C is significantly related to both temperatures of June (with r= -0.65) and the total precipitation of May, June and July (r= -0.46). This suggests that δ13C records reflects some features of the East Asian summer monsoon. In addition, temperature departure for June is reconstructed from a transfer function developed with δ13C-climatic response.
- Leavitt, S. W. (1994). Major wet interval in White Mountains Medieval warm period evidenced in delta13C of bristlecone pine tree rings. Climatic Change, 26(2-3), 299-307.
- Leavitt, S. W., & Lara, A. (1994). South American tree rings show declining δ13C trend. Tellus, Series B, 46 B(2), 152-157.More infoAbstract: A composite, 290-year tree-ring δ13C chronology was developed from a site in Chile where 5 Fitzroya cupressoides (alerce) trees were sampled. This chronology shows a decreasing δ13C trend of approximately 1.2‰, primarily since the turn of this century. This is the first evidence for any interhemispheric reproducibility of tree-ring δ13C chronologies, and furthermore, the Fitzroya δ13C trend conforms well to that of δ13C of atmospheric CO2 determined from ice cores and direct measurements. This correspondence suggests the alerce δ13C trend has not been substantially influenced by systematic changes in environmental factors such light, relative humidity and soil moisture or by changing atmospheric CO2 concentration, all of which are, in theory, capable of altering Ci/Ca ratios and obscuring the atmospheric δ13C record contained in the tree rings. -from Authors
- Leavitt, S. W., Paul, E. A., Kimball, B. A., Hendrey, G. R., Mauney, J. R., Rauschkolb, R., Rogers, H., Lewin, K. F., Nagy, J., Pinter Jr., P. J., & Johnson, H. B. (1994). Carbon isotope dynamics of free-air CO2-enriched cotton and soils. Agricultural and Forest Meteorology, 70(1-4), 87-101.More infoAbstract: A role for soils as global carbon sink or source under increasing atmospheric CO2 concentrations has been speculative. Free-air carbon dioxide enrichment (FACE) experiments with cotton, conducted from 1989 to 1991 at the Maricopa Agricultural Center in Arizona, maintained circular plots at 550 μmol mol-1 CO2 with tank CO2 while adjacent ambient control plots averaged about 370 μmol mol-1 CO2. This provided an exceptional test for entry of carbon into soils because the petrochemically derived tank CO2 used to enrich the air above the FACE plots was depleted in both radiocarbon (14C content was 0% modern carbon (pmC)) and 13C (δ13C≈ -36‰) relative to background air, thus serving as a potent isotopic tracer. Flask air samples, and plant and soil samples were collected in conjunction with the 1991 experiment. Most of the isotopic analyses on the plants were performed on the holocellulose component. Soil organic carbon was obtained by first removing carbonate with HCl, floating off plant fragments with a NaCl solution, and picking out remaining plant fragments under magnification. The δ13C of the air above the FACE plots was approximately -15 to -19‰, i.e. much more 13C depleted than the background air of approximately -7.5‰. The δ13C values of plants and soils in the FACE plots were 10-12‰ and 2‰13C-depleted, respectively, compared with their control counterparts. The 14C content of the FACE cotton plants was approximately 40 pmC lower than tha tof the control cotton, but the 14C results from soils were conflicting and therefore not as revealing as the δ13C of soils. Soil stable-carbon isotope patterns were consistent, and mass balance calculations indicate that about 10% of the present organic carbon content in the FACE soil derived from the 3 year FACE experiment. At a minimum, this is an important quantitative measure of carbon turnover, but the presence of 13C-depleted carbon, even in the recalcitrant 6 N HCl resistant soil organic fraction (average age 2200 years before present (BP)), suggests that at least some portion of this 10% is an actual increase in carbon accumulation. Similar isotopic studies on FACE experiments in different ecosystems could permit more definitive assessment of carbon turnover rates and perhaps provide insight into the extent to which soil organic matter can accommodate the 'missing' carbon in the global carbon cycle. © 1994.
- Van, P., Leavitt, S. W., & Betancourt, J. L. (1994). Trends in stomatal density and 13C/12C ratios of Pinus flexilis needles during last glacial-interglacial cycle. Science, 264(5156), 239-243.More infoAbstract: Measurements of stomatal density and δ13C of limber pine (Pinus flexilis) needles (leaves) preserved in pack rat middens from the Great Basin reveal shifts in plant physiology and leaf morphology during the last 30,000 years. Sites were selected so as to offset glacial to Holocene climatic differences and thus to isolate the effects of changing atmospheric CO2 levels. Stomatal density decreased ∼17 percent and δ13C decreased ∼1.5 per mil during deglaciation from 15,000 to 12,000 years ago, concomitant with a 30 percent increase in atmospheric CO2. Water-use efficiency increased ∼15 percent during deglaciation, if temperature and humidity were held constant and the proxy values for CO2 and δ13C of past atmospheres are accurate. The δ13C variations may help constrain hypotheses about the redistribution of carbon between the atmosphere and biosphere during the last glacial-interglacial cycle.
- Dupouey, J. L., Leavitt, S., Choisnel, E., & Jourdain, S. (1993). Modelling carbon isotope fractionation in tree rings based on effective evapotranspiration and soil water status. Plant, Cell and Environment, 16(8), 939-947.
- Leavitt, S. W., & Danzer, S. R. (1993). Method for batch processing small wood samples to holocellulose for stable-carbon isotope analysis. Analytical Chemistry, 65(1), 87-88.
- Leavitt, S. W., & Kalin, R. M. (1992). A new tree-ring width ,δ13C and 14C investigation of the Two Creeks site. Radiocarbon, 34(3), 792-797.More infoAbstract: We have made isotopic and dendrochronologic measurements on material collected from the Two Creeks site. Radiocarbon dating of outside wood of four logs yielded an average of 11 760±100 BP, in good agreement with results of Broecker & Ferrand (1963) over 25 yr ago. The range of 11 640±160 to 11 900±160 BP suggests a period of forest growth of 200-300 yrs. Stable-carbon isotope chronologies on cellulose from 5-yr ring groups show δ13C scatter among trees typical of that found within modern sites, with some matches of isotopic maxima nd minima. Some downward δ13C trends may result from physiological response to rising lake levels (and/or cooling temperatures) at the site, which also produced very narrow rings in the outer ca 50±20 yrs. -from Authors
- Leavitt, S. W., & Long, A. (1992). Altitudinal differences in δ13c of bristlecone pine tree rings. Naturwissenschaften, 79(4), 178-180.
- Leavitt, S. W., & Danzer, S. R. (1991). Chronology from plant matter . Nature, 352(6337), 671-.
- Leavitt, S. W., & Lone, A. (1991). Seasonal stable-carbon isotope variability in tree rings: possible paleoenvironmental signals. Chemical Geology: Isotope Geoscience Section, 87(1), 59-70.More infoAbstract: Plant stable-carbon isotope fractionation models indicate that δ13C of atmospheric CO2, CO2 concentration, light and moisture stress, among other factors, may potentially affect the δ13C of fixed carbon. Seasonal δ13C variations in tree rings may therefore represent a new tool for paleoenvironmental reconstruction. The seasonal δ13C patterns in growth rings exist in trees (conifer and hardwood) from tropical and temperate localities, and isotopic variation is even seen in trees which are lacking or have poorly-defined rings. The patterns in different rings from a single tree are usually similar, although differences in amplitude and timing of maxima and/or minima are common. Some of the differences may be attributable to radial variation of the δ13C patterns which was found to be substantial in a severely water-stressed tree. Correlations of δ13C patterns and corresponding seasonal environmental variation for one well-monitored tree showed greater response of the δ13C change to measured soil moisture and precipitation than to temperature, calculated soil moisture, solar radiation, or net photosynthesis (as estimated from CO2 release). © 1991.
- Leavitt, S. W., & Long, A. (1990). Accelerator-measured 14C activity in tree rings from the vicinity of the first atomic bomb test. Radiocarbon, 31(3), 762-765.More infoAbstract: Detonation of the first fission bomb at White Sands, New Mexico, on July 16, 1945, produced a tremendous neutron flux capable of creating tritium and radiocarbon byproducts. The authors sampled a 115-year-old pinyon Pinus edulis 10km E of the Trinity test site to determine 14C evidence of this event. The most likely mechanism for this enrichment in the 1945 tree ring would be fixation of 14CO2 produced at the blast site and carried with the fallout cloud over the pinyon site. Analysis of cellulose of the 1944 and 1945 rings shows δ13C values of -19.9 and -19.5‰, respectively, and 14C activity (fraction of modern uncorrected for δ13C) as 0.991 ± .005 and 0.991 ± .006, respectively. It is likely that the duration and/or concentration of the 14CO2 exposure was not sufficient to increase 14C activity expected for that year. -Authors
- Leavitt, S. W., & Long, A. (1990). The atmospheric δ13C record as derived from 56 pinyon trees at 14 sites in the southwestern United States. Radiocarbon, 31(3), 469-474.More infoAbstract: Regressions of first differences of ring-width indices and δ13C values from each site were used to "correct' individual δ13C chronologies for climate effects which appear primarily related to high-frequency δ13C fluctuations. These climate-corrected chronologies were normalized as deviations from their respective 1800-1849 δ13C means, and these normalized chronologies were averaged into the master. The overall δ13C drop from 1600 to the present is c1.2-1.4‰, consistent with recent ice-core data showing a drop of 1.14±0.15‰ from 1740 to present. However, the δ13C decline in the late 19th and early 20th centuries is greater in the pinyon chronology than that of the ice cores. -from Authors
- Leavitt, S. W., & Long, A. (1990). Variation of concentration, 14C activity and 13C/12C ratios of CO2 in air samples from Kitt Peak, Arizona. Radiocarbon, 31(3), 464-468.More infoAbstract: Air was sampled with 5L flasks at Kitt Peak (2100 m elev) from 1983 through 1984 at approximately monthly intervals. The Kitt Peak CO2 concentrations, represented by a yield measurement, fluctuated c25% over the monitoring period. The δ13C values (uncorrected for N2O) varied from c-7.6 to -9.0‰, with high values (and low CO2 yields) in the late summer consistent with hemispheric seasonal biosphere effects. 14C activity of four Kitt Peak samples range from 1.158±.007 to 1.223±.008 as uncorrected fraction of modern, below free air activity of c1.250 for 1984 even after correcting for fractionation. -from Authors
- Leavitt, S. W., & Long, A. (1989). Drought indicated in carbon-13/carbon-12 ratios of southwestern tree rings. Water Resources Bulletin, 25(2), 341-347.More infoAbstract: Stomatal closure during periods of moisture deficiency should theoretically lead to elevated 13C/12C ratios as reduction of available CO2 leads to diminished photosynthetic discrimination against 13C in favor of 12C. Stable-carbon isotope ratio chronologies developed from 5-yr tree-ring groups at 17 sites in six southwestern states were tested for a drought relationship by first fitting a spline curve to each chronology to remove the long-term trend and calculating indices as the ratio of actual to spline curve value. The time series of 'Del Indices' so developed are significantly correlated with 5-yr mean Palmer Hydrological Drought Indices (post-1930 period). These Del Index drought reconstructions may provide a useful measure of past physiological response to drought (stomatal closure).
- Leavitt, S. W., & Long, A. (1984). Sampling strategy for stable carbon isotope analysis of tree rings in pine. Nature, 311(5982), 145-147.More infoAbstract: Stable carbon isotope measurements in tree rings have been explored as potential indicators of past climate1,2, and as proxy data for reconstructions of the history of 13C/12C variations of atmospheric carbon dioxide as related to the global carbon cycle3-6. Results of these studies are often conflicting, perhaps partly due to the limited style of sampling natural systems with large inherent spatial variability. We have examined stable carbon isotopic trends in tree rings among several radii of the same tree (Pinus edulis Engelm.) and among several trees from the same site, to determine the extent of this variability. The circumferential range in δ13C values of cellulose is ∼1-1.5‰, whereas among individuals it is ∼2-3‰. Tests to determine how well various combinations of cores fit the trend of an individual tree or of the site as a whole indicate that pooling four cores from four trees accurately represent site δ13C trends and absolute values. © 1984 Nature Publishing Group.
- Leavitt, S. W., & Long, A. (1983). An atmospheric C13/12C reconstruction generated through removal of climate effects from tree-ring 13C/12C measurements.. Tellus, Series B, 35 B(2), 92-102.More infoAbstract: An accurate reconstruction of past 13C/12C ratios of atmospheric CO2 may provide key constraints on the historical activity of the biosphere as CO2 source or sink. Tree rings appear to be a repository of this information but there is much noise in the collection of previous reconstructions, presumably associated with site selection, radial variability, choice of representative wood chemical constituent, and subtle effects of climate on fractionation. -from Authors
- Leavitt, S. W., & Long, A. (1983). Possible climatic response of delta 13C in leaf cellulose of pinyon pine in Arizona, U.S.A.. Isotope Geoscience, 1(2), 169-180.More infoAbstract: Leaves of pinyon pine (Pinus edulis) were collected from 10 living trees at 9 sites in Arizona and New Mexico, representing elevations from 1200 to 2200 m. Stable-carbon isotopic analysis was performed on cellulose isolated from the whole-leaf material. In spite of the range of conditions, the delta 13C response varies only over approx 1per mille, with an average variance among leaves of the same tree of 0.4per mille. These values were regressed with elevation and with monthly, seasonal and annual climatic data from nearby climatological stations. Linear relations of delta with T and precipitation yield mostly positive correlations, not significant at 95%. Because of high delta 13C values for the the highest elevation sites, the regressions improve greatly for second-order polynomials of elevation and T. The mechanism for higher delta 13C at higher elevations is not clear, but may be related to length of growing season. There may be some potential for reconstructions based on strong linear relations derived from the lower elevation trees.-P.Br.
- Leavitt, S. W. (1982). Annual volcanic carbon dioxide emission: An estimate from eruption chronologies. Environmental Geology, 4(1), 15-21.More infoAbstract: Continuing interest in the effects of carbon dioxide on climate has been promoted by the exponentially increasing anthropogenic production of CO2. Volcanoes are also a major source of carbon dioxide, but their average input to the atmosphere is generally considered minor relative to anthropogenic input. This study examines eruption chronologies to determine a new estimate of the volcanic CO2 input and to test if temporal fluctuations may be resolved. Employing representative average values of 2.7 g cm-3 as density of erupted material, 0.2 wt percent CO2 in the original melt, 60 percent degassing during eruption, and an average volume of 0.1 km3 for each of the eruptions in the recently published eruption chronology of Hirschboeck (1980), a volcanic input of about 1.5 · 1011 moles CO2 yr-1 was determined for the period 1800-1969. The period 1800-1899 had a somewhat lower input than 1900-1969, which could well be related more to completeness of observational data than to a real increase in volcanic CO2. This input is well below man's current CO2 production of 4-5 · 1014 moles CO2 yr-1. The average values above together with specific volumetric estimates were employed to calculate CO2 input from individual historic eruptions, massive flood basalts, and ash-flow eruptions. Total CO2 release from the largest of flood basalt and ash-flow sequences was 1015-1016 moles of CO2. The impact of these sources on global atmospheric CO2 and climate, however, will be limited by the duration and spacing of the major individual eruptive periods in the sequences. © 1982 Springer-Verlag New York Inc.
- Leavitt, S. W., & Long, A. (1982). Evidence for 13C/12C fractionation between tree leaves and wood. Nature, 298(5876), 742-744.More infoAbstract: An isotopic fractionation step between carbon fixed in the leaves and that assimilated into wood cellulose has not previously been included in atmosphere-plant fractionation models. Data presented here, however, show that the magnitude of such a fractionation step may be significant. In an experiment which involved 10 juniper trees from around Arizona, cellulose of leaves was found to be isotopically lighter than that of the corresponding tree rings in all trees by an average of 2‰. In a second experiment, the δ13C of leaf material from a juniper tree sampled at monthly intervals was compared with the δ13C of the corresponding ring. For cellulose, δ13C of the wood varied with changes in δ13C of the leaves and was persistently isotopically heavier by 3-4‰. Furthermore, the observed change in δ13C through the growing season suggests a temperature coefficient of about -0.27‰ °C-1. The direction of this leaf-wood fractionation precludes diffusion processes as the primary cause. © 1982 Nature Publishing Group.
- Holle, R. L., Simpson, J., & Leavitt, S. W. (1979). GATE-B scale cloudiness from whole-sky cameras on four US ships.. Monthly Weather Review, 107(7), 874-895.More infoAbstract: The largest network of surface cameras ever established in the tropics for studies of cloud cover was deployed during the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE) in 1974. Analysis was made of 2572 hourly whole-sky photographs taken aboard four U.S. ships during the daytime hours of nearly every day covering the three phases of GATE. The cloud-cover analyses were made on a grid divided into 100 squares covering most of the overhead sky, much as was previously made for Barbados and adjacent Atlantic Ocean cloudiness. Analyzed cloud types include low, middle, high, two kinds of total, and two kinds of combined upper cloudiness. Rainfall duration was obtained from drops impinging on the glass dome covering the whole-sky lens. - from Author
- Leavitt, S. W., & Goodell, H. (1979). Evaluation of biogeochemical prospecting methods in the search for sulfide deposits in the Appalachian piedmont, Virginia, U.S.A.. Journal of Geochemical Exploration, 11(1), 89-100.More infoAbstract: Soil and stream sediment sampling have been the primary geochemical exploration tools in the Appalachian piedmont to date. However, the great thicknesses of soil and saprolite found in the region coupled with the dense vegetation frequently encountered favor biogeochemistry as an alternative or supplemental method since deep-rooted plants sample closer to bedrock. To evaluate this method, an orientation survey was performed in which soils and vegetation at 17 sites north of Mineral, Virginia, were sampled and analyzed for Ag, Cd, Cu, Pb and Zn. The traverse included stations over the host rocks of massive sulfide mineralization, as well as over apparent "barren" country rock. Samples were analyzed by atomic absorption spectrophotometry using standard digestion and analytical techniques. Both A- and B-horizon soil metals generally appear to be reliable indicators of mineralization, with soils developed over sulfides showing up to three-fold enrichment in metal content relative to the average soils developed on the country rock. Correlation of metal concentrations in vegetation to soil metal concentrations reveal plant concentrations expressed on a dry-weight basis correlate stronger and more frequently to soil metals than do ash-weight concentrations. Copper shows some promise in selected organs and species, Ag appears fair but data are limited to one organ of one species, and plant Pb seems totally unresponsive to soil metal concentrations perhaps because foliar absorption is an important plant uptake mechanism here. However, Zn and Cd in organs of the oak group, especially mature leaves and twigs of the current year's growth show the greatest promise as prospecting tools. They correlate well with soil metals and when compared directly to the geology they reliably reflect mineralization. Although results using White oak were slightly less profound than those obtained from the Black-Red oak group, White oak may be preferred as it is a single, more widespread, easily-identifiable species. Copper and especially Zn although essential elements to plants, do not appear to be "difficult" elements for biogeochemical prospecting in the Appalachian piedmont. © 1979.
- Leavitt, S. W., Treydte, K., & Liu, Y. u. (2010, January). Environment in time and space: Opportunities from tree-ring isotope networks. In Isoscapes: Understanding movement, pattern, and process on Earth through isotope mapping, 113-135.More infoAbstract: Light stable isotopes in plants have been related to various climate parameters such as temperature, precipitation, relative humidity, and soil moisture, and to ecophysiological variables such as photosynthetic rate, stomatal conductance and intrinsic water-use efficiency. By collecting radial growth rings of trees in networks, it is not only possible to characterize these variables around the geographical area, but also to reconstruct the variation of these parameters through time. The real power of dendrochronologically dated tree rings is in providing certainty in assignment of an isotopic value to a particular year and perhaps even a sub-annual period. Additionally, the growth rings integrate isotopic signals from around the tree crown and may smooth out some of the high intra-crown variability associated with microenvironments. Furthermore, woody plants are widespread on six of seven continents, thereby supporting widespread opportunities for network development. Examples of existing and emerging tree-ring isotope networks on three continents are used to illustrate the findings, successes and limitations of this methodology. The goals of such studies will ultimately determine the methods used in developing the network, but analysis of multiple, dendrochronologically dated trees at each site, sampling of the same species at all sites, and analysis of the same wood component in all trees would be ideal in most cases. © 2010 Springer Science+Business Media B.V.
- Leavitt, S. W., Panyushkina, I. P., & Gutiérrez García, G. (2018, June). Tree, tree-ring and isotope prospects for identifying Holocene abrupt climate change in North America. Holocene Abrupt Climatic Events and the Environmental Effects Workshop. Xi'an, China: Institute of Earth Environment, CAS, Xi'an, China.
- Panyushkina, I. P., Leavitt, S. W., & Mode, W. (2017, August). Late Pleistocene millennium-scale tree-ring record from North America. The 14th International Conference on Accelerator Mass Spectrometry, IntCal and Dendrochronology Workshop, 14-18 August 2017. University of Ottawa, Canada.
- Detizio, M. L., Zawiskie, J. M., Panyushkina, I. P., & Leavitt, S. W. (2016, March). New AMS radiocarbon bone and tusk dates for Late Pleistocene mastodons in southeast Michigan. Michigan Academy of Science Arts & Letters Conference, 4 March 2016. Saginaw Valley State University.
- Detizio, M. L., Zawiskie, J. M., Panyushkina, I. P., & Leavitt, S. W. (2016, March). New AMS radiocarbon bone and tusk dates for Late Pleistocene mastodons in southeast Michigan. Michigan Academy of Science Arts & Letters Conference. Saginaw Valley State University.
- Gutierrez Garcia, G., Carriquiry-Beltran, J. D., Leavitt, S. W., & Trouet, V. M. (2016, March). Hydroclimatic variability of the Baja California Peninsula reconstructed from tree-rings of Pinus lagunae and Pinus monophylla. Third American Dendrochronology Conference (AmeriDendro). Mendoza, Argentina.
- Johnson, J. E., Tipple, B. J., Betancourt, J. L., Ehleringer, J. R., Leavitt, S. W., & Monson, R. K. (2016, December). Variation in the apparent biosynthetic fractionation for N-alkane δD among terrestrial plants: Patterns, mechanisms, and implications. AGU Fall Meeting. San Francisco, CA: American Geophysical Union.
- Leavitt, S. W., & Panyushkina, I. P. (2016, April). The stable-isotopic view of tree-ring series from the Great Lakes area between ca. 12 ka and 13.8 ka. Geological Society of America, North-Central Section - 50th Annual Meeting, 18 to 19 April 2016. Champaign, IL.
- Leavitt, S. W., & Panyushkina, I. P. (2016, April). The stable-isotopic view of tree-ring series from the Great Lakes area between ca. 12 ka and 13.8 ka. Geological Society of America, North-Central Section - 50th Annual Meeting. Champaign, IL: Geological Society of America.
- Panyushkina, I. P., Leavitt, S. W., & Zawiskie, J. (2016, April). Annually-resolved environmental proxies in the Great Lakes Area, 14 ka to 10 ka BP: A time of rising human population and mega-fauna extinction. Society for American Archaeology (SAA) 81st Annual Meeting, 6-10 April 2016. Orlando, Florida.
- Panyushkina, I. P., Leavitt, S. W., & Zawiskie, J. M. (2016, April). Annually-Resolved Environmental proxies in the Great Lakes Area, 14 ka to 10 ka BP: A Time of Rising Human Population and Mega-Fauna Extinction. Society for American Archaeology (SAA) 81st Annual Meeting. Orlando, Florida: Society for American Archaeology.
- Panyushkina, I. P., Livina, V. N., & Leavitt, S. W. (2016, April). Climate variability of Late Pleistocene deglaciation in the North American midcontinent derived from tree rings. European Geosciences Union (EGU) General Assembly 2016, 17–22 April 2016. Vienna, Austria.
- Szejner, P., Wright, W. E., Babst, F., Belmecheri, S., Trouet, V. M., Leavitt, S. W., Ehleringer, J. R., & Monson, R. K. (2016, December). Latitudinal gradients in tree ring stable carbon and oxygen isotopes reveal differential climate influences of the North American Monsoon system. AGU Fall Meeting. San Francisco, CA: American Geophysical Union.
- Szejner, P., Wright, W. E., Leavitt, S. W., & Monson, R. K. (2016, Summer). Using stable isotopes in tree-rings to understand tree performance. Isotope Seminar. Georg-August-University, Göttingen, Germany.
- García, G. G., Carriquiry Beltrán, J. D., Leavitt, S. W., & Trouet, V. (2015, 2-7 November, 2015). Ariabilidad hidroclimática de la península de Baja California reconstruida a partir de anillos de crecimiento de Pinus lagunae y Pinus monophylla. Reunión Anual 2015 de la Unión Geofísica Mexicana. Puerto Vallarta, Jalisco, México.
- Leavitt, S. W. (2015, 13 March 2015). Tree rings and the tales they tell of drought, floods, fires, and times gone by. Cochise College, High Desert Gardening Workshop. Cochise College, Sierra Vista, AZ.
- Leavitt, S. W. (2015, 25-26 April 2015). Radiocarbon dating at LTRR and the birth of calibration. Workshop- A Comprehensive Guide to Bayesian Modeling of Radiocarbon Dates using OxCal. Tucson, AZ: UA AMS Laboratory.
- Leavitt, S. W. (2015, Fall). Connecting Trees and Space: The A.E. Douglass Story. Moon Tree Celebration. Space Sciences Building, University of Arizona.
- Leavitt, S. W. (2015, Summer). Long tree-ring isotope chronologies in the U.S. Southwest and North American Monsoon hydroclimate. 10th North American Forest Ecology Workshop (NAFEW). Veracruz, Mexico.
- Leavitt, S. W., & Panyushkina, I. P. (2015, 12-17 April 2015). Climatic signals in tree-ring stable isotope records from the U.S. Great Lakes subfossil wood network: Successes and limitations. European Geoscience Union General Assembly (EGU). Vienna, Austria.
- Leavitt, S. W., & Panyushkina, I. P. (2015, 26 July to 2 August 2015). Tree-ring evidence of non-stationary environment in the U.S. Great Lakes area between 15ka-7ka: A time of human transcontinental migrations and mega-fauna extinctions. XIX INQUA Congress. Nagoya, Japan: The International Union for Quaternary Science.
- Leavitt, S. W., & Panyushkina, I. P. (2015, Summer). Tree-ring evidence of non-stationary environment in the U.S. Great Lakes area between 15ka-7ka: A time of human transcontinental migrations and mega-fauna extinctions. XIX INQUA Congress (Quaternary Perspectives on Climate Change, Natural Hazards and Civilization). Nagoya, Japan: INQUA.
- Panyushkina, I. P., & Leavitt, S. W. (2015, Summer). Pan-Pacific tree-ring records in excess of 50 ka. XIX INQUA Congress (Quaternary Perspectives on Climate Change, Natural Hazards and Civilization). Nagoya, Japan.
- Panyushkina, I. P., Leavitt, S. W., Lara, A., Roig, F., Palmer, J., Turney, C. S., Jull, T., & Van de Water, P. K. (2015, 26 July to 2 August 2015). Pan-Pacific tree-ring records in excess of 50 ka. XIX INQUA Congress. Nagoya, Japan: The International Union for Quaternary Science.
- Panyushkina, I. P., authors, F. o., Leavitt, S. W., Hughes, M. K., Panyushkina, I. P., authors, F. o., Leavitt, S. W., & Hughes, M. K. (2014, January). Tree-ring evidence of wood property changes caused by modern acidification of arctic soils.. World Dendro 2014. Melbourne, Australia.
- Monson, R. K., Szejner, P., Belmecheri, S., Morino, K., Wright, w., Babst, F., Hughes, M. K., Leavitt, S. W., Trouet, V. M., & Ehleringer, J. (2017, August). Reconstructing drought legacies in the North American Monsoon climate system using tree ring stable isotopes. ESA meeting. Portland, OR.
- Hamann, L., Johnson, J. E., Dettman, D. L., Kim-Hak, D., Leavitt, S. W., Monson, R. K., Pope, A. J., & Papuga, S. A. (2016, April). Quantifying differences between cryogenic distillation and induction methods for stable isotope analysis: Implications for understanding plant-water dynamics. Earth Week. University of Arizona.
- Meixner, T., Leavitt, S. W., & Morino, K. (2016, December). Water sources over time for a semi-arid river- Implications for water resources. AGU Fall Meeting. San Francisco, CA: American Geophysical Union.
- Panyushkina, I. P., Livina, V. N., & Leavitt, S. W. (2016, April). Climate variability of Late Pleistocene deglaciation in the North American midcontinent derived from tree rings. European Geosciences Union (EGU) General Assembly. Vienna, Austria: EGU.
- Johnson, J. E., Tipple, B. J., Betancourt, J. L., Ehleringer, J. R., Leavitt, S. W., & Monson, R. K. (2015, Winter). Do interspeciﬁc differences in the stable hydrogen isotopic composition of n-alkanes reﬂect variation in plant water sources or in biosynthetic fractionation?. AGU Fall Meeting. San Francisco, CA: American Geophysical Union.
- Csank, A., Woodhouse, C., Pederson, G., Danhole, J., & Leavitt, S. (2014, August). Assessing the ability of tree-ring isotopes to improve our understanding of the climatic drivers of streamflow in the Upper-Colorado River Basin. AMQUA 2014 Biennial Meeting. Seattle, WA.More infoDates: 08/07-08/10
- Csank, A., Woodhouse, C., Pederson, G., Leavitt, S., & Danloe, J. (2014, January). Proxy cool-season temperatures in the Upper-Colorado River headwaters from tree-ring δ18O. 9th International Conference on Dendrochronology (WorldDendro). Melbourne, Australia.
- Griffin, D., Woodhouse, C., Leavitt, S., & Graham, R. (2014). North American Monsoon climate signal in a multi-proxy dataset from tree-ring latewood. 2014 AAG Annual Meeting. Tampa, FL.
- Jull, T., Panyuskina, I., Lange, T., Chang, C., Myglan, V., Kukarskih, V., Burr, G., Leavitt, S., & Swetnam, T. (2014, January). Evaluating the possible size of excursions in the 14C record due to cosmic events. 9th International Conference on Dendrochronology (WorldDendro). Melbourne, Australia.
- Leavitt, S., & Panyushkina, I. (2014, January). Radiocarbon age control of tree-ring chronologies from a network of ancient wood sites in the U.S. Great Lakes area. 9th International Conference on Dendrochronology (WorldDendro). Melbourne, Australia.
- Liu, T., Li, Q., Wang, Y., Song, H., Leavitt, S., Linderholm, H., Wang, R., & An, Z. (2014, January). The superiority of numerical mix method in tree-ring stable-carbon isotope based May-July temperature reconstruction over Nanwutai, China for the past century. 9th International Conference on Dendrochronology (WorldDendro). Melbourne, Australia.
- Panyushkina, I., Grachev, A., Shishov, V., Kirdynov, A., Chebykin, E., Vaganov, E., Leavitt, S., & Hughes, M. (2014, January). Tree-ring evidence of wood property changes caused by modern acidification of Arctic soils. 9th International Conference on Dendrochronology (WorldDendro). Melbourne, Australia.
- Wright, W., Leavitt, S., Woodhouse, C., Monson, R., & Szejner, P. (2014, January). Tree-ring isotopes applied to understanding the North American Monsoon macrosystem. 9th International Conference on Dendrochronology (WorldDendro). Melbourne, Australia.
- Griffin, D., Woodhouse, C., Meko, D., Stahle, D., Faulstich, H., Carrillo, C., Touchan, R., Castro, C., & Leavitt, S. (2013, May). North American monsoon precipitation reconstructed from tree-ring latewood. 2nd AmeriDendro Conference. Tucson, AZ.
- Leavitt, S., Woodhouse, C., Griffin, D., Touchan, R., Carrillo, C., Castro, C., & Meko, D. (2013, May). Monsoon Rainfall from Annual ?13C Chronologies of Pseudotsuga menziesii and Pinus ponderosa in Southern Arizona, USA. 2nd AmeriDendro Conference. Tucson, AZ.
- Panyushkina, I., & Leavitt, S. (2013, May). Tree-ring perspectives on short-term climate variability during the transition during Late Pleistocene to Holocene from the Great Lakes Network of buried timbers. 2nd AmeriDendro Conference. Tucson, AZ.
- Panyushkina, I., & Leavitt, S. (2013, October). Abrupt changes of moisture variability in the Great Lakes region at ca. 13.7ka, 12ka, 11.5ka and 8.2ka: A new perspective from subfossil tree rings. PaleoAmerican Odyssey Conference on the First Americans. Santa Fe, NM.More infoDates: 10/17-10/20
- Wright, W., Leavitt, S., Woodhouse, C., & Monson, R. (2013, May). Spatial changes in the seasonal mixing proportions of available soil moisture in the high elevations of Arizona and New Mexico. 2nd AmeriDendro Conference. Tucson, AZ.
- Graham, R., Woodhouse, C., Griffin, D., Meko, D., Touchan, R., Leavitt, S., & Castro, C. (2012, December). Reconstruction of Winter and July Precipitation in the US Southwest using minimum blue intensity measurements from Pseudotsuga menziesii. AGU Fall Meeting. San Francisco, CA.More infoDates: 12/03-12/07
- Jull, A., Panyushkina, I., Van, d., Leavitt, S., Squire, J., Testa, N., Wie, A., Armstrong, A., Diaz-Gomez, C., & Leonard, A. (2012, July). High-resolution terrestrial MIS3 environment from trees recovered from landslide deposits in Oregon. 21st International Radiocarbon Conference. Paris, France.
- Leavitt, S., Panyushkina, I., Zawiskie, J., & Wiedenhoeft, A. (2012, July). Wood from the Shelton Mastodon Site (Michigan): Radiocarbon and tree-ring dating. 21st International Radiocarbon Conference. Paris, France.
- Panyushkina, I., & Leavitt, S. (2012, June). Progress with Ancient Great Lakes Wood: Radiocarbon and Tree-Ring Analysis. 2012 American Quaternary Association Conference. Duluth, MN.More infoDates: 06/21-06/24
- Panyushkina, I., Leavitt, S., Schmierer, J., & Wiedenhoeft, A. (2012, July). Mystery of ancient buried wood in the Arnheim “Swamp”, Upper Peninsula Michigan, USA. 21st International Radiocarbon Conference. Paris, France.
- Leavitt, S. W. (2016, April). Global Change in the Great Lakes Region 14ka to 6ka from the Annals of Ancient Wood. Seminar- Geology Department, University of Wisconsin-Oshkosh.
- Leavitt, S. (2014, Fall). Ancient Environmental Change from Tree Rings: Paleoenvironment in the American Great Lakes (5-14ka BP) and in Khazakhstan (Iron Age). 11 February 2014, Climate Change Research Center, University of New South Wales, Australia..More infoCo-presentation with Irina Panyushkina; Date: 02/11/2014; Conference/Event: Climate Change Research Center; Location: University of New South Wales, Australia
- Leavitt, S. (2014, Fall). Stable isotopes in tree rings: Background and applications. Environmental Workshop, 6 January 2014, Universidad Austral de Chile, Valdivia..More infoConference/Event: Environmental Workshop; Date: 01/06/2014; Location: Universidad Austral de Chile, Valdivia
- Leavitt, S. (2014, Fall). Tree rings and the tales they tell of drought, floods, fires, and times gone by. Shade Tree Short Course “Respect Your Elders” (invited) Feb 25-27, 2014, Ames, Iowa..More infoConference/Event: Shade Tree Short Course “Respect Your Elders”; Date: 02/25-27/2014; Location: Ames, Iowa