Jia Hu

Interests

Research

Ecohydrology, Plant Ecophysiology, Biogeochemistry

Teaching

Plant Ecophysiology, Climate Change, Global Change Biology, Ecosystem Ecology

Courses

Scholarly Contributions

Chapters

• Jia, H. u. (2020). How Do Non-Native Plants Influence Soil Nutrients Along a Hydroclimate Gradient on San Cristobal Island?. In Land Cover and Land Use Change on Islands. Social and Ecological Interactions in the Galapagos Islands.
• Jia, H. u. (2020). The Carbon Balance of Tropical Islands: Lessons from Soil Respiration. In Land Cover and Land Use Change on Islands. Social and Ecological Interactions in the Galapagos Islands.

Journals/Publications

• Huang, J., Ladd, S. N., Ingrisch, J., Kübert, A., Meredith, L. K., van Haren, J., Bamberger, I., Daber, L. E., Kühnhammer, K., Bailey, K., Hu, J., Fudyma, J., Shi, L., Dippold, M. A., Meeran, K., Miller, L., O’Brien, M. J., Yang, H., Herrera-Ramírez, D., , Hartmann, H., et al. (2024). The mobilization and transport of newly fixed carbon are driven by plant water use in an experimental rainforest under drought. Journal of Experimental Botany, 75(8), 2545-2557. doi:10.1093/jxb/erae030
• Steiner, B., Scott, R. L., Hu, J., MacBean, N., Richardson, A., & Moore, D. J. (2024). Using phenology to unravel differential soil water use and productivity in a semiarid savanna. Ecosphere, 15(2). doi:10.1002/ecs2.4762
• , ., Ochoa‐Hueso, R., Delgado‐Baquerizo, M., Risch, A. C., Ashton, L., Augustine, D., Bélanger, N., Bridgham, S., Britton, A. J., Bruckman, V. J., Camarero, J. J., Cornelissen, G., Crawford, J. A., Dijkstra, F. A., Diochon, A., Earl, S., Edgerley, J., Epstein, H., Felton, A., , Fortier, J., et al. (2023). Bioavailability of Macro and Micronutrients Across Global Topsoils: Main Drivers and Global Change Impacts. Global Biogeochemical Cycles, 37(6). doi:10.1029/2022gb007680
• Bailey, K., Szejner, P., Strange, B., Monson, R. K., & Hu, J. (2023). The Influence of Winter Snowpack on the Use of Summer Rains in Montane Pine Forests Across the Southwest U.S.. Journal of Geophysical Research: Biogeosciences, 128(9). doi:10.1029/2023jg007494
• Strange, B. M., Monson, R. K., Szejner, P., Ehleringer, J., & Hu, J. (2023). The North American Monsoon buffers forests against the ongoing megadrought in the Southwestern United States. Global Change Biology, 29(15), 4354-4367. doi:10.1111/gcb.16762
• Bailey, K., Bamberger, I., Beyer, M., Dubbert, M., Hu, J., Kübert, A., Kühnhammer, K., Meredith, L. K., Nemiah Ladd, S., Werner, C., & van Haren, J. (2022). Tracing plant source water dynamics during drought by continuous transpiration measurements: An in‐situ stable isotope approach. Plant, Cell & Environment, 46(1), 133-149. doi:10.1111/pce.14475
• Bailey, K., Harris, S., Hu, J., Korb, N., & Kruse, C. (2022). Water use strategies between two co‐occurring woody species in a riparian area: Naturally occurring willow, Salix exigua , and expanding juniper, Juniperus scopulorum , in central Montana. Ecohydrology, 15(3). doi:10.1002/eco.2402
• Chen, T., , ., Dorado-Liñán, I., Gagen, M., Hu, J., Liu, X., Szejner, P., Trouet, V., & Xu, G. (2022). Intra-annual tree-ring δ18O and δ13C reveal a trade-off between isotopic source and humidity in moist environments. Tree Physiology. doi:10.1093/treephys/tpac076
• Clute, T., Hu, J., Looker, N., & Martin, J. T. (2021). Correction to: Hydraulic traits of co-existing conifers do not correlate with local hydroclimate condition: a case study in the northern Rocky Mountains, U.S.A.. Oecologia, 196(2), 605. doi:10.1007/s00442-020-04837-z
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  In this study, we examined the inter- and intra-specific variation of hydraulic traits of three conifers of the Northern Rockies: Pinus ponderosa, Picea engelmannii, and Pseudotsuga menziesii to understand the mechanisms that allow different plant species to co-exist across a watershed. We quantified differences in plant xylem water potential (ψx), xylem tissue vulnerability to cavitation (P50, or ψ causing 50% loss of hydraulic conductivity), and safety margins for co-occurring trees from low and high elevations. We then investigated xylem vulnerability to cavitation with rooting depth. We found that xylem vulnerability to cavitation did not correspond to where tree species were found in the landscape. For example, P. ponderosa grew in more xeric locations, while P. engelmannii were largely confined to more mesic locations, yet P. engelmannii had more negative P50 values. P. menziesii had the lowest P50 value, but displayed little variation in vulnerability to cavitation across the hydroclimatic gradient. These patterns were also reflected in the safety margins; P. menziesii had the widest safety margin, P. engelmannii was intermediate, and P. ponderosa displayed the narrowest safety margin. All three species were also using water sources deeper than 30 cm in depth, allowing them to persist throughout the mid-summer drought. Overall, species-specific hydraulic traits did not necessarily follow a predictable response to the environment; instead, a combination of physiological and morphological traits likely allow trees to persist across a broader hydroclimatic gradient than would be otherwise expected from hydraulic trait measurements alone.
• Garcia, M. N., Hu, J., Domingues, T. F., Groenendijk, P., Oliveira, R. S., & Costa, F. R. (2021). Local hydrological gradients structure high intraspecific variability in plant hydraulic traits in two dominant central Amazonian tree species. Journal of Experimental Botany, 73(3), 939-952. doi:10.1093/jxb/erab432
• Garcia, M. N., Hu, J., Tomas, D. F., Peter, G., Rafael, O. S., & Costa, F. F. (2021). Local hydrological gradients structure high intra-species variability in plant hydraulic traits in two dominant central Amazonian tree species. Journal of Experimental Biology, 73(3), 939–952. doi:https://doi.org/10.1093/jxb/erab43
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  Maquelle Garcia was a visiting PhD student from Brazil, who came to work in my lab for 8 months. This publication was a result of her visit to UA.
• Jia, H. u. (2021). Nitrogen acquisition strategies of mature Douglas‐fir: a case study in the northern Rocky Mountains. Ecosphere.
• Qubain, C. A., Yano, Y., & Hu, J. (2021). Nitrogen acquisition strategies of mature Douglas‐fir: a case study in the northern Rocky Mountains. Ecosphere, 12(1). doi:10.1002/ecs2.3338
• Qubain, C., Yano, Y., & Hu, J. (2021). Nitrogen acquisition strategies of Douglas fir: A case study in the northern Rocky Mountains. Ecosphere, 121, e03338. doi:doi.org/10.1002/ecs2.3338
• Szejner, P., Belmecheri, S., Babst, F., Wright, J. E., Frank, D. C., Hu, J., & Monson, R. K. (2021). Stable isotopes of tree rings reveal seasonal-to-decadal patterns during the emergence of a megadrought in the Southwestern US. Oecologia, 197(4), 16. doi:doi.org/10.1007/s00442-021-04916-9
• Werner, C., , ., Meredith, L. K., Ladd, S. N., Ingrisch, J., Kübert, A., van Haren, J., Bahn, M., Bailey, K., Bamberger, I., Beyer, M., Blomdahl, D., Byron, J., Daber, E., Deleeuw, J., Dippold, M. A., Fudyma, J., Gil-Loaiza, J., Honeker, L. K., , Hu, J., et al. (2021). Ecosystem fluxes during drought and recovery in an experimental forest. Science, 374(6574), 1514-1518. doi:10.1126/science.abj6789
• Werner, C., Meredith, L. K., Ladd, S. N., Ingrisch, J., Kübert, A., van Haren, J., Bahn, M., Bailey, K., Bamberger, I., Beyer, M., Blomdahl, D., Byron, J., Daber, E., Deleeuw, J., Dippold, M. A., Fudyma, J., Gil-Loaiza, J., Honeker, L. K., Hu, J., , Huang, J., et al. (2021). Ecosystem fluxes during drought and recovery in an experimental forest. Science (New York, N.Y.), 374(6574), 1514-1518.
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  [Figure: see text].
• Jia, H. u. (2020). Biophysical Gradients and Performance of Whitebark Pine Plantings in the Greater Yellowstone Ecosystem. Forests.
• Jia, H. u. (2020). Hydraulic traits of co-existing conifers do not correlate with local hydroclimate condition: a case study in the northern Rocky Mountains, U.S.A. Oecologia.
• Jia, H. u. (2020). Reduction in lumen area is associated with the δ 18 O exchange between sugars and source water during cellulose synthesis. New Phytologist.
• Szejner, P., Clute, T., Anderson, E., Evans, M. N., & Hu, J. (2020). Reduction in lumen area is associated with the δ¹⁸O exchange between sugars and source water during cellulose synthesis. New Phytologist, 226(6), 1583-1593. doi:10.1111/nph.16484
• Anderson, D. E., Anderson, D. E., Burns, S. P., Burns, S. P., Hu, J., Hu, J., Lenschow, D. H., Lenschow, D. H., Monson, R. K., Monson, R. K., Oncley, S. P., Oncley, S. P., Stephens, B. B., Stephens, B. B., Sun, J., Sun, J., Yi, C., & Yi, C. (2019). Correction to: Atmospheric Stability Effects on Wind Fields and Scalar Mixing Within and Just Above a Subalpine Forest in Sloping Terrain. Boundary-Layer Meteorology, 172(3), 481-484. doi:10.1007/s10546-019-00436-5
• Dwivedi, R., Eastoe, C., Knowles, J. F., Wright, W. E., Hamann, L., Minor, R., Mitra, B., Meixner, T., McIntosh, J., Ty Ferre, P. A., Castro, C., Niu, G., Barron‐Gafford, G. A., Abramson, N., Papuga, S. A., Stanley, M., Hu, J., & Chorover, J. (2019). Vegetation source water identification using isotopic and hydrometric observations from a subhumid mountain catchment. Ecohydrology, 13(1). doi:10.1002/eco.2167
• Dwivedi, R., Eastoe, C., Knowles, J., Wright, W., Hammon, L., Minor, R., Mitra, B., Ferre, P., Castro, C., Nui, G., Barron-Gafford, G. A., Meixner, T., Abramson, N., Papuga, S., Stanley, M., Hu, J., & Chorover, J. (2019). Vegetation source water identification using isotopic and hydrometric observations from a sub-humid mountain Catchment. Ecohydrology, e2167, 17. doi:10.1002/eco.2167
• Hoylman, Z. H., Jencso, K. G., Hu, J., Holden, Z. A., Allred, B., Dobrowski, S., Robinson, N., Martin, J. T., Affleck, D., & Seielstad, C. (2019). The Topographic Signature of Ecosystem Climate Sensitivity in the Western United States. Geophysical Research Letters, 46(24), 14508-14520. doi:10.1029/2019gl085546
• Hoylman, Z. H., Jencso, K. G., Hu, J., Holden, Z. A., Martin, J. T., & Gardner, W. P. (2019). The Climatic Water Balance and Topography Control Spatial Patterns of Atmospheric Demand, Soil Moisture, and Shallow Subsurface Flow. Water Resources Research, 55(3), 2370-2389. doi:10.1029/2018wr023302
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  USDA NIFA McIntire Stennis award [233327]; NSF grants [DEB-1457749, DEB-1457720]; NASA applied science program Wildland Fire award [NNH11ZDA001N-FIRES]; NSF EPSCoR through the Montana Institute on Ecosystems
• Hoylman, Z., Jencso, K., Hu, J., Holden, Z., Allred, B., Dobrowski, S., Robinson, N., Martin, J., Affleck, D., & Seielstad, C. (2019). The topographic signature of ecosystem climate sensitivities in the western U.S.. Geophysical Research Letters, 46, 14,508–14,520. doi:10.1029/2019GL085546
• Yano, Y., Qubain, C., Holyman, Z., Jencso, K., & Hu, J. (2019). Snowpack influences spatial and temporal soil nitrogen dynamics in a western U.S. montane forested watershed. Ecosphere, 10(7). doi:10.1002/ecs2.2794
• Yano, Y., Qubain, C., Hoylman, Z., Jencso, K., & Hu, J. (2019). Climate and topographic controls on seasonal and spatial nitrogen availability in a snow-dominated western US forest. Ecosphere, 10(7), 20. doi:10.1002/ecs2.2794
• Barnard, D. M., Knowles, J. F., Barnard, H. R., Goulden, M. L., Hu, J., Litvak, M. E., & Molotch, N. P. (2018). Reevaluating growing season length controls on net ecosystem production in evergreen conifer forests. SCIENTIFIC REPORTS, 8.
• Hoylman, Z. H., Jencso, K. G., Hu, J., Martin, J. T., Holden, Z. A., Seielstad, C. A., & Rowell, E. M. (2018). Hillslope Topography Mediates Spatial Patterns of Ecosystem Sensitivity to Climate. JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES, 123(2), 353-371.
• Looker, N., , ., Martin, J., Hoylman, Z., Jencso, K., & Hu, J. (2018). Diurnal and seasonal coupling of conifer sap flow and vapour pressure deficit across topoclimatic gradients in a subalpine catchment. Ecohydrology, 11(7). doi:10.1002/eco.1994
• Looker, N., Martin, J., Hoylman, Z., Jencso, K., & Hu, J. (2018). Diurnal and seasonal coupling of conifer sap flow and vapour pressure deficit across topoclimatic gradients in a subalpine catchment. ECOHYDROLOGY, 11(7).
• Martin, J., , ., Looker, N., Hoylman, Z., Jencso, K., & Hu, J. (2018). Differential use of winter precipitation by upper and lower elevation Douglas fir in the Northern Rockies. Global Change Biology, 24(12), 5607-5621. doi:10.1111/gcb.14435
• Martin, J., Looker, N., Hoylman, Z., Jencso, K., & Hu, J. (2018). Differential use of winter precipitation by upper and lower elevation Douglas fir in the Northern Rockies. GLOBAL CHANGE BIOLOGY, 24(12), 5607-5621.
• Riveros-Iregui, D. A., Lorenzo, T. M., Liang, L. L., & Hu, J. (2018). Summer dry-down modulates the isotopic composition of soil CO2 production in snow-dominated landscapes. PLOS ONE, 13(5).
• Schmitt, S. R., Riveros-Iregui, D. A., & Hu, J. (2018). The role of fog, orography, and seasonality on precipitation in a semiarid, tropical island. HYDROLOGICAL PROCESSES, 32(18), 2792-2805.
• Schmitt, S. R., Riveros‐Iregui, D. A., & Hu, J. (2018). The role of fog, orography, and seasonality on precipitation in a semiarid, tropical island. Hydrological Processes, 32(18), 2792-2805. doi:10.1002/hyp.13228
• Looker, N., Looker, N., Hoylman, Z., Hoylman, Z., Jencso, K., Jencso, K., Hu, J., Hu, J., Martin, J. T., & Martin, J. T. (2017). Hydrometeorology organizes intra-annual patterns of tree growth across time, space and species in a montane watershed.. The New phytologist, 215(4), 1387-1398. doi:10.1111/nph.14668
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  Tree radial growth is often systematically limited by water availability, as is evident in tree ring records. However, the physiological nature of observed tree growth limitation is often uncertain outside of the laboratory. To further explore the physiology of water limitation, we observed intra-annual growth rates of four conifer species using point dendrometers and microcores, and coupled these data to observations of water potential, soil moisture, and vapor pressure deficit over 2 yr in the Northern Rocky Mountains, USA. The onset of growth limitation in four species was well explained by a critical balance between soil moisture supply and atmospheric demand representing relatively mesic conditions, despite the timing of this threshold response varying by up to 2 months across topographic and elevation gradients, growing locations, and study years. Our findings suggest that critical water deficits impeding tissue growth occurred at relatively high water potential values, often occurring when hydrometeorological conditions were relatively wet during the growing season (e.g. in early spring in some cases). This suggests that species-specific differences in water use strategies may not necessarily affect tree growth, and that tissue growth may be more directly linked to environmental moisture conditions than might otherwise be expected.
• Hu, J., & Riveros-iregui, D. A. (2016). Life in the clouds: are tropical montane cloud forests responding to changes in climate?. Oecologia, 180(4), 1061-73. doi:10.1007/s00442-015-3533-x
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  The humid tropics represent only one example of the many places worldwide where anthropogenic disturbance and climate change are quickly affecting the feedbacks between water and trees. In this article, we address the need for a more long-term perspective on the effects of climate change on tropical montane cloud forests (TMCF) in order to fully assess the combined vulnerability and long-term response of tropical trees to changes in precipitation regimes, including cloud immersion. We first review the ecophysiological benefits that cloud water interception offers to trees in TMCF and then examine current climatological evidence that suggests changes in cloud base height and impending changes in cloud immersion for TMCF. Finally, we propose an experimental approach to examine the long-term dynamics of tropical trees in TMCF in response to environmental conditions on decade-to-century time scales. This information is important to assess the vulnerability and long-term response of TMCF to changes in cloud cover and fog frequency and duration.
• Looker, N., Looker, N., Jencso, K., Jencso, K., Hu, J., Hu, J., Martin, J. T., & Martin, J. T. (2016). Contribution of sapwood traits to uncertainty in conifer sap flow as estimated with the heat-ratio method. Agricultural and Forest Meteorology, 223, 60-71. doi:10.1016/j.agrformet.2016.03.014
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  Abstract Inferring whole-tree sap flow rates (Q) with thermometric sap flow sensors requires specification of physiological and structural attributes of trees. Using sap temperature measurements to estimate Q with the heat-ratio method (HRM) requires quantification of the water content (mc), basic density (ρb), and depth (Rs) of sapwood. Values of mc and ρb serve to estimate sapwood thermal diffusivity (k), a necessary variable in the calculation of heat-pulse velocity (Vh) that is often set to a nominal value (k_nom); mc and ρb are also used to convert Vh to sap velocity (Vs). The sapwood area across which Vs is integrated is often estimated on the basis of Rs. Because mc and ρb are correlated and influence Q through estimation of k and the conversion of Vh to Vs, we sought to quantify the potential error introduced when Q is calculated with k_nom rather than with k as estimated from measurements of mc and ρb in five coniferous species. We also examined how variation in mc, ρb, and Rs across sampling scales may contribute to uncertainty in estimated Q. Across the observed range of mc and ρb, the two traits contributed to a net decline in the process of estimating Vs from Vh (with k_nom). This suggests that the use of k_nom rather than a calculated k may result in overestimation of Q when mc and ρb co-vary as they did in our study. Variability in mc and ρb across sampling scales could induce errors greater than 10% in Vs (and hence Q), while within- and among-tree variability in Rs could impart even greater errors (up to 130%). We propose that this uncertainty be represented in the expression of error in whole-tree sap flow estimates and in statistical analyses involving those estimates.
• Hu, J., Blanken, P. D., Burns, S. P., Monson, R. K., & Turnipseed, A. A. (2015). The influence of warm-season precipitation on the diel cycle of the surface energy balance and carbon dioxide at a Colorado subalpine forest site. Biogeosciences, 12(23), 7349-7377. doi:10.5194/bg-12-7349-2015
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  Abstract. Precipitation changes the physical and biological characteristics of an ecosystem. Using a precipitation-based conditional sampling technique and a 14 year data set from a 25 m micrometeorological tower in a high-elevation subalpine forest, we examined how warm-season precipitation affected the above-canopy diel cycle of wind and turbulence, net radiation Rnet, ecosystem eddy covariance fluxes (sensible heat H, latent heat LE, and CO2 net ecosystem exchange NEE) and vertical profiles of scalars (air temperature Ta, specific humidity q, and CO2 dry mole fraction χc). This analysis allowed us to examine how precipitation modified these variables from hourly (i.e., the diel cycle) to multi-day time-scales (i.e., typical of a weather-system frontal passage). During mid-day we found the following: (i) even though precipitation caused mean changes on the order of 50–70 % to Rnet, H, and LE, the surface energy balance (SEB) was relatively insensitive to precipitation with mid-day closure values ranging between 90 and 110 %, and (ii) compared to a typical dry day, a day following a rainy day was characterized by increased ecosystem uptake of CO2 (NEE increased by a 10 %), enhanced evaporative cooling (mid-day LE increased by a 30 W m−2), and a smaller amount of sensible heat transfer (mid-day H decreased by a 70 W m−2). Based on the mean diel cycle, the evaporative contribution to total evapotranspiration was, on average, around 6 % in dry conditions and between 15 and 25 % in partially wet conditions. Furthermore, increased LE lasted at least 18 h following a rain event. At night, even though precipitation (and accompanying clouds) reduced the magnitude of Rnet, LE increased from a 10 to over 20 W m−2 due to increased evaporation. Any effect of precipitation on the nocturnal SEB closure and NEE was overshadowed by atmospheric phenomena such as horizontal advection and decoupling that create measurement difficulties. Above-canopy mean χc during wet conditions was found to be about 2–3 μmol mol−1 larger than χc on dry days. This difference was fairly constant over the full diel cycle suggesting that it was due to synoptic weather patterns (different air masses and/or effects of barometric pressure). Finally, the effect of clouds on the timing and magnitude of daytime ecosystem fluxes is described.
• Berkelhammer, M., Hu, J., Bailey, A., Noone, D. C., Still, C. J., Gochis, D., Hsiao, G. S., Rahn, T., Turnipseed, A., & Barnard, H. R. (2013). The nocturnal water cycle in an open-canopy forest. Journal of Geophysical Research, 118(17), 10225-10242. doi:10.1002/jgrd.50701
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  [1] The movement of moisture into, out-of, and within forest ecosystems is modulated by feedbacks that stem from processes which couple plants, soil, and the atmosphere. While an understanding of these processes has been gleaned from Eddy Covariance techniques, the reliability of the method suffers at night because of weak turbulence. During the summer of 2011, continuous profiles of the isotopic composition (i.e., δ18O and δD) of water vapor and periodic measurements of soil, leaf, and precipitation pools were measured in an open-canopy ponderosa pine forest in central Colorado to study within-canopy nocturnal water cycling. The isotopic composition of the nocturnal water vapor varies significantly based on the relative contributions of the three major hydrological processes acting on the forest: dewfall, exchange of moisture between leaf waters and canopy vapor, and periodic mixing between the canopy and background air. Dewfall proved to be surprisingly common (∼30% of the nights) and detectable on both the surface and within the canopy through the isotopic measurements. While surface dew could be observed using leaf wetness and soil moisture sensors, dew in the foliage was only measurable through isotopic analysis of the vapor and often occurred even when no dew accumulated on the surface. Nocturnal moisture cycling plays a critical role in water availability in forest ecosystems through foliar absorption and transpiration, and assessing these dynamics, as done here, is necessary for fully characterizing the hydrological controls on terrestrial productivity.
• Hu, J., Hu, J., Hopping, K. A., Hopping, K. A., Bump, J. K., Bump, J. K., Kang, S., Kang, S., Klein, J. A., & Klein, J. A. (2013). Climate change and water use partitioning by different plant functional groups in a grassland on the Tibetan Plateau.. PloS one, 8(9), e75503. doi:10.1371/journal.pone.0075503
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  The Tibetan Plateau (TP) is predicted to experience increases in air temperature, increases in snowfall, and decreases in monsoon rains; however, there is currently a paucity of data that examine the ecological responses to such climate changes. In this study, we examined the effects of increased air temperature and snowfall on: 1) water use partitioning by different plant functional groups, and 2) ecosystem CO2 fluxes throughout the growing season. At the individual plant scale, we used stable hydrogen isotopes (δD) to partition water use between shallow- and deep-rooted species. Prior to the arrival of summer precipitation (typically mid-July), snowmelt was the main water source in the soils. During this time, shallow and deep-rooted species partitioned water use by accessing water from shallow and deep soils, respectively. However, once the monsoon rains arrived, all plants used rainwater from the upper soils as the main water source. Snow addition did not result in increased snowmelt use throughout the growing season; instead, snowmelt water was pushed down into deeper soils when the rains arrived. At the larger plot scale, CO2 flux measurements demonstrated that rain was the main driver for net ecosystem productivity (NEP). NEP rates were low during June and July and reached a maximum during the monsoon season in August. Warming decreased NEP through a reduction in gross primary productivity (GPP), and snow additions did not mitigate the negative effects of warming by increasing NEP or GPP. Both the isotope and CO2 flux results suggest that rain drives productivity in the Nam Tso region on the TP. This also suggests that the effects of warming-induced drought on the TP may not be mitigated by increased snowfall. Further decreases in summer monsoon rains may affect ecosystem productivity, with large implications for livestock-based livelihoods.
• Simonin, K. A., Simonin, K. A., Roddy, A. B., Roddy, A. B., Link, P., Link, P., Apodaca, R., Apodaca, R., Tu, K. P., Tu, K. P., Hu, J., Hu, J., Dawson, T. E., Dawson, T. E., Barbour, M. M., & Barbour, M. M. (2013). Isotopic composition of transpiration and rates of change in leaf water isotopologue storage in response to environmental variables.. Plant, cell & environment, 36(12), 2190-206. doi:10.1111/pce.12129
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  During daylight hours, the isotope composition of leaf water generally approximates steady-state leaf water isotope enrichment model predictions. However, until very recently there was little direct confirmation that isotopic steady-state (ISS) transpiration in fact exists. Using isotope ratio infrared spectroscopy (IRIS) and leaf gas exchange systems we evaluated the isotope composition of transpiration and the rate of change in leaf water isotopologue storage (isostorage) when leaves were exposed to variable environments. In doing so, we developed a method for controlling the absolute humidity entering the gas exchange cuvette for a wide range of concentrations without changing the isotope composition of water vapour. The measurement system allowed estimation of (18)O enrichment both at the evaporation site and for bulk leaf water, in the steady state and the non-steady state. We show that non-steady-state effects dominate the transpiration isoflux even when leaves are at physiological steady state. Our results suggest that a variable environment likely prevents ISS transpiration from being achieved and that this effect may be exacerbated by lengthy leaf water turnover times due to high leaf water contents.
• Hu, J., Bowling, D. R., Burns, S. P., Monson, R. K., & Riveros-iregui, D. A. (2011). An interannual assessment of the relationship between the stable carbon isotopic composition of ecosystem respiration and climate in a high-elevation subalpine forest. Journal of Geophysical Research, 116(2). doi:10.1029/2010jg001556
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  [1] We measured the carbon isotopic composition (δ13C) of ecosystem respiration (δ13CR) in a subalpine forest across four growing seasons to examine whether patterns in δ13CR were consistent with those expected based on leaf-level gas-exchange theory, and in agreement with past studies of the relation between δ13CR and climate conducted across broad geographic regions. Conventional trends (i.e., less negative δ13CR with increased vapor pressure deficit (VPD) and air temperature (TAIR), and decreased soil moisture (θ)) were observed when we focused on the driest portions of average-wetness years and when δ13CR was positively correlated with nighttime ecosystem respiration (RE). Nonconventional trends (i.e., more negative δ13CR with decreased θ, and increased VPD and TAIR) were observed under specific climatic conditions (e.g., late snowmelt; extreme TAIR late in the growing season), and when δ13CR was negatively correlated with RE. These nonconventional trends were independently corroborated using δ13C of extracted sugars from needles of dominant tree species at the site. Our results clearly demonstrate that the commonly reported relations between δ13CR and climate may break down depending on the interactions among environmental conditions. Efforts to model and predict the variability of δ13CR under changing climatic variables must characterize and parameterize the effects of unique combinations of weather conditions and variable hydrologic regimes, in combination with the susceptibility of photosynthetic isotope discrimination to extreme air temperatures.
• Hu, J., Hu, J., Anderson, D. E., Anderson, D. E., Burns, S. P., Burns, S. P., Lenschow, D. H., Lenschow, D. H., Monson, R. K., Monson, R. K., Oncley, S. P., Oncley, S. P., Stephens, B. B., Stephens, B. B., Sun, J., Sun, J., Yi, C., & Yi, C. (2011). Atmospheric stability effects on wind fields and scalar mixing within and just above a subalpine forest in sloping terrain. Boundary-Layer Meteorology, 138(2), 231-262. doi:10.1007/s10546-010-9560-6
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  Air temperature Ta, specific humidity q,CO2 mole fraction χc, and three-dimen- sional winds were measured in mountainous terrain from five tall towers within a 1 km region encompassing a wide range of canopy densities. The measurements were sorted by a bulk Richardson number Rib. For stable conditions, we found vertical scalar differences devel- oped over a "transition" region between 0.05 1), the vertical scalar differences reached a maximum and remained fairly constant with increasing stability. The relationships q and χc have with Rib are explained by con- sidering their sources and sinks. For winds, the strong momentum absorption in the upper canopy allows the canopy sublayer to be influenced by pressure gradient forces and terrain effects that lead to complex subcanopy flow patterns. At the dense-canopy sites, soil respi- ration coupled with wind-sheltering resulted in CO2 near the ground being 5-7 µmol mol −1 larger than aloft, even with strong above-canopy winds (near-neutral conditions). We found Rib-binning to be a useful tool for evaluating vertical scalar mixing; however, additional information (e.g., pressure gradients, detailed vegetation/topography, etc.) is needed to fully explain the subcanopy wind patterns. Implications of our results for CO2 advection over heterogenous, complex terrain are discussed.
• Hu, J., Allwine, E., Anderson, D. E., Aulenbach, S., Burns, S. P., Campos, T., Clements, C. B., Coons, T., Ellsworth, P. Z., Lai, C., Lamb, B., Lenschow, D. H., Monson, R. K., Moore, D. J., Ojima, D. S., Oncley, S. P., Sacks, W. J., Schimel, D. S., Stephens, B. B., , Sternberg, L. D., et al. (2010). A Multiscale and Multidisciplinary Investigation Of Ecosystem–Atmosphere CO2 Exchange Over the Rocky Mountains of Colorado. Bulletin of the American Meteorological Society, 91(2), 209-230. doi:10.1175/2009bams2733.1
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  A significant fraction of Earth consists of mountainous terrain. However, the question of how to monitor the surface–atmosphere carbon exchange over complex terrain has not been fully explored. This article reports on studies by a team of investigators from U.S. universities and research institutes who carried out a multiscale and multidisciplinary field and modeling investigation of the CO2 exchange between ecosystems and the atmosphere and of CO2 transport over complex mountainous terrain in the Rocky Mountain region of Colorado. The goals of the field campaign, which included ground and airborne in situ and remote-sensing measurements, were to characterize unique features of the local CO2 exchange and to find effective methods to measure regional ecosystem–atmosphere CO2 exchange over complex terrain. The modeling effort included atmospheric and ecological numerical modeling and data assimilation to investigate regional CO2 transport and biological processes involved in ecosystem–atmosphere carbon exch...
• Hu, J., Burns, S. P., Monson, R. K., & Moore, D. J. (2010). Longer growing seasons lead to less carbon sequestration by a subalpine forest. Global Change Biology, 16(2), 771-783. doi:10.1111/j.1365-2486.2009.01967.x
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  As global temperatures increase, the potential for longer growing seasons to enhance the terrestrial carbon sink has been proposed as a mechanism to reduce the rate of further warming. At the Niwot Ridge AmeriFlux site, a subalpine forest in the Colorado Rocky Mountains, we used a 9-year record (1999-2007) of continuous eddy flux observations to show that longer growing season length (GSL) actually resulted in less annual CO 2 uptake. Years with a longer GSL were correlated with a shallower snow pack, as measured using snow water equivalent (SWE). Furthermore, years with a lower SWE correlated with an earlier start of spring. For three years, 2005, 2006, and 2007, we used observations of stable hydrogen isotopes (δD) of snow vs. rain, and extracted xylem water from the three dominant tree species, lodgepole pine, Engelmann spruce, and subalpine fir, to show that the trees relied heavily on snow melt water even late into the growing season. By mid-August, 57% to 68% of xylem water reflected the isotopic signature of snow melt. By coupling the isotopic water measurements with an ecosystem model, SIPNET, we found that annual forest carbon uptake was highly dependent on snow water, which decreases in abundance during years with longer growing seasons. Once again, for the 3 years 2005, 2006, and 2007, annual gross primary productivity, which was derived as an optimized parameter from the SIPNET model was estimated to be 67% 77%, and 71% dependent on snow melt water, respectively. Past studies have shown that the mean winter snow pack in mountain ecosystems of the Western US has been declining for decades and is correlated with positive winter temperature anomalies. Since climate change models predict continuation of winter warming and reduced snow in mountains of the Western US, the strength of the forest carbon sink is likely to decline further.
• Hu, J., Burns, S. P., Monson, R. K., Moore, D. J., & Riveros-iregui, D. A. (2010). Modeling whole-tree carbon assimilation rate using observed transpiration rates and needle sugar carbon isotope ratios.. The New phytologist, 185(4), 1000-15. doi:10.1111/j.1469-8137.2009.03154.x
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  *Understanding controls over plant-atmosphere CO(2) exchange is important for quantifying carbon budgets across a range of spatial and temporal scales. In this study, we used a simple approach to estimate whole-tree CO(2) assimilation rate (A(Tree)) in a subalpine forest ecosystem. *We analysed the carbon isotope ratio (delta(13)C) of extracted needle sugars and combined it with the daytime leaf-to-air vapor pressure deficit to estimate tree water-use efficiency (WUE). The estimated WUE was then combined with observations of tree transpiration rate (E) using sap flow techniques to estimate A(Tree). Estimates of A(Tree) for the three dominant tree species in the forest were combined with species distribution and tree size to estimate and gross primary productivity (GPP) using an ecosystem process model. *A sensitivity analysis showed that estimates of A(Tree) were more sensitive to dynamics in E than delta(13)C. At the ecosystem scale, the abundance of lodgepole pine trees influenced seasonal dynamics in GPP considerably more than Engelmann spruce and subalpine fir because of its greater sensitivity of E to seasonal climate variation. *The results provide the framework for a nondestructive method for estimating whole-tree carbon assimilation rate and ecosystem GPP over daily-to weekly time scales.
• Hu, J., Burns, S. P., Monson, R. K., Prater, M. R., Scott-denton, L. E., Sparks, K. L., & Sparks, J. P. (2010). Tree species effects on ecosystem water-use efficiency in a high-elevation, subalpine forest.. Oecologia, 162(2), 491-504. doi:10.1007/s00442-009-1465-z
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  Ecosystem water-use efficiency (eWUE; the ratio of net ecosystem productivity to evapotranspiration rate) is a complex landscape-scale parameter controlled by both physical and biological processes occurring in soil and plants. Leaf WUE (lWUE; the ratio of leaf CO(2) assimilation rate to transpiration rate) is controlled at short time scales principally by leaf stomatal dynamics and this control varies among plant species. Little is known about how leaf-scale variation in lWUE influences landscape-scale variation in eWUE. We analyzed approximately seven thousand 30-min averaged eddy covariance observations distributed across 9 years in order to assess eWUE in two neighboring forest communities. Mean eWUE was 19% lower for the community in which Engelmann spruce and subalpine fir were dominant, compared to the community in which lodgepole pine was dominant. Of that 19% difference, 8% was attributed to residual bias in the analysis that favored periods with slightly drier winds for the spruce-fir community. In an effort to explain the remaining 11% difference, we assessed patterns in lWUE using C isotope ratios. When we focused on bulk tissue from older needles we detected significant differences in lWUE among tree species and between upper and lower canopy needles. However, when these differences were scaled to reflect vertical and horizontal leaf area distributions within the two communities, they provided no power to explain differences in eWUE that we observed in the eddy covariance data. When we focused only on bulk needle tissue of current-year needles for 3 of the 9 years, we also observed differences in lWUE among species and in needles from upper and lower parts of the canopy. When these differences in lWUE were scaled to reflect leaf area distributions within the two communities, we were able to explain 6.3% of the differences in eWUE in 1 year (2006), but there was no power to explain differences in the other 2 years (2003 and 2007). When we examined sugars extracted from needles at 3 different times during the growing season of 2007, we could explain 3.8-6.0% of the differences in eWUE between the two communities, but the difference in eWUE obtained from the eddy covariance record, and averaged over the growing season for this single year, was 32%. Thus, overall, after accounting for species effects on lWUE, we could explain little of the difference in eWUE between the two forest communities observed in the eddy covariance record. It is likely that water and C fluxes from soil, understory plants, and non-needle tissues, account for most of the differences observed in the eddy covariance data. For those cases where we could explain some of the difference in eWUE on the basis of species effects, we partitioned the scaled patterns in lWUE into two components: a component that is independent of canopy leaf area distribution, and therefore only dependent on species-specific differences in needle physiology; and a component that is independent of species differences in needle physiology, and only dependent on species-specific influences on canopy leaf area distribution. Only the component that is dependent on species influences on canopy leaf area distribution, and independent of inherent species differences in needle physiology, had potential to explain differences in eWUE between the two communities. Thus, when tree species effects are important, canopy structure, rather than species-specific needle physiology, has more potential to explain patterns in eWUE.
• Hu, J., Monson, R. K., & Moore, D. J. (2010). Weather and climate controls over the seasonal carbon isotope dynamics of sugars from subalpine forest trees.. Plant, cell & environment, 33(1), 35-47. doi:10.1111/j.1365-3040.2009.02049.x
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  We examined the environmental variables that influence the delta(13)C value of needle and phloem sugars in trees in a subalpine forest. We collected sugars from Pinus contorta, Picea engelmannii and Abies lasiocarpa from 2006 to 2008. Phloem and needle sugars were enriched in (13)C during the autumn, winter and early spring, but depleted during the growing season. We hypothesized that the late-winter and early-spring (13)C enrichment was due to the mobilization of carbon assimilated the previous autumn; however, needle starch concentrations were completely exhausted by autumn, and we observed evidence of new starch production during episodic warm weather events during the winter and early-spring. Instead, we found that (13)C enrichment was best explained by the occurrence of cold night-time temperatures. We also observed seasonal decoupling in the (13)C/(12)C ratios of needle and phloem sugars. We hypothesized that this was due to seasonally-changing source-sink patterns, which drove carbon translocation from the needles towards the roots early in the season, before bud break, but from the roots towards the needles later in the season, after bud break. Overall, our results demonstrate that the (13)C/(12)C ratio of recently-assimilated sugars can provide a sensitive record of the short-term coupling between climate and tree physiology.
• Hu, J., Hu, J., Burns, S. P., Burns, S. P., Guenther, A. B., Guenther, A. B., Monson, R. K., Monson, R. K., Moore, D. J., Moore, D. J., Turnipseed, A. A., & Turnipseed, A. A. (2009). Controls over ozone deposition to a high elevation subalpine forest. Agricultural and Forest Meteorology, 149(9), 1447-1459. doi:10.1016/j.agrformet.2009.04.001
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  Ecosystem level ozone (O3) fluxes during four different years were examined at a subalpine forest site in the Colorado Rocky Mountains. The local mountain–valley wind system and the proximity of the Denver Metropolitan area leads to high summertime ozone episodes on many afternoons. The timing between these episodes and the ecosystem processes controlling photosynthesis during the growing season plays a critical role in determining the amount of ozone deposition. Light and vapor pressure deficit (VPD) were the most dominant environmental drivers controlling the deposition of O3 at this site through their influence on stomatal conductance. 81% of the daytime O3 uptake was predicted to occur through the stomata. Stomatal uptake decreased at high VPD and temperatures leading to an overall decrease in O3 flux; however, we did observe a non-stomatal conductance for O3 that increased slightly with temperature before leveling off at higher values. During the growing season, O3 deposition fluxes were enhanced after midday precipitation events and continued at elevated levels throughout the following night, implying a role for surface wetness. From nighttime data, evidence for both the presence of water films on the needles and non-closure of the plant stomata were observed. During the winter (nongrowing) season, the ozone deposition velocity showed a consistent dependency on the latent heat flux. Although the mechanism is unclear, it is apparent that precipitation events play a role here through their influence on latent heat flux.
• Hu, J., Hu, J., Monson, R. K., Monson, R. K., Moore, D. J., Moore, D. J., Sacks, W. J., Sacks, W. J., Schimel, D. S., & Schimel, D. S. (2008). Estimating transpiration and the sensitivity of carbon uptake to water availability in a subalpine forest using a simple ecosystem process model informed by measured net CO2 and H2O fluxes. Agricultural and Forest Meteorology, 148(10), 1467-1477. doi:10.1016/j.agrformet.2008.04.013
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  Modeling how the role of forests in the carbon cycle will respond to predicted changes in water availability hinges on an understanding of the processes controlling water use in ecosystems. Recent studies in forest ecosystem modeling have employed data-assimilation techniques to generate parameter sets that conform to observations, and predict net ecosystem CO2 exchange (NEE) and its component processes. Since the carbon and water cycles are linked, there should be additional process information available from ecosystem H2O exchange. We coupled SIPNET (Simple Photosynthesis EvapoTranspiration), a simplified model of ecosystem function, with a data-assimilation system to estimate parameters leading to model predictions most closely matching the net CO2 and H2O fluxes measured by eddy covariance in a high-elevation, subalpine forest ecosystem. When optimized using measurements of CO2 exchange, the model matched observed NEE (RMSE = 0.49 g C m−2) but underestimated transpiration calculated independently from sap flow measurements by a factor of 4. Consequently, the carbon-only optimization was insensitive to imposed changes in water availability. Including eddy flux data from both CO2 and H2O exchange to the optimization reduced the model fit to the observed NEE fluxes only slightly (RME = 0.53 g C m−2), however this parameterization also reproduced transpiration calculated from independent sap flow measurements (r2 = 0.67, slope = 0.6). A significant amount of information can be extracted from simultaneous analysis of CO2 and H2O exchange, which improved the accuracy of transpiration estimates from measured evapotranspiration. Conversely, failure to include both CO2 and H2O data streams can generate results that mask the responses of ecosystem carbon cycling to variation in the precipitation. In applying the model conditioned on both CO2 and H2O fluxes to the subalpine forest at the Niwot Ridge AmeriFlux site, we observed that the onset of transpiration is coincident with warm soil temperatures. However, after snow has covered the ground in the fall, we observed significant inter-annual variability in the fraction of evapotranspiration composed of transpiration; evapotranspiration was dominated by transpiration in years when late fall air temperatures were high enough to maintain photosynthesis, but by sublimation from the surface of the snowpack in years when late fall air temperatures were colder and forest photosynthetic activity had ceased. Data-assimilation techniques and simultaneous measurements of carbon and water exchange can be used to quantify the response of net carbon uptake to changes in water availability by using an ecosystem model where the carbon and water cycles are linked.
• Hu, J., Hu, J., Backlund, B., Backlund, B., Burns, S. P., Burns, S. P., Harley, P. C., Harley, P. C., Huxman, T. E., Huxman, T. E., Monson, R. K., Monson, R. K., Rosenstiel, T. N., Rosenstiel, T. N., Scott-denton, L. E., Scott-denton, L. E., Sparks, J. P., Sparks, J. P., Turnipseed, A. A., & Turnipseed, A. A. (2005). Climatic influences on net ecosystem CO2 exchange during the transition from wintertime carbon source to springtime carbon sink in a high-elevation, subalpine forest.. Oecologia, 146(1), 130-47. doi:10.1007/s00442-005-0169-2
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  The transition between wintertime net carbon loss and springtime net carbon assimilation has an important role in controlling the annual rate of carbon uptake in coniferous forest ecosystems. We studied the contributions of springtime carbon assimilation to the total annual rate of carbon uptake and the processes involved in the winter-to-spring transition across a range of scales from ecosystem CO2 fluxes to chloroplast photochemistry in a coniferous, subalpine forest. We observed numerous initiations and reversals in the recovery of photosynthetic CO2 uptake during the initial phase of springtime recovery in response to the passage of alternating warm- and cold-weather systems. Full recovery of ecosystem carbon uptake, whereby the 24-h cumulative sum of NEE (NEEdaily) was consistently negative, did not occur until 3-4 weeks after the first signs of photosynthetic recovery. A key event that preceded full recovery was the occurrence of isothermality in the vertical profile of snow temperature across the snow pack; thus, providing consistent daytime percolation of melted snow water through the snow pack. Interannual variation in the cumulative annual NEE (NEEannual) was mostly explained by variation in NEE during the snow-melt period (NEEsnow-melt), not variation in NEE during the snow-free part of the growing season (NEEsnow-free). NEEsnow-melt was highest in those years when the snow melt occurred later in the spring, leading us to conclude that in this ecosystem, years with earlier springs are characterized by lower rates of NEEannual, a conclusion that contrasts with those from past studies in deciduous forest ecosystems. Using studies on isolated branches we showed that the recovery of photosynthesis occurred through a series of coordinated physiological and biochemical events. Increasing air temperatures initiated recovery through the upregulation of PSII electron transport caused in part by disengagement of thermal energy dissipation by the carotenoid, zeaxanthin. The availability of liquid water permitted a slightly slower recovery phase involving increased stomatal conductance. The most rate-limiting step in the recovery process was an increase in the capacity for the needles to use intercellular CO2, presumably due to slow recovery of Rubisco activity. Interspecific differences were observed in the timing of photosynthetic recovery for the dominant tree species. The results of our study provide (1) a context for springtime CO2 uptake within the broader perspective of the annual carbon budget in this subalpine forest, and (2) a mechanistic explanation across a range of scales for the coupling between springtime climate and the carbon cycle of high-elevation coniferous forest ecosystems.

Proceedings Publications

• Musselman, K. N., Kueppers, L. M., Hu, J., & Kaiser, K. E. (2019). Blue and Green Water in the Mountains: Water Supply, Extreme Events, and Ecological Responses in Snow- and Glacier-Fed Catchments II Posters. In American Geophysical Union.
• Anderson, T., Hoylman, Z., Hu, J., & Jencso, K. (2015). Affordable Open-Source Data Loggers for Distributed Measurements of Sap-Flux, Stem Growth, Relative Humidity, Temperature, and Soil Water Content. In American Geophysical Union.
• Hoylman, Z., Hu, J., Jencso, K., Looker, N., & Martin, J. T. (2015). Linking Tree Growth Response to Measured Microclimate - A Field Based Approach. In American Geophysical Union.
• Hu, J., Jencso, K. G., & Yano, Y. (2015). Landscape controls on nitrogen availability in a western forest watershed. In American Geophysical Union.
• Schmitt, S., Riveros-iregui, D. A., & Hu, J. (2015). Elucidating Native and Non-Native Plant-Fog Interactions Across Microclimatic Zones in San Cristobal Island, Galapagos. In American Geophysical Union.
• Hu, J., Jencso, K., Looker, N., & Martin, J. T. (2014). Precipitation and Topography as Drivers of Tree Water Use and Productivity at Multiple Scales. In American Geophysical Union.
• Looker, N., Hu, J., Jencso, K., & Martin, J. T. (2014). Uncertainty in sap flow-based transpiration due to xylem properties. In American Geophysical Union.

Presentations

• Hu, J. (2021). Interpreting tree ring records using a plant ecophysiological approach. University of Nevada Reno. Tuson, AZ (virtual): University of Reno Nevada.
• Hu, J. (2021). The Role of Fog on the Galapagos Islands. Invited seminar, University of Arizona. Tuson, AZ (virtual): University of Arizona.
• Hu, J., Yano, Y., Qubain, C., Hoylman, Z., & Jencso, K. (2019, December). Differential use of winter precipitation by upper and lower elevation Douglas fir in the Northern Rockies. American Geophysical Union Meeting. San Francisco, CA: American Geophysical Union.

Poster Presentations

• Bailey, K., Korb, N., & Hu, J. (2019, December). Ecophysiological comparison within a riparian area between two co-occurring woody species: juniper (encroached) and willow (naturally occurring). American Geophysical Union Annual Meeting. San Francisco, CA: American Geophysical Union.
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  This poster presentation was led by my PhD student, Kinzie Bailey.
• Strange, B., Knowles, J., Barron-Gafford, G. A., & Hu, J. (2019, December). Increased spatiotemporal resolution of whole-tree carbon assimilation estimated with stable isotopes from tree-ring cellulose. American Geophysical Union annual meeting. San Francisco, CA: American Geophysical Union.
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  This poster was presented by my PhD student, Brandon Strange at the annual AGU meeting.