Thomas W Swetnam

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Books

• Swetnam, T. (2003). Fire and Climatic Change in Temperate Ecosystems of the Western Americas.

Journals/Publications

• Arizpe, A. H., Falk, D. A., Woodhouse, C. A., & Swetnam, T. W. (2020). Widespread fire years in the US-Mexico Sky Islands are contingent on both winter and monsoon precipitation. INTERNATIONAL JOURNAL OF WILDLAND FIRE.
• Guiterman, C. H., Baisan, C. H., English, N. B., Quade, J., Dean, J. S., & Swetnam, T. W. (2020). Convergence of Evidence Supports a Chuska Mountains Origin for the Plaza Tree of Pueblo Bonito, Chaco Canyon. AMERICAN ANTIQUITY, 85(2), 331-346.
• Swetnam, T. (2020). Historical Stand-Replacing Fire in Upper Montane Forests of the Madrean Sky Islands and Mogollon Plateau, Southwestern USA.
• Swetnam, T. (2020). Spatially and Temporally Variable Fire Regime on Rincon Peak, Arizona, USA.
• Swetnam, T. (2020). Special Issue: Fire History in California.
• Swetnam, T. (2020). Using Bigcone Douglas-fir Fire Scars and Tree Rings to Reconstruct Interior Chaparral Fire History.
• Swetnam, T. (2020). Widespread fire years in the US–Mexico Sky Islands are contingent on both winter and monsoon precipitation. International Journal of Wildland Fire.
• Guiterman, C. H., Margolis, E. Q., Baisan, C. H., Falk, D. A., Allen, C. D., & Swetnam, T. W. (2019). Spatiotemporal variability of human-fire interactions on the Navajo Nation. ECOSPHERE, 10(11).
• Swetnam, T., Allen, C. D., Falk, D. A., Baisan, C. H., Margolis, E., & Guiterman, C. H. (2019). Spatio-temporal variability of human-fire interactions on the Navajo Nation.. EcoSphere, 10(11), e02932. doi:https://doi.org/10.1002/ecs2.2932
• Guiterman, C. H., Margolis, E. Q., Allen, C. D., Falk, D. A., & Swetnam, T. W. (2018). Long-Term Persistence and Fire Resilience of Oak Shrubfields in Dry Conifer Forests of Northern New Mexico. ECOSYSTEMS, 21(5), 943-959.
• Hanna, D. P., Falk, D. A., Swetnam, T. W., & Romme, W. (2018). Age-related climate sensitivity in Pinus Edulis at Dinosaur National Monument, Colorado, USA. DENDROCHRONOLOGIA, 52, 40-47.
• Power, M. J., Codding, B. F., Taylor, A. H., Swetnam, T. W., Magargal, K. E., Bird, D. W., & O'Connell, J. F. (2018). Human Fire Legacies on Ecological Landscapes. FRONTIERS IN EARTH SCIENCE, 6.
• Bigio, E. R., Swetnam, T. W., & Pearthree, P. A. (2017). Late Holocene fire-climate relationships of the western San Juan Mountains, Colorado. INTERNATIONAL JOURNAL OF WILDLAND FIRE, 26(11), 944-962.
• Guiterman, C. H., Margolis, E. Q., Allen, C. D., Falk, D. A., & Swetnam, T. (2017). Long-term persistence and frequent fire suggest future increased landscape dominance of shrubfields in northern New Mexico. Ecosystems, 1-17. doi:http://DOI:10.1007/s10021-017-0192-2
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  Guiterman CH, EQ Margolis, CD Allen, DA Falk, and TW Swetnam. Ecosystems. In review.
• Kitzberger, T., Falk, D. A., Westerling, A. L., & Swetnam, T. W. (2017). Direct and indirect climate controls predict heterogeneous early-mid 21st century wildfire burned area across western and boreal North America. PLOS ONE, 12(12).
• O'Connor, C. D., Falk, D. A., Lynch, A. M., Swetnam, T. W., & Wilcox, C. P. (2017). Disturbance and productivity interactions mediate stability of forest composition and structure. ECOLOGICAL APPLICATIONS, 27(3), 900-915.
• Bigio, E. R., Swetnam, T. W., & Baisan, C. H. (2016). Local-scale and regional climate controls on historical fire regimes in the San Juan Mountains, Colorado. FOREST ECOLOGY AND MANAGEMENT, 360, 311-322.
• Guiterman, C. H., Swetnam, T. W., & Dean, J. S. (2016). Eleventh-century shift in timber procurement areas for the great houses of Chaco Canyon (vol 113, pg 1186, 2015). PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 113(6), E811-E811.
• Guiterman, C. H., Swetnam, T. W., & Dean, J. S. (2016). Eleventh-century shift in timber procurement areas for the great houses of Chaco Canyon. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 113(5), 1186-1190.
• Iniguez, J. M., Swetnam, T. W., & Baisan, C. H. (2016). Fire history and moisture influences on historical forest age structure in the sky islands of southern Arizona, USA. JOURNAL OF BIOGEOGRAPHY, 43(1), 85-95.
• Liebmann, M. J., Farella, J., Roos, C. I., Stack, A., Martini, S., & Swetnam, T. W. (2016). Native American depopulation, reforestation, and fire regimes in the Southwest United States, 1492-1900 CE. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 113(6), E696-E704.
• Stevens, J. T., Safford, H. D., North, M. P., Fried, J. S., Gray, A. N., Brown, P. M., Dolanc, C. R., Dobrowski, S. Z., Falk, D. A., Farris, C. A., Franklin, J. F., Fule, P. Z., Hagmann, R. K., Knapp, E. E., Miller, J. D., Smith, D. F., Swetnam, T. W., & Taylor, A. H. (2016). Average Stand Age from Forest Inventory Plots Does Not Describe Historical Fire Regimes in Ponderosa Pine and Mixed-Conifer Forests of Western North America. PLOS ONE, 11(5).
• Swetnam, T. (2016). Multiscale perspectives of fire, climate and humans in western North America and the Jemez Mountains, USA. Philosophical Transactions of the Royal Society B: Biological Sciences.
• Swetnam, T. (2016). Native American depopulation, reforestation, and fire regimes in the Southwest United States, 1492–1900 CE. Proceedings of the National Academy of Sciences.
• Swetnam, T. W., Farella, J., Roos, C. I., Liebmann, M. J., Falk, D. A., & Allen, C. D. (2016). Multiscale perspectives of fire, climate and humans in western North America and the Jemez Mountains, USA. PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY B-BIOLOGICAL SCIENCES, 371(1696).
• Guiterman, C. H., Margolis, E. Q., & Swetnam, T. W. (2015). DENDROECOLOGICAL METHODS FOR RECONSTRUCTING HIGH-SEVERITY FIRE IN PINE-OAK FORESTS. TREE-RING RESEARCH, 71(2), 67-77.
• O'Conner, C. D., Lynch, A. M., Falk, D. A., & Swetnam, T. W. (2015). Post-fire forest dynamics and climate variability affect spatial and temporal properties of spruce beetle outbreaks on a Sky Island mountain range. Forest Ecology and Management, 336, 148-162.
• O'Connor, C. D., Lynch, A. M., Falk, D. A., & Swetnam, T. W. (2015). Post-fire forest dynamics and climate variability affect spatial and temporal properties of spruce beetle outbreaks on a Sky Island mountain range. FOREST ECOLOGY AND MANAGEMENT, 336, 148-162.
• O'Connor, C. D., Swetnam, T. W., Lynch, A. M., Falk, D. A., Lynch, A. M., Falk, D. A., O'Connor, C. D., & Swetnam, T. W. (2015). Post-fire forest dynamics and climate variability affect spatial and temporal properties of spruce beetle outbreaks on a Sky Island mountain range.. Forest Ecology and Management, 336, 148-162. doi:http://dx.doi.org/10.1016/j.foreco.2014.10.021
• Williams, A. P., Seager, R., Macalady, A. K., Berkelhammer, M., Crimmins, M. A., Swetnam, T. W., Trugman, A. T., Buenning, N., Noone, D., McDowell, N. G., Hryniw, N., Mora, C. I., & Rahn, T. (2015). Correlations between components of the water balance and burned area reveal new insights for predicting forest fire area in the southwest United States. INTERNATIONAL JOURNAL OF WILDLAND FIRE, 24(1), 14-26.
• Fule, P. Z., Swetnam, T. W., Brown, P. M., Falk, D. A., Peterson, D. L., Allen, C. D., Aplet, G. H., Battaglia, M. A., Binkley, D., Farris, C., Keane, R. E., Margolis, E. Q., Grissino-Mayer, H., Miller, C., Sieg, C. H., Skinner, C., Stephens, S. L., & Taylor, A. (2014). Unsupported inferences of high-severity fire in historical dry forests of the western United States: response to Williams and Baker. GLOBAL ECOLOGY AND BIOGEOGRAPHY, 23(7), 825-830.
• O'Connor, C. D., Falk, D. A., Lynch, A. M., & Swetnam, T. W. (2014). Fire severity, size, and climate associations diverge from historical precedent along an ecological gradient in the Pinaleno Mountains, Arizona, USA. FOREST ECOLOGY AND MANAGEMENT, 329, 264-278.
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  In recent decades fire size and severity have been increasing in high elevation forests of the American Southwest. Ecological outcomes of these increases are difficult to gauge without an historical context for the role of fire in these systems prior to interruption by Euro-American land uses. Across the gradient of forest types in the Pinaleno Mountains, a Sky Island system in southeast Arizona that experienced two relatively large high-severity fires in the last two decades, we compared fire characteristics and climate associations before and after the onset of fire exclusion to determine the degree of similarity between past and recent fires. We use a gridded fire scar and demography sampling network to reconstruct spatially explicit estimates of fire extent and burn severity, as well as climate associations of fires from individual site to landscape scales from 1640 to 2008 C.E. We found that patterns of fire frequency, size, and severity were relatively stable for at least several centuries prior to 1880. A combination of livestock grazing and active fire suppression after circa 1880 led to (1) a significant reduction in fire spread but not fire ignition, (2) a conversion of more than 80% of the landscape from a frequent, low to mixed-severity fire regime to an infrequent mixed to high-severity fire regime, and (3) an increase in fuel continuity within a mid-elevation zone of dry mixed-conifer forest, resulting in increased opportunities for surface and crown fire spread into higher elevation mesic forests. The two most recent fires affecting mesic forests were associated with drought and temperature conditions that were not exceptional in the historical record but that resulted in a relative proportion of high burn severity up to four times that of previous large fires. The ecological effects of these recent fires appear to be more severe than any fire in the reconstructed period, casting uncertainty upon the recovery of historical species composition in high-severity burn patches. Significant changes to the spatial pattern, frequency, and climate associations of spreading fires after 1880 suggest that limits to fuel loading and fuel connectivity sustained by frequent fire have been removed. Coinciding factors of high fuel continuity and fuel loading, projected lengthening of the fire season, and increased variability in seasonal precipitation suggest that large high-severity fires, especially in mixed-conifer forests, will become the predominant fire type without aggressive actions to reduce fuel continuity and restore fire-resilient forest structure and species composition. (C) 2014 The Authors. Published by Elsevier B.V.
• Roos, C. I., Bowman, D. M., Balch, J. K., Artaxo, P., Bond, W. J., Cochrane, M., D'Antonio, C. M., DeFries, R., Mack, M., Johnston, F. H., Krawchuk, M. A., Kull, C. A., Moritz, M. A., Pyne, S., Scott, A. C., & Swetnam, T. W. (2014). Pyrogeography, historical ecology, and the human dimensions of fire regimes. JOURNAL OF BIOGEOGRAPHY.
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  In our 2011 synthesis (Bowman etal., Journal of Biogeography, 2011, 38, 2223-2236), we argued for a holistic approach to human issues in fire science that we term pyrogeography'. Coughlan & Petty (Journal of Biogeography, 2013, 40, 1010-1012) critiqued our paper on the grounds that our pyric phase' model was built on outdated views of cultural development, claiming we developed it to be the unifying explanatory framework for all human-fire sciences. Rather, they suggest that historical ecology' could provide such a framework. We used the pyric transition' for multiple purposes but did not offer it as an exclusive explanatory framework for pyrogeography. Although historical ecology' is one of many useful approaches to studying human-fire relationships, scholars should also look to political and evolutionary ecology, ecosystems and complexity theories, as well as empirical generalizations to build an interdisciplinary fire science that incorporates human, ecological and biophysical dimensions of fire regimes.
• Williams, A. P., Seager, R., Berkelhammer, M., Macalady, A. K., Crimmins, M. A., Swetnam, T. W., Trugman, A. T., Buenning, N., Hryniw, N., McDowell, N. G., Noone, D., Mora, C. I., & Rahn, T. (2014). Causes and Implications of Extreme Atmospheric Moisture Demand during the Record-Breaking 2011 Wildfire Season in the Southwestern United States. JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY, 53(12), 2671-2684.
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  In 2011, exceptionally low atmospheric moisture content combined with moderately high temperatures to produce a record-high vapor pressure deficit (VPD) in the southwestern United States (SW). These conditions combined with record-low cold-season precipitation to cause widespread drought and extreme wildfires. Although interannual VPD variability is generally dominated by temperature, high VPD in 2011 was also driven by a lack of atmospheric moisture. The May-July 2011 dewpoint in the SW was 4.5 standard deviations below the long-term mean. Lack of atmospheric moisture was promoted by already very dry soils and amplified by a strong ocean-to-continent sea level pressure gradient and upper-level convergence that drove dry northerly winds and subsidence upwind of and over the SW. Subsidence drove divergence of rapid and dry surface winds over the SW, suppressing southerly moisture imports and removing moisture from already dry soils. Model projections developed for the fifth phase of the Coupled Model Intercomparison Project (CMIP5) suggest that by the 2050s warming trends will cause mean warm-season VPD to be comparable to the record-high VPD observed in 2011. CMIP5 projections also suggest increased interannual variability of VPD, independent of trends in background mean levels, as a result of increased variability of dewpoint, temperature, vapor pressure, and saturation vapor pressure. Increased variability in VPD translates to increased probability of 2011-type VPD anomalies, which would be superimposed on ever-greater background VPD levels. Although temperature will continue to be the primary driver of interannual VPD variability, 2011 served as an important reminder that atmospheric moisture content can also drive impactful VPD anomalies.
• Williams, A. P., Seager, R., Macalady, A. K., Berkelhammer, M., Crimmins, M. A., Swetnam, T. W., Trugman, A. T., Beunning, N., Hryniw, N., McDowell, N. G., Noone, D., Mora, C. I., & Rahn, T. (2014). Correlations between components of the water balance and burned area reveal new insights for predicting forest fire area in the southwest United States.. International Journal of Wildlnd Fire.
• Diaz, H. F., & Swetnam, T. W. (2013). THE WILDFIRES OF 1910 Climatology of an Extreme Early Twentieth-Century Event and Comparison with More Recent Extremes. BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY, 94(9), 1361-1370.
• Diaz, H. F., & Swetnam, T. W. (2013). The wildfires of 1910: Climatology of an extreme early twentieth-century event and comparison with more recent extremes. Bulletin of the American Meteorological Society, 94(9), 1361-1370.
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  Abstract: The Great Fire of 1910 was a wildfire that burned about three million acres in northeast Washington, northern Idaho, and western Montana. Accidental fires were also common, especially from sparks along railways from wood-burning locomotives. The fledgling Forest Service had a tiny force of rangers with the responsibility for detecting and suppressing wildfires over enormous and remote areas. The national forests increased by 16 million acres in 1907 by executive order of Theodore Roosevelt in the last days of his presidency. The largest burned areas were in the northern Rockies and particularly in Idaho, where most fatalities occurred. March 1910 was an exceptionally warm month, as illustrated by the spatial pattern and magnitude of the temperature departure from the long-term average and the time series of area-weighted mean temperature over the contiguous US.
• Farris, C. A., Baisan, C. H., Falk, D. A., L., M., Fulé, P. Z., & Swetnam, T. W. (2013). A comparison of targeted and systematic fire-scar sampling for estimating historical fire frequency in south-western ponderosa pine forests. International Journal of Wildland Fire, 22(8), 1021-1033.
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  Abstract: Fire history researchers employ various forms of search-based sampling to target specimens that contain visible evidence of well preserved fire scars. Targeted sampling is considered to be the most efficient way to increase the completeness and length of the fire-scar record, but the accuracy of this method for estimating landscape-scale fire frequency parameters compared with probabilistic (i.e. systematic and random) sampling is poorly understood. In this study we compared metrics of temporal and spatial fire occurrence reconstructed independently from targeted and probabilistic fire-scar sampling to identify potential differences in parameter estimation in south-western ponderosa pine forests. Data were analysed for three case studies spanning a broad geographic range of ponderosa pine ecosystems across the US Southwest at multiple spatial scales: Centennial Forest in northern Arizona (100ha); Monument Canyon Research Natural Area (RNA) in central New Mexico (256ha); and Mica Mountain in southern Arizona (2780ha). We found that the percentage of available samples that recorded individual fire years (i.e. fire-scar synchrony) was correlated strongly between targeted and probabilistic datasets at all three study areas (r≤0.85, 0.96 and 0.91 respectively). These strong positive correlations resulted predictably in similar estimates of commonly used statistical measures of fire frequency and cumulative area burned, including Mean Fire Return Interval (MFI) and Natural Fire Rotation (NFR). Consistent with theoretical expectations, targeted fire-scar sampling resulted in greater overall sampling efficiency and lower rates of sample attrition. Our findings demonstrate that targeted sampling in these systems can produce accurate estimates of landscape-scale fire frequency parameters relative to intensive probabilistic sampling. © 2013 IAWF.
• Farris, C. A., Baisan, C. H., Falk, D. A., Van Horne, M. L., Fule, P. Z., & Swetnam, T. W. (2013). Targeted fire-scar sampling can accurately and efficiently estimate past fire regime patterns in Southwestern ponderosa pine forests.. International Journal of Wildland Fire.
• Farris, C. A., Baisan, C. H., Falk, D. A., Van, H., Fule, P. Z., & Swetnam, T. W. (2013). A comparison of targeted and systematic fire-scar sampling for estimating historical fire frequency in south-western ponderosa pine forests. INTERNATIONAL JOURNAL OF WILDLAND FIRE, 22(8), 1021-1033.
• Farris, C. A., Swetnam, T. W., Fulé, P. Z., Baisan, C. H., Van Horne, M. L., Falk, D. A., Van Horne, M. L., Falk, D. A., Baisan, C. H., Fulé, P. Z., Farris, C. A., & Swetnam, T. W. (2013). A comparison of targeted and systematic fire-scar sampling for estimating historical fire frequency in south-western ponderosa pine forests. International Journal of Wildland Fire, 22(8), 1021-1033. doi:http://dx.doi.org/10.1071/WF13026
• Fulé, P. Z., Swetnam, T. W., Brown, P. M., Falk, D. A., Peterson, D. L., Allen, C. D., Aplet, G. H., Battaglia, M. A., Binkley, D., Farris, C., Keane, R. E., Margolis, E. Q., Grissino-Mayer, H., Miller, C., Sieg, C. H., Skinner, C., Stephens, S. L., & Taylor, A. (2014). Unsupported inferences of high-severity fire in historical dry forests of the western United States: Response to Williams and Baker. Global Ecology and Biogeography, 23(7), 825-830.
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  Abstract: Reconstructions of dry western US forests in the late 19th century in Arizona, Colorado and Oregon based on General Land Office records were used by Williams & Baker (2012; Global Ecology and Biogeography, 21, 1042-1052; hereafter W&B) to infer past fire regimes with substantial moderate and high-severity burning. The authors concluded that present-day large, high-severity fires are not distinguishable from historical patterns. We present evidence of important errors in their study. First, the use of tree size distributions to reconstruct past fire severity and extent is not supported by empirical age-size relationships nor by studies that directly quantified disturbance history in these forests. Second, the fire severity classification of W&B is qualitatively different from most modern classification schemes, and is based on different types of data, leading to an inappropriate comparison. Third, we note that while W&B asserted 'surprising' heterogeneity in their reconstructions of stand density and species composition, their data are not substantially different from many previous studies which reached very different conclusions about subsequent forest and fire behaviour changes. Contrary to the conclusions of W&B, the preponderance of scientific evidence indicates that conservation of dry forest ecosystems in the western United States and their ecological, social and economic value is not consistent with a present-day disturbance regime of large, high-severity fires, especially under changing climate. © 2013 John Wiley & Sons Ltd.
• Margolis, E. Q., & Swetnam, T. W. (2013). Historical fire-climate relationships of upper elevation fire regimes in the south-western United States. INTERNATIONAL JOURNAL OF WILDLAND FIRE, 22(5), 588-598.
• Margolis, E. Q., & Swetnam, T. W. (2013). Historical fire-climate relationships of upper elevation fire regimes in the south-western United States. International Journal of Wildland Fire, 22(5), 588-598.
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  Abstract: Understanding relationships between variability in historical fire occurrence and ocean-atmosphere oscillations provides opportunities for fire forecasting and projecting changes in fire regimes under climate change scenarios. We analysed tree-ring reconstructed regional climate teleconnections and fire-climate relationships in upper elevation forests (>2700m) from 16 sites in eight mountain ranges in the south-western USA. Climate teleconnections were identified by testing for associations between regional Palmer Drought Severity Index (PDSI) and individual and combined phases of El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) indices for both the fire exclusion (1905-1978) and reconstructed fire periods (1700-1904). Fire-climate relationships were identified by comparing reconstructed fires (84 fire years) in three classes (all, synchronous and stand-replacing fires) with PDSI, precipitation, temperature, and individual and combined phases of ENSO, PDO and AMO indices. Individual and phase combinations of ENSO, PDO and AMO were associated with variability in regional PDSI. Upper elevation fire occurrence was related to variability in regional drought, ENSO phase and phase combinations of ENSO and PDO. We conclude that ENSO most consistently influenced variability in moisture and upper elevation fire occurrence, including stand-replacing fires, but this relationship was potentially modulated by phases of the PDO. © IAWF 2013.
• SWETNAM, T., & LYNCH, A. (1993). MULTICENTURY, REGIONAL-SCALE PATTERNS OF WESTERN SPRUCE BUDWORM OUTBREAKS. Ecological Monographs, 63(4), 399-424.
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  Tree ring chronologies from 24 mixed-conifer stands were used to reconstruct the long-term history of western spruce budworm (Choristoneura occidentalis) in northern New Mexico. Temporal and spatial patterns of budworm infestations (within-stand occurrences) and outbreaks (more-or-less synchronous infestations across many stands) were investigated to identify local-scale to regional-scale forest disturbance patterns. Nine regional-scale outbreaks were identified from 1690 to 1989. One ancient stand of Douglas-fir trees (Pseudotsuga menziesii) exceeding 700 yr in age revealed that budworms and overstory trees can coexist for extraordinary lengths of time. Using spectral analysis we found that the regional outbreak record contained important cyclical components with periods varying from almost-equal-to 20 to 33 yr. The statistically significant (P < .05) but variable periodicity of regional outbreaks suggests the forest-budworm dynamic is pseudoperiodic (i.e., a stable limit cycle or damped oscillator perturbed by noise).
• Stephens, S. L., Agee, J. K., Fulé, P., North, M. P., Romme, W. H., Swetnam, T. W., & Turner, M. G. (2013). Managing forests and fire in changing climates. Science, 342(6154), 41-42.
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  PMID: 24092714;Abstract: Policy focused on fire suppression only delays the inevitable.
• Swetnam, T. (2013). A comparison of targeted and systematic fire-scar sampling for estimating historical fire frequency in south-western ponderosa pine forests.
• Swetnam, T. (2013). Historical fire?climate relationships of upper elevation fire regimes in the south-western United States.
• Swetnam, T. (2013). The Wildfires of 1910: Climatology of an Extreme Early Twentieth-Century Event and Comparison with More Recent Extremes.
• Swetnam, T. W. (2013). THOMAS P. HARLAN 1935-2013 In Memoriam. TREE-RING RESEARCH, 69(2), 101-102.
• Taylor, A., Stephens, S. L., Skinner, C., Sieg, C. H., Miller, C., Grissino-Mayer, H., Margolis, E. Q., Keane, R. E., Farris, C., Binkley, D., Battaglia, M. A., Aplet, G. H., Allen, C. D., Peterson, D. L., Falk, D. A., Brown, P. M., Swetnam, T. W., & Fulé, P. Z. (2013). Unsupported inferences of high-severity fire in historical dry forests of the western United States: Response to Williams and Baker. Global Ecology and Biogeography, 23(7), 825-830. doi:http://doi:10.1111/geb.12136
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  Abstract: Reconstructions of dry western US forests in the late 19th century in Arizona, Colorado and Oregon based on General Land Office records were used by Williams & Baker (2012; Global Ecology and Biogeography, 21, 1042-1052; hereafter W&B) to infer past fire regimes with substantial moderate and high-severity burning. The authors concluded that present-day large, high-severity fires are not distinguishable from historical patterns. We present evidence of important errors in their study. First, the use of tree size distributions to reconstruct past fire severity and extent is not supported by empirical age-size relationships nor by studies that directly quantified disturbance history in these forests. Second, the fire severity classification of W&B is qualitatively different from most modern classification schemes, and is based on different types of data, leading to an inappropriate comparison. Third, we note that while W&B asserted 'surprising' heterogeneity in their reconstructions of stand density and species composition, their data are not substantially different from many previous studies which reached very different conclusions about subsequent forest and fire behaviour changes. Contrary to the conclusions of W&B, the preponderance of scientific evidence indicates that conservation of dry forest ecosystems in the western United States and their ecological, social and economic value is not consistent with a present-day disturbance regime of large, high-severity fires, especially under changing climate. © 2013 John Wiley & Sons Ltd.
• Williams, A. P., Allen, C. D., Macalady, A. K., Griffin, D., Woodhouse, C. A., Meko, D. M., Swetnam, T. W., Rauscher, S. A., Seager, R., Grissino-Mayer, H. D., Dean, J. S., Cook, E. R., Gangodagamage, C., Cai, M., & McDowell, N. G. (2013). Temperature as a potent driver of regional forest drought stress and tree mortality. NATURE CLIMATE CHANGE, 3(3), 292-297.
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  As the climate changes, drought may reduce tree productivity and survival across many forest ecosystems; however, the relative influence of specific climate parameters on forest decline is poorly understood. We derive a forest drought-stress index (FDSI) for the southwestern United States using a comprehensive tree-ring data set representing AD 1000-2007. The FDSI is approximately equally influenced by the warm-season vapour-pressure deficit (largely controlled by temperature) and cold-season precipitation, together explaining 82% of the FDSI variability. Correspondence between the FDSI and measures of forest productivity, mortality, bark-beetle outbreak and wildfire validate the FDSI as a holistic forest-vigour indicator. If the vapour-pressure deficit continues increasing as projected by climate models, the mean forest drought-stress by the 2050s will exceed that of the most severe droughts in the past 1,000 years. Collectively, the results foreshadow twenty-first-century changes in forest structures and compositions, with transition of forests in the southwestern United States, and perhaps water-limited forests globally, towards distributions unfamiliar to modern civilization.
• Williams, A., Allen, C. D., Macalady, A. K., Griffin, D., Woodhouse, C. A., Meko, D. M., Swetnam, T. W., Rauscher, S. A., Seager, R., Grissino-Mayer, H. D., Dean, J. S., Cook, E. R., Gangodagamage, C., Cai, M., & Mcdowell, N. G. (2013). Temperature as a potent driver of regional forest drought stress and tree mortality. Nature Climate Change, 3(3), 292-297.
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  Abstract: As the climate changes, drought may reduce tree productivity and survival across many forest ecosystems; however, the relative influence of specific climate parameters on forest decline is poorly understood. We derive a forest drought-stress index (FDSI) for the southwestern United States using a comprehensive tree-ring data set representing AD 1000-2007. The FDSI is approximately equally influenced by the warm-season vapour-pressure deficit (largely controlled by temperature) and cold-season precipitation, together explaining 82% of the FDSI variability. Correspondence between the FDSI and measures of forest productivity, mortality, bark-beetle outbreak and wildfire validate the FDSI as a holistic forest-vigour indicator. If the vapour-pressure deficit continues increasing as projected by climate models, the mean forest drought-stress by the 2050s will exceed that of the most severe droughts in the past 1,000 years. Collectively, the results foreshadow twenty-first-century changes in forest structures and compositions, with transition of forests in the southwestern United States, and perhaps water-limited forests globally, towards distributions unfamiliar to modern civilization. Copyright © 2013 Macmillan Publishers Limited.
• BROWN, P., & SWETNAM, T. (2012). A CROSS-DATED FIRE HISTORY FROM COAST REDWOOD NEAR REDWOOD NATIONAL-PARK, CALIFORNIA. CANADIAN JOURNAL OF FOREST RESEARCH-REVUE CANADIENNE DE RECHERCHE FORESTIERE, 24(1), 21-31.
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  Cross sections from coast redwood trees (Sequoia sempervirens (D.Don.)Endl.) in and near Redwood National Park were dendrochronologically cross-dated and used to develop a fire history from 1714 to 1985. A master chronology for the study area was first developed from old-growth trees and provided dating control for fire-scarred samples. Redwood offers a challenge for dendrochronology owing to partially absent rings (ring wedging) and uniform ring widths (complacency). Cross dating was successful in portions of 12 of 24 fire-scarred trees. Fire events were dated by noting the position of fire scars and other fire-associated ring structures (resin ducts, double latewood, growth releases, and ring separations) in the cross-dated ring series. Using only dates of fire scars, the mean fire interval (MFI) was 9.9 years from the first recorded fire in 1714 to the last in 1962. The MFI was 8.0 years for the best represented (greatest sample depth) presettlement period from 1714 to 1881. Using dates for all fire-associated ring features, the MFI from 1714 to 1962 was 7.0 years and from 1714 to 1881 was 6.0 years. Use of all fire-associated ring characteristics is argued to be a more complete representation of past fire frequency due to possible under-representation of fire-scar records from stump-top samples. Based upon scar positions within annual rings, fires occurred predominately late in the growing season or after growth ceased for the year. The mean fire intervals determined are shorter than those reported in all except one other fire history study from coast redwood and suggest that fire frequency in redwood may have been underestimated in many past studies.
• Farris, C. A., Baisan, C. H., Falk, D. A., Yool, S. R., & Swetnam, T. W. (2012). Spatial and temporal corroboration of a fire-scar-based fire history in a frequently burned ponderosa pine forest. ECOLOGICAL APPLICATIONS, 20(6), 1598-1614.
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  Fire scars are used widely to reconstruct historical fire regime parameters in forests around the world. Because fire scars provide incomplete records of past fire occurrence at discrete points in space, inferences must be made to reconstruct fire frequency and extent across landscapes using spatial networks of fire-scar samples. Assessing the relative accuracy of fire-scar fire history reconstructions has been hampered due to a lack of empirical comparisons with independent fire history data sources. We carried out such a comparison in a 2780-ha ponderosa pine forest on Mica Mountain in southern Arizona (USA) for the time period 1937-2000. Using documentary records of fire perimeter maps and ignition locations, we compared reconstructions of key spatial and temporal fire regime parameters developed from documentary fire maps and independently collected fire-scar data (n = 60 plots). We found that fire-scar data provided spatially representative and complete inventories of all major fire years (>100 ha) in the study area but failed to detect most small fires. There was a strong linear relationship between the percentage of samples recording fire scars in a given year (i.e., fire-scar synchrony) and total area burned for that year (y = 0.0003x+0.0087, r(2) = 0.96). There was also strong spatial coherence between cumulative fire frequency maps interpolated from fire-scar data and ground-mapped fire perimeters. Widely reported fire frequency summary statistics varied little between fire history data sets: fire-scar natural fire rotations (NFR) differed by = 25% of study area burned) were identical between data sets (25.5 yr); fire-scar MFIs for all fire years differed by 1.2 yr from documentary records. The known seasonal timing of past fires based on documentary records was furthermore reconstructed accurately by observing intra-annual ring position of fire scars and using knowledge of tree-ring growth phenology in the Southwest. Our results demonstrate clearly that representative landscape-scale fire histories can be reconstructed accurately from spatially distributed fire-scar samples.
• Roos, C. I., & Swetnam, T. W. (2012). A 1416-year reconstruction of annual, multidecadal, and centennial variability in area burned for ponderosa pine forests of the southern Colorado Plateau region, Southwest USA. HOLOCENE, 22(3), 281-290.
• Roos, C. I., & Swetnam, T. W. (2012). A 1416-year reconstruction of annual, multidecadal, and centennial variability in area burned for ponderosa pine forests of the southern Colorado Plateau region, Southwest USA. Holocene, 22(3), 281-290.
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  Abstract: Fire history reconstructions from fire scars in tree rings have been valuable for assessing fire regime changes and their climatic controls. It has been asserted, however, that these two to four-century long records from the western USA are unrepresentative of longer periods of the Holocene and are of limited use for understanding current or future fire regimes. The Medieval Climate Anomaly (800-1300 CE) is often suggested as a better analog for future Southwestern US climates but is beyond the chronological range of most fire-scar studies in this region. To evaluate fire regime changes over the past millennium, we build on centennial-length fire-climate studies to generate a 1416 year long reconstruction of fire activity in ponderosa pine forests of the Southern Colorado Plateau region of Arizona and New Mexico. We used a split-period calibration and verification protocol to test the reliability of a multiple regression model using annual and antecedent precipitation (reconstructed from tree-ring width chronologies) to predict the percentage of fire-scar localities (i.e. sites, N=45) that recorded extensive fires within those sites (>25% of recorder trees scarred) each year between 1700 and 1899 CE. The model explains approximately 50% of the variation in annual fire activity. Applying the model to the entire precipitation reconstruction provides a proxy for annual area burned since 572 CE. There are no statistically significant differences between the period available for fire-scar study (1600 ce-present) and the Medieval Climate Anomaly (800-1300 CE) in terms of predicted annual area burned or the frequencies of regional fire years. Multidecadal and centennial variation in the frequencies of regional fire years, however, does indicate reduced surface fire frequencies from approximately 700-800 CE and 1360-1455 CE. We hypothesize that these were periods when some forests were vulnerable to altered canopy structure, accumulated fuels, and increased fire severity. © SAGE Publications 2011.
• Swetnam, T. (2012). Dendroclimatology. Progress and Prospects. Malcolm K. Hughes, Thomas W. Swetnam, Henry F. Diaz (Eds.) Developments in Paleoenvironmental Research, Vol. 11. 1st Edition, 2011, XII, 365 p. Hardcover, ISBN 978-1-4020-4010-8..
• Swetnam, T. (2012). Temperature as a potent driver of regional forest drought stress and tree mortality.
• Sánchez-Salguero, R., Navarro-Cerrillo, R. M., Swetnam, T. W., & Zavala, M. A. (2012). Is drought the main decline factor at the rear edge of Europe? The case of southern Iberian pine plantations. Forest Ecology and Management, 271, 158-169.
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  Abstract: Drought has been frequently discussed as a trigger for forest decline. Today, large-scale forest decline is observed at the rear edge of Mediterranean forests, with drought identified as the most likely driver. The vulnerability of Mediterranean mountain plantations to regional climatic variations; however, is poorly understood. In this paper, we analyze the impact of biotic and abiotic factors on the growth and vigor of two pine species in drought-prone areas. We assess the main factors influencing crown defoliation and radial growth to develop a predictive model of forest decline risk for pine plantations at the dry edge of the species range. Dendrochronological data were collected on 50 plots for Pinus nigra subsp. salzmanii and 40 plots for Pinus sylvestris. We examined tree size, competition, site characteristics and climate variables related to decline of pine plantations. Correlation and principal component analysis (PCA) were used to identify the correlates of decline with crown condition and growth, separately. Logistic regression and generalized linear models were used to study the relationship between canopy defoliation and growth, respectively, for P. nigra and P. sylvestris. Explanatory variables were introduced in a stepwise selection. The PCA revealed three main axes, associated with climate, competition and physiographic variables. Those three axes were associated with crown damage and basal area growth, respectively. Probability of crown damage was associated with tree size, competition and climate conditions. P. sylvestris was more sensitive than P. nigra to summer potential evapotranspiration effects. Also, climate and competition were the two main drivers affecting basal area growth. Unlike crown defoliation, physiography had an important effect. Within each species, there was a divergence between healthy and damaged trees, mainly related to competition factors. On the other hand, growth trends as an independent variable were not included in the crown damage probability model in a stepwise selection. However, declining growth found in trees at higher crown defoliation and drier sites may imply a greater vulnerability to decline, suggesting an enhanced die-off risk. The sharp growth reduction and widespread defoliation in declining pine plantations make their future persistence in xeric sites subject to frequent and severe droughts unlikely under expected warmer and drier conditions in the future. © 2012 Elsevier B.V.
• Bowman, D. M., Balch, J., Artaxo, P., Bond, W. J., Cochrane, M. A., D'Antonio, C. M., Defries, R., Johnston, F. H., Keeley, J. E., Krawchuk, M. A., Kull, C. A., Mack, M., Moritz, M. A., Pyne, S., Roos, C. I., Scott, A. C., Sodhi, N. S., & Swetnam, T. W. (2011). The human dimension of fire regimes on Earth. Journal of Biogeography, 38(12), 2223-2236.
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  Abstract: Humans and their ancestors are unique in being a fire-making species, but 'natural' (i.e. independent of humans) fires have an ancient, geological history on Earth. Natural fires have influenced biological evolution and global biogeochemical cycles, making fire integral to the functioning of some biomes. Globally, debate rages about the impact on ecosystems of prehistoric human-set fires, with views ranging from catastrophic to negligible. Understanding of the diversity of human fire regimes on Earth in the past, present and future remains rudimentary. It remains uncertain how humans have caused a departure from 'natural' background levels that vary with climate change. Available evidence shows that modern humans can increase or decrease background levels of natural fire activity by clearing forests, promoting grazing, dispersing plants, altering ignition patterns and actively suppressing fires, thereby causing substantial ecosystem changes and loss of biodiversity. Some of these contemporary fire regimes cause substantial economic disruptions owing to the destruction of infrastructure, degradation of ecosystem services, loss of life, and smoke-related health effects. These episodic disasters help frame negative public attitudes towards landscape fires, despite the need for burning to sustain some ecosystems. Greenhouse gas-induced warming and changes in the hydrological cycle may increase the occurrence of large, severe fires, with potentially significant feedbacks to the Earth system. Improved understanding of human fire regimes demands: (1) better data on past and current human influences on fire regimes to enable global comparative analyses, (2) a greater understanding of different cultural traditions of landscape burning and their positive and negative social, economic and ecological effects, and (3) more realistic representations of anthropogenic fire in global vegetation and climate change models. We provide an historical framework to promote understanding of the development and diversification of fire regimes, covering the pre-human period, human domestication of fire, and the subsequent transition from subsistence agriculture to industrial economies. All of these phases still occur on Earth, providing opportunities for comparative research. © 2011 Blackwell Publishing Ltd.
• Falk, D. A., Heyerdahl, E. K., Brown, P. M., Farris, C., Fulé, P. Z., McKenzie, D., Swetnam, T. W., Taylor, A. H., & L., M. (2011). Multi-scale controls of historical forest-fire regimes: New insights from fire-scar networks. Frontiers in Ecology and the Environment, 9(8), 446-454.
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  Abstract: Anticipating future forest-fire regimes under changing climate requires that scientists and natural resource managers understand the factors that control fire across space and time. Fire scars - proxy records of fires, formed in the growth rings of long-lived trees - provide an annually accurate window into past low-severity fire regimes. In western North America, networks of the fire-scar records spanning centuries to millennia now include hundreds to thousands of trees sampled across hundreds to many thousands of hectares. Development of these local and regional fire-scar networks has created a new data type for ecologists interested in landscape and climate regulation of ecosystem processes - which, for example, may help to explain why forest fires are widespread during certain years but not others. These data also offer crucial reference information on fire as a dynamic landscape process for use in ecosystem management, especially when managing for forest structure and resilience to climate change. © The Ecological Society of America.
• Margolis, E. Q., Swetnam, T. W., & Allen, C. D. (2011). Historical stand-replacing fire in upper montane forests of the Madrean Sky Islands and Mogollon Plateau, Southwestern USA. Fire Ecology, 7(3), 88-107.
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  Abstract: The recent occurrence of large fires with a substantial stand-replacing component in the southwestern United States (e.g., Cerro Grande, 2000; Rodeo-Chedeski, 2002; Aspen, 2003; Horseshoe 2, Las Conchas, and Wallow, 2011) has raised questions about the historical role of stand-replacing fire in the region. We reconstructed fire dates and stand-replacing fire patch sizes using four lines of tree-ring evidence at four upper montane forest sites (>2600 m) in the Madrean Sky Islands and Mogollon Plateau of Arizona and New Mexico, USA. The four lines of tree-ring evidence include: (1) quaking aspen (Populus tremuloides) and spruce-fir age structure, (2) conifer death dates, (3) traumatic resin ducts and ring-width changes, and (4) conifer fire scars. Pre-1905 fire regimes in the upper montane forest sites were variable, with drier, south-facing portions of some sites recording frequent, low-severity fire (mean fire interval of all fires ranging from 5 yr to 11 yr among sites), others burning with stand-replacing severity, and others with no evidence of fire for >300 yr. Reconstructed fires at three of the four sites (Pinaleño Mountains, San Francisco Peaks, and Gila Wilderness) had stand-replacing fire patches >200 ha, with maximum patch sizes ranging from 286 ha in mixed conifer-aspen forests to 521 ha in spruce-fir forests. These data suggest that recent stand-replacing fire patches as large as 200 ha to 500 ha burning in upper elevation (>2600 m) mixed conifer-aspen and spruce-fir forests may be within the historical range of variability.
• O'Connor, C. D., Garfin, G. M., Falk, D. A., & Swetnam, T. W. (2011). Human Pyrogeography: A New Synergy of Fire, Climate and People is Reshaping Ecosystems across the Globe. Geography Compass, 5(6), 329-350.
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  Abstract: Climate and fire have shaped global ecosystems for millennia. Today human influence on both of these components is causing changes to ecosystems at a scale and pace not previously seen. This article reviews trends in pyrogeography research, through the lens of interactions between fire, climate and society. We synthesize research on the occurrence and extent of wildland fire, the historic role of climate as a driver of fire regimes, the increasing role of humans in shaping ecosystems and accelerating fire ignitions, and projections of future interactions among these factors. We emphasize an ongoing evolution in the roles that humans play in mediating fire occurrence, behavior and feedbacks to the climate system. We outline the necessary elements for the development of a mechanistic model of human, fire and climate interactions, and discuss the role geographers can play in the development of sound theoretical underpinnings for a new paradigm of human pyrogeography. Disciplines such as geography that encourage science-society research can contribute significantly to policy discussions and the development of frameworks for adapting fire management for the preservation of societal and natural system priorities. © 2011 The Authors. Geography Compass © 2011 Blackwell Publishing Ltd.
• Rollins, M., Morgan, P., & Swetnam, T. (2011). Landscape-scale controls over 20(th) century fire occurrence in two large Rocky Mountain (USA) wilderness areas. LANDSCAPE ECOLOGY, 17(6), 539-557.
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  Topography, vegetation, and climate act together to determine the spatial patterns of fires at landscape scales. Knowledge of landscape- fire- climate relations at these broad scales (1,000s ha to 100,000s ha) is limited and is largely based on inferences and extrapolations from fire histories reconstructed from finer scales. In this study, we used long time series of fire perimeter data (fire atlases) and data for topography, vegetation, and climate to evaluate relationships between large 20 (th) century fires and landscape characteristics in two contrasting areas: the 486,673- ha Gila/ Aldo Leopold Wilderness Complex (GALWC) in New Mexico, USA, and the 785,090- ha Selway- Bitterroot Wilderness Complex (SBWC) in Idaho and Montana, USA. There were important similarities and differences in gradients of topography, vegetation, and climate for areas with different fire frequencies, both within and between study areas. These unique and general relationships, when compared between study areas, highlight important characteristics of fire regimes in the Northern and Southern Rocky Mountains of the Western United States. Results suggest that amount and horizontal continuity of herbaceous fuels limit the frequency and spread of surface fires in the GALWC, while the moisture status of large fuels and crown fuels limits the frequency of moderate- to- high severity fires in the SBWC. These empirically described spatial and temporal relationships between fire, landscape attributes, and climate increase understanding of interactions among broad- scale ecosystem processes. Results also provide a historical baseline for fire management planning over broad spatial and temporal scales in each wilderness complex.
• Swetnam, T. (2011). Human pyrogeography.
• Swetnam, T. (2011). Multi-scale controls of historical forest-fire regimes: new insights from fire-scar networks.
• Swetnam, T. (2011). The human dimension of fire regimes on Earth.
• Swetnam, T., Allen, C., & Betancourt, J. (2011). Applied historical ecology: Using the past to manage for the future. ECOLOGICAL APPLICATIONS, 9(4), 1189-1206.
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  Applied historical ecology is the use of historical knowledge in the management of ecosystems. Historical perspectives increase our understanding of the dynamic nature of landscapes and provide a frame of reference for assessing modern patterns and processes. Historical records, however, are often too brief or fragmentary to be useful, or they are not obtainable for the process or structure of interest. Even where long historical time series can be assembled, selection of appropriate reference conditions may be complicated by the past influence of humans and the many potential reference conditions encompassed by nonequilibrium dynamics. These complications, however, do not lessen the value of history; rather they underscore the need for multiple, comparative histories from many locations for evaluating both cultural and natural causes of variability, as well as for characterizing the overall dynamical properties of ecosystems. Historical knowledge may not simplify the task of setting management goals and making decisions, but 20th century trends, such as increasingly severe wildfires, suggest that disregarding history can be perilous.
• Westerling, A. L., Hidalgo, H. G., Cayan, D. R., & Swetnam, T. W. (2011). Warming and earlier spring increase western US forest wildfire activity. SCIENCE, 313(5789), 940-943.
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  Western United States forest wildfire activity is widely thought to have increased in recent decades, yet neither the extent of recent changes nor the degree to which climate may be driving regional changes in wildfire has been systematically documented. Much of the public and scientific discussion of changes in western United States wildfire has focused instead on the effects of 19th- and 20th-century land-use history. We compiled a comprehensive database of large wildfires in western United States forests since 1970 and compared it with hydroclimatic and land-surface data. Here, we show that large wildfire activity increased suddenly and markedly in the mid-1980s, with higher large-wildfire frequency, longer wildfire durations, and longer wildfire seasons. The greatest increases occurred in mid-elevation, Northern Rockies forests, where land-use histories have relatively little effect on fire risks and are strongly associated with increased spring and summer temperatures and an earlier spring snowmelt.
• Bigio, E., Swetnam, T. W., & Baisan, C. H. (2010). A comparison and integration of tree-ring and alluvial records of fire history at the Missionary Ridge Fire, Durango, Colorado, USA. Holocene, 20(7), 1047-1061.
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  Abstract: We used tree-ring and alluvial sediment methods to reconstruct past fire regimes for a mixed conifer forest within a 1 km2 drainage basin which was severely burned by a wildfire near Durango, Colorado. Post-fire debris flow events incised the valley-filling alluvial sediments in the lower basin, and created exposures of fire-related of deposits of late-Holocene age. Tree-ring and alluvial sediment fire history records were created separately, and then compared and integrated to create a ~ 3000 year record of past fire activity. The tree-ring record showed that from AD 1679 to 1879, there were frequent surface fires, while patches of high-severity fire occurred during widespread fire years. The alluvial record showed that a low- to moderate-and mixed-severity fire regime has likely been dominant over the past ~ 2600 calibrated calendar years before present, as shown by locally episodic deposition of charcoal-rich, fine-grained sediments. Radiocarbon dating suggested that in two stratigraphic sections, there was rapid deposition of several fine-grained sediment layers. One of these episodes occurred during the Medieval Climatic Anomaly (AD 900-1300). A charcoal-rich debris flow deposit in the oldest exposed part of the stratigraphic record dated to ~ 2600 calibrated calendar years before present. This event was potentially equivalent in magnitude to the debris-flow events following the recent wildfire in the study area, and is evidence of a high-severity fire that burned a large proportion of the study basin. The timing of this event coincides with a period of less frequent, yet more severe wildfires in a nearby lake sediment record, and is associated with the end of a Neoglacial period of cooler and wetter temperatures. © The Author(s) 2010.
• Brown, T. J., Falk, D. A., Falk, D. A., Yocom, L., Heyerdahl, E. K., Heyerdahl, E. K., Gedalof, Z., Brown, P. M., Brown, P. M., Sutherland, E. K., Swetnam, T. W., Swetnam, T. W., Sutherland, E. K., Sutherland, E. K., Swetnam, T. W., Brown, P. M., Gedalof, Z., Gedalof, Z., Heyerdahl, E. K., , Yocom, L., et al. (2010). Fire and climate variation in western North America from fire scar networks.. Past Global Climates, 18(2), 70-72.
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  Falk, D. A., E. K. Heyerdahl, P. M. Brown, T. W. Swetnam, E. K. Sutherland, Z. Gedalof, L. Yocom, and T. J. Brown. 2010. Past Global Climates 18(2): 70-72.
• Diaz, S., Touchan, R., & Swetnam, T. (2010). A tree-ring reconstruction of past precipitation for Baja California Sur, Mexico. INTERNATIONAL JOURNAL OF CLIMATOLOGY, 21(8), 1007-1019.
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  There is great interest in the climatic variability of Baja California and the Sea of Cortes, but long-term information is limited because instrumental climate records begin in the 1940s or 1960s. The first tree-ring chronology of Pinus lagunae was developed from the southern part of the Baja California Peninsula and the chronology is used to reconstruct the history of precipitation variations. A September-July precipitation reconstruction is developed for the period AD 1862-1996 (R = 0.71, p < 0.0001, n = 56, cross-validation = 0.68. This reconstruction is used to assess precipitation variability over the past two centuries, including the relationship with ENSO events. The reconstructed precipitation series indicates a long drought period from 1939 to 1958. It also shows that 1983, one of the strongest El Nino events of the 20th century, is the wettest year. El Nino events during the 20th century are associated with above-normal precipitation, whereas La Nina events are characterized by below-normal precipitation. Four of the most extreme wet years occurred in association with these warm events (1905, 1912, 1919 and 1983). Seventy-one percent of La Nina events are characterized by below-normal precipitation. Sixty-two percent of Fl Nino events are characterized by above-normal precipitation. Tree-ring growth of P. lagunae is most strongly correlated with winter precipitation in Sonora, Sinaloa and southern Baja California Sur. Precipitation data from meteorological stations in northern Baja California do not correlate well with the tree-ring chronology because this zone has a Mediterranean climate, which differs from the rest of northwest Mexico. Copyright (C) 2001 Royal Meteorological Society.
• Farris, C. A., Baisan, C. H., Falk, D. A., Yool, S. R., & Swetnam, T. W. (2010). Spatial and temporal corroboration of a fire-scar-based fire history in a frequently burned ponderosa pine forest. Ecological Applications, 20(6), 1598-1614.
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  PMID: 20945762;Abstract: Fire scars are used widely to reconstruct historical, fire regime parameters in forests around the world. Because fire scars provide incomplete records of past fire occurrence at discrete points in space, inferences must be made to reconstruct fire frequency and extent across landscapes using spatial networks of fire-scar samples. Assessing the relative accuracy of fire-scar fire history reconstructions has been hampered due to a lack of empirical comparisons with independent fire history data sources. We carried out such a comparison in a 2780-ha ponderosa pine forest on Mica Mountain in southern Arizona (USA) for the time period 1937-2000. Using documentary records of fire perimeter maps and ignition locations, we compared reconstructions of key spatial and temporal fire regime parameters developed from documentary fire maps and independently collected fire-scar data (= 60 plots). We found that fire-scar data provided spatially representative and complete inventories of all major fire years (>100 ha) in the study area but failed to detect most small fires. There was a strong linear relationship between the percentage of samples recording fire scars in a given year (i.e., fire-scar synchrony) and total area burned for that year (y = 0.0003x + 0.0087, r2 = 0.96). There was also strong spatial coherence between cumulative fire frequency maps interpolated from fire-scar data and ground-mapped fire perimeters. Widely reported fire frequency summary statistics varied little between fire history data sets: fire-scar natural fire rotations (NFR) differed by 25% of study area burned) were identical between data sets (25.5 yr); fire-scar MFIs for all fire years differed by 1.2 yr from documentary records. The known seasonal timing of past fires based on documentary records was furthermore reconstructed accurately by observing intra-annual ring position of fire scars and using knowledge of treering growth phenology in the Southwest. Our results demonstrate clearly that representative landscape-scale fire histories can be reconstructed accurately from spatially distributed firescar samples. © 2010 by the Ecological Society of America empirical corroboration, fire. © 2010 by the Ecological Society of America.
• Kitzberger, T., Swetnam, T., & Veblen, T. (2010). Inter-hemispheric synchrony of forest fires and the El Nino-Southern Oscillation. GLOBAL ECOLOGY AND BIOGEOGRAPHY, 10(3), 315-326.
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  Fire histories were compared between the southwestern United States and northern Patagonia, Argentina using both documentary records (1914-87 and 1938-96, respectively) and tree-ring reconstructions over the past several centuries. The two regions share similar fire-climate relationships and similar relationships of climatic anomalies to the El Nino-Southern Oscillation (ENSO). In both regions, El Nino events coincide with above-average cool season precipitation and increased moisture availability to plants during the growing season. Conversely, La Nina events correspond with drought conditions. Monthly patterns of ENSO indicators (southern oscillation indices and tropical Pacific sea surface temperatures) preceding years of exceptionally widespread fires are highly similar in both regions during the 20th century. Major fire years tend to follow the switching from El Nino to La Nina conditions. El Nino conditions enhance the production of fine fuels, which when desiccated by La Nina conditions create conditions for widespread wildfires. Decadal-scale patterns of fire occurrence since the mid-17th century are highly similar in both regions. A period of decreased fire occurrence in both regions from c. 1780-1830 coincides with decreased amplitude and/or frequency of ENSO events. The interhemispheric synchrony of lire regimes in these two distant regions is tentatively interpreted to be a response to decadal-scale changes in ENSO activity. The ENSO-fire relationships of the south-western USA and northern Patagonia document the importance of high-frequency climatic variation to fire hazard. Thus, in addition to long-term trends in mean climatic conditions, multi-decadal scale changes in year-to-year variability need to be considered in assessments of the potential influence of climatic change on fire regimes.
• Speer, J., Swetnam, T., Wickman, B., & Youngblood, A. (2010). Changes in pandora moth outbreak dynamics during the past 622 years. ECOLOGY, 82(3), 679-697.
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  Episodic outbreaks of pandora moth (Coloradia pandora Blake), a forest insect that defoliates ponderosa pine (Pinus panderosa Dougl. ex Laws.) and other pine species in the western United States, have recurred several times during the 20th century in forests of south-central Oregon. We collected and analyzed tree-ring samples from stands affected by recent outbreaks of pandora moth to develop a long-term record of outbreaks. Outbreaks were evident in tree-ring series as a characteristic "signature" of sharply reduced latewood width within a ring, followed by reduced ring widths lasting 4-20 yr. We verified that this tree-ring signature was unrelated to drought or other climatic fluctuations by comparing the timing of known and inferred outbreaks with independent climatic data. Using the pandora moth tree-ring signature, we reconstructed a 622-year record of 22 individual outbreaks in 14 old-growth ponderosa pine stands. This is currently the longest regional reconstruction of forest insect outbreak history in North America. Intervals between pandora moth outbreaks were highly variable within individual forest stands, ranging from 9 yr to 156 yr. Spectral analyses of a composite time series from all stands, however, showed more consistent intervals between outbreaks, suggesting quasicyclical population dynamics at regional and decadal scales. Waveforms extracted from the regional outbreak time series had periods ranging over similar to 18-24 yr (39.7% variance explained) and similar to 37-41 yr (37.3% variance explained). The periods and strengths of these cycles varied across the centuries, with the largest outbreaks occurring when relatively high-amplitude periods of the dominant cycles were in phase. Twentieth-century outbreaks were not more synchronous (extensive), severe, or longer in duration than outbreaks in previous centuries, but there was an unusual 60-yr reduction in regional activity during similar to 1920-1980. The changing dynamical behavior of pandora moth populations highlights the need to evaluate historical factors that may have influenced this system, such as climatic variations, forest fires, and human land uses. Although cyclical dynamics in animal populations have most commonly been attributed to endogenous, ecological processes (e.g., "delayed density dependence," predators, pathogens, and parasites) our findings suggest that exogenous processes (e.g., climatic oscillations) may also be involved.
• Swetnam, T. (2010). Fire and the Environment.
• Swetnam, T. (2010). Spatial and temporal corroboration of a fire-scar-based fire history in a frequently burned ponderosa pine forest.
• Swetnam, T., Farris, C. A., Baisan, C. H., Falk, D. A., Yool, S. R., & Swetnam, T. W. (2010). Spatial and temporal corroboration of a fire-scar-based fire history in a frequently burned ponderosa pine forest. Ecological applications : a publication of the Ecological Society of America, 20(6).
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  Fire scars are used widely to reconstruct historical fire regime parameters in forests around the world. Because fire scars provide incomplete records of past fire occurrence at discrete points in space, inferences must be made to reconstruct fire frequency and extent across landscapes using spatial networks of fire-scar samples. Assessing the relative accuracy of fire-scar fire history reconstructions has been hampered due to a lack of empirical comparisons with independent fire history data sources. We carried out such a comparison in a 2780-ha ponderosa pine forest on Mica Mountain in southern Arizona (USA) for the time period 1937-2000. Using documentary records of fire perimeter maps and ignition locations, we compared reconstructions of key spatial and temporal fire regime parameters developed from documentary fire maps and independently collected fire-scar data (n = 60 plots). We found that fire-scar data provided spatially representative and complete inventories of all major fire years (> 100 ha) in the study area but failed to detect most small fires. There was a strong linear relationship between the percentage of samples recording fire scars in a given year (i.e., fire-scar synchrony) and total area burned for that year (y = 0.0003x + 0.0087, r2 = 0.96). There was also strong spatial coherence between cumulative fire frequency maps interpolated from fire-scar data and ground-mapped fire perimeters. Widely reported fire frequency summary statistics varied little between fire history data sets: fire-scar natural fire rotations (NFR) differed by < 3 yr from documentary records (29.6 yr); mean fire return intervals (MFI) for large-fire years (i.e., > or = 25% of study area burned) were identical between data sets (25.5 yr); fire-scar MFIs for all fire years differed by 1.2 yr from documentary records. The known seasonal timing of past fires based on documentary records was furthermore reconstructed accurately by observing intra-annual ring position of fire scars and using knowledge of tree-ring growth phenology in the Southwest. Our results demonstrate clearly that representative landscape-scale fire histories can be reconstructed accurately from spatially distributed fire-scar samples.
• Williams, A. P., Allen, C. D., Millar, C. I., Swetnam, T. W., Michaelsen, J., Still, C. J., Leavitt, S. W., 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.
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  PMID: 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.
• Iniguez, J. M., Swetnam, T. W., & Baisan, C. H. (2009). Spatially and temporally variable fire regime on Rincon Peak, Arizona, USA. Fire Ecology, 5(1), 3-21.
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  Abstract: Spatial and temporal patterns of fire history are affected by factors such as topography, vegetation, and climate. It is unclear, however, how these factors influenced fire history patterns in small isolated forests, such as that found on Rincon Peak, a "sky island" mountain range in southern Arizona, USA. We reconstructed the fire history of Rincon Peak to evaluate the influences of broad-scale (i.e., climate) versus local-scale (i.e., topographic) factors on fire occurrence and extent. We evaluated both fire scars and tree demography (natality and mortality) to investigate surface fire and crown fire events. The fire history of a 310 ha study area surrounding the top of Rincon Peak was reconstructed by tree-ring sampling in 21 plots. Between 1648 and 1763, spreading fires on Rincon Peak were controlled primarily by regional climate. Widespread surface fires occurred during drought years, and were generally synchronized with regional fire events known from an extensive network of other fire history studies. After 1763, fire extent was apparently limited by local factors (i.e., fuels) as frequent fires continued to burn, but were limited to the southern part of the study area until a widespread fire occurred in 1819. Landscape fires (i.e., fires that scarred ≥2 plots) were absent from the entire study area between 1819 and 1867 despite continued burning in adjacent mountain ranges. Multiple lines of evidence indicate that the 1867 fire was both a surface and a stand-replacing event that killed most trees within a 60 ha patch. Our findings suggest that past climatic variations had important effects on fire regimes and age structures of small, fragmented ponderosa pine (Pinus ponderosa) landscapes like Rincon Peak. Given anticipated climate changes, the rich biodiversity harbored in these steep, isolated landscapes will be critical habitat in the migration of species and should therefore be considered high conservation priority.
• Iniguez, J. M., Swetnam, T. W., & Yool, S. R. (2009). Topography affected landscape fire history patterns in southern Arizona, USA. FOREST ECOLOGY AND MANAGEMENT, 256(3), 295-303.
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  Fire histories contribute important information to contemporary fire planning, however, our knowledge is not comprehensive geographically. We evaluated the influence of topography on fire history patterns in two contrasting landscapes within the Santa Catalina Mountains of southeastern Arizona. Multiple fire-scarred trees from randomly selected 2-ha plots were used to develop plot composite mean fire intervals (PCMFIs) within the Butterfly Peak (BP) and Rose Canyon (RC) landscapes. BP is dominated by steep, northerly aspects and presence of potential fire spread barriers (exposed rock bluffs and scree slopes). RC is dominated by more gentle and southerly aspects with relatively few fire barriers. Within each landscape, PCMFIs did not differ significantly between aspect classes from A.D. 1748 to 1910 (BP: p = 0.73 and RC: p = 0.57). Pooled PCMFIs in the gentler RC landscape were, however, significantly shorter (p < 0.001) than in the steeper BP landscape. The frequency of relatively widespread fires (i.e., number of fire years when >= 2 plots scarred) was similar between landscapes, but fires in the gentler RC landscape were significantly larger (p = 0.033). The higher frequency of large fires (i.e., fires that burned >75% of the landscape) in RC resulted in more area burned over time and shorter fire intervals at individual plots. Conversely, smaller fires in the dissected BP landscape resulted in less area burned and longer periods between fires at individual plots. The different topographies in the two landscapes likely result in different wind intensities, fuel moistures, and fuel/vegetation types-and consequently, different historical fire spread patterns. Our conclusion is that fire history patterns are not influenced primarily by stand-scale topography, but rather by the topographic characteristics of the broader, surrounding landscape. Published by Elsevier B.V.
• Lombardo, K. J., Swetnam, T. W., Baisan, C. H., & Borchert, M. I. (2009). Using bigcone douglas-fir fire scars and tree rings to reconstruct interior chaparral fire history. Fire Ecology, 5(3), 35-56.
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  Abstract: Bigcone Douglas-fir (Pseudotsuga macrocarpa [Vasey] Mayr) is a long-lived, fire-adapted conifer that is endemic to the Transverse Ranges of southern California. At the lower and middle reaches of its elevational distribution, isolated stands of bigcone Douglas-fir are surrounded by extensive stands of chaparral. Our dendrochronology investigations have revealed that these ancient trees commonly record multiple past fires as fire scars in their lower boles. We hypothesized that the fire-scar record found within and among bigcone Douglas-fir stands reflects the temporal and spatial patterns of fire in the surrounding chaparral. We compared the fire scar results with independent, twentieth century fire atlas data to assess our interpretations. Using fire scars and ring-growth changes, we reconstructed fire history in Los Padres National Forest and investigated changes in fire regime characteristics over the past several centuries. Our analyses confirm that the tree-ring record can be used to accurately reconstruct past fire occurrence and extent patterns both within bigcone Douglas-fir stands and surrounding chaparral stands. Many extensive fires were apparent in both the preand post-twentieth century period indicating that such events were a natural component of the system. However, many smaller fires were also evident in the tree-ring record, and more of these types of events occurred during the nineteenth century (and earlier) than during the twentieth century. We also identified a shift after the late nineteenth century to potentially more severe fires within and among stands, and by inference the surrounding chaparral. These findings suggest that land management policies, rates of human-set fires, or climatic variations may have played a role in shaping the contemporary fire regime, and that this recent period is different in some respects from the pre-twentieth century regime. Replication of this work in other mountain ranges, in addition to comparisons with climate and human histories, will provide valuable insights into our understanding of the relative roles of humans versus climate in changing bigcone Douglas-fir and chaparral fire regimes.
• M., D., Balch, J. K., Artaxo, P., Bond, W. J., Carlson, J. M., Cochrane, M. A., D'Antonio, C. M., DeFries, R. S., Doyle, J. C., Harrison, S. P., Johnston, F. H., Keeley, J. E., Krawchuk, M. A., Kull, C. A., Marston, J. B., Moritz, M. A., Prentice, I. C., Roos, C. I., Scott, A. C., , Swetnam, T. W., et al. (2009). Fire in the earth system. Science, 324(5926), 481-484.
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  PMID: 19390038;Abstract: Fire is a worldwide phenomenon that appears in the geological record soon after the appearance of terrestrial plants. Fire influences global ecosystem patterns and processes, including vegetation distribution and structure, the carbon cycle, and climate. Although humans and fire have always coexisted, our capacity to manage fire remains imperfect and may become more difficult in the future as climate change alters fire regimes. This risk is difficult to assess, however, because fires are still poorly represented in global models. Here, we discuss some of the most important issues involved in developing a better understanding of the role of fire in the Earth system.
• Romme, W. H., Allen, C. D., Bailey, J. D., Baker, W. L., Bestelmeyer, B. T., Brown, P. M., Eisenhart, K. S., Floyd, M. L., Huffman, D. W., Jacobs, B. F., Miller, R. F., Muldavin, E. H., Swetnam, T. W., Tausch, R. J., & Weisberg, P. J. (2009). Historical and modern disturbance regimes, stand structures, and landscape dynamics in piñon-juniper vegetation of the western united states. Rangeland Ecology and Management, 62(3), 203-222.
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  Abstract: Piñon-juniper is a major vegetation type in western North America. Effective management of these ecosystems has been hindered by inadequate understanding of 1) the variability in ecosystem structure and ecological processes that exists among the diverse combinations of piñons, junipers, and associated shrubs, herbs, and soil organisms; 2) the prehistoric and historic disturbance regimes; and 3) the mechanisms driving changes in vegetation structure and composition during the past 150 yr. This article summarizes what we know (and don't know) about three fundamentally different kinds of piñon-juniper vegetation. Persistent woodlands are found where local soils, climate, and disturbance regimes are favorable for piñon, juniper, or a mix of both; fires have always been infrequent in these woodlands. Piñon-juniper savannas are found where local soils and climate are suitable for both trees and grasses; it is logical that low-severity fires may have maintained low tree densities before disruption of fire regimes following Euro-American settlement, but information is insufficient to support any confident statements about historical disturbance regimes in these savannas. Wooded shrublands are found where local soils and climate support a shrub community, but trees can increase during moist climatic conditions and periods without disturbance and decrease during droughts and following disturbance. Dramatic increases in tree density have occurred in portions of all three types of piñon-juniper vegetation, although equally dramatic mortality events have also occurred in some areas. The potential mechanisms driving increases in tree density-such as recovery from past disturbance, natural range expansion, livestock grazing, fire exclusion, climatic variability, and CO2 fertilization-generally have not received enough empirical or experimental investigation to predict which is most important in any given location. The intent of this synthesis is 1) to provide a source of information for managers and policy makers; and 2) to stimulate researchers to address the most important unanswered questions.
• Swetnam, T. (2009). Historical and Modern Disturbance Regimes, Stand Structures, and Landscape Dynamics in Piñon–Juniper Vegetation of the Western United States.
• Swetnam, T. W. (2009). Special issue: Fire history in California. Fire Ecology, 5(3), 1-3.
• Swetnam, T. W., Baisan, C. H., Caprio, A. C., Brown, P. M., Touchan, R., Anderson, R. S., & Hallett, D. J. (2009). Multi-millennial fire history of the giant forest, Sequoia National Park, California, USA. Fire Ecology, 5(3), 120-150.
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  Abstract: Giant sequoias (Sequoiadendron giganteum [Lindl.] J. Buchholz) preserve a detailed history of fire within their annual rings. We developed a 3000 year chronology of fire events in one of the largest extant groves of ancient giant sequoias, the Giant Forest, by sampling and tree-ring dating fire scars and other fire-related indicators from 52 trees distributed over an area of about 350 ha. When all fire events were included in composite chronologies, the mean fire intervals (years between fires of any size) declined as a function of in-creasing spatial extent from tree, to group, to multiple groups, to grove scales: 15.5 yr (0.1 ha), 7.4 yr (1 ha.), 3.0 yr (70 ha), and 2.2 yr (350 ha), respectively. We interpreted wide-spread fires (i.e., fire events recorded on ≥2 trees, or ≥25% of all trees recording fires within composites) to have occurred in areas of 70 ha to 350 ha at mean intervals ranging from about 6 yr to 35 yr. We compared the annual, multi-decadal and centennial variations in Giant Forest fire frequency with those documented in tree-ring and charcoal-based fire chronologies from four other giant sequoia groves in the Sierra Nevada, and with independent tree-ring-based reconstructions of summer drought and temperatures. The other giant sequoia fire histories (tree rings and charcoal-based) were significantly (P < 0.001) correlated with the Giant Forest fire frequency record and independent climate reconstructions, and confirm a maximum fire frequency during the warm and drought-prone period from 800 C.E. to 1300 C.E. (Common Era). This was the driest period of the past two millennia, and it may serve as an analog for warming and drying effects of anthropogenic greenhouse gases in the next few decades. Sequoias can sustain very high fire frequencies, and historically they have done so during warm, dry times. We suggest that preparation of sequoia groves for anticipated warming may call for increasing the rate of prescribed burning in most parts of the Giant Forest.
• BAISAN, C., & SWETNAM, T. (2008). FIRE HISTORY ON A DESERT MOUNTAIN-RANGE - RINCON MOUNTAIN WILDERNESS, ARIZONA, USA. CANADIAN JOURNAL OF FOREST RESEARCH-REVUE CANADIENNE DE RECHERCHE FORESTIERE, 20(10), 1559-1569.
• Iniguez, J. M., Swetnam, T. W., & Yool, S. R. (2008). Topography affected landscape fire history patterns in southern Arizona, USA. Forest Ecology and Management, 256(3), 295-303.
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  Abstract: Fire histories contribute important information to contemporary fire planning, however, our knowledge is not comprehensive geographically. We evaluated the influence of topography on fire history patterns in two contrasting landscapes within the Santa Catalina Mountains of southeastern Arizona. Multiple fire-scarred trees from randomly selected 2-ha plots were used to develop plot composite mean fire intervals (PCMFIs) within the Butterfly Peak (BP) and Rose Canyon (RC) landscapes. BP is dominated by steep, northerly aspects and presence of potential fire spread barriers (exposed rock bluffs and scree slopes). RC is dominated by more gentle and southerly aspects with relatively few fire barriers. Within each landscape, PCMFIs did not differ significantly between aspect classes from A.D. 1748 to 1910 (BP: p = 0.73 and RC: p = 0.57). Pooled PCMFIs in the gentler RC landscape were, however, significantly shorter (p < 0.001) than in the steeper BP landscape. The frequency of relatively widespread fires (i.e., number of fire years when ≥2 plots scarred) was similar between landscapes, but fires in the gentler RC landscape were significantly larger (p = 0.033). The higher frequency of large fires (i.e., fires that burned >75% of the landscape) in RC resulted in more area burned over time and shorter fire intervals at individual plots. Conversely, smaller fires in the dissected BP landscape resulted in less area burned and longer periods between fires at individual plots. The different topographies in the two landscapes likely result in different wind intensities, fuel moistures, and fuel/vegetation types-and consequently, different historical fire spread patterns. Our conclusion is that fire history patterns are not influenced primarily by stand-scale topography, but rather by the topographic characteristics of the broader, surrounding landscape.
• Swetnam, T. (2008). Fire Climatology in the western United States: introduction to special issue.
• Swetnam, T. (2008). The perfect firestorm: Climate change, fuels build-up & land use among causes of rising megafire trend. Wildland Firefighter, 12(4), 18-.
• Swetnam, T. (2008). Topography affected landscape fire history patterns in southern Arizona, USA.
• Swetnam, T. W., & Anderson, R. S. (2008). Fire Climatology in the western United States: Introduction to special issue. International Journal of Wildland Fire, 17(1), 1-7.
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  Abstract: Advances in fire climatology have derived from recent studies of modern and paleoecological records. We convened a series of workshops and a conference session to report and review regional-scale findings, and these meetings led to the 10 papers in this special issue. Two papers focus on fire and climate patterns in the modern era using documentary records, four papers utilise tree rings to evaluate recent centuries of change, and four papers evaluate charcoal and pollen in lake, bog, and alluvial sediments over the Holocene. Here we summarise some of the key findings from these papers in the context of other recent fire climatology literature. These studies illustrate the value of long-term perspectives and spatial networks of fire and climate data in discovering the patterns and modes of past fire regime and climate variations. © IAWF 2008.
• Kaufmann, M. R., Binkley, D., Fulé, P. Z., Johnson, M., Stephens, S. L., & Swetnam, T. W. (2007). Defining old growth for fire-adapted forests of the western United States. Ecology and Society, 12(2).
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  Abstract: There are varying definitions of old-growth forests because of differences in environment and differing fire influence across the Intermountain West. Two general types of forests reflect the role of fire: 4) forests shaped by natural changes in structure and species, makeup - plant succession - that are driven by competitive differences among species and individual trees and by small-scale disturbances, and 2) forests where plant succession processes are disrupted by major biological disturbances (fire, insects, wind, or drought) extending across larger areas. Some case examples of old-growth forests where fire was historically frequent are used. The examples sketch out the typical biophysical settings, fire regime, natural disturbance factors, spatial features of patches, and the processes and conditions that produce spatial changes of the landscape over time. These examples confirm the complexity of describing or defining old growth in frequent-fire forests. We define fire-adapted forests at three spatial scales, whereas the standard definition of old growth refers to a patch or stand condition. Our definition is based on ecological principles rather than on the cultural aspects of old growth. It focuses on central tendencies, given all the possible combinations of conditions and processes, that move forests toward old growth in the fire-adapted forests of the Intermountain West. Copyright © 2007 by the author(s).
• Kitzberger, T., Brown, P. M., Heyerdahl, E. K., Swetnam, T. W., & Veblen, T. T. (2007). Contingent Pacific-Atlantic Ocean influence on multicentury wildfire synchrony over western North America. Proceedings of the National Academy of Sciences of the United States of America, 104(2), 543-548.
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  PMID: 17197425;PMCID: PMC1766421;Abstract: Widespread synchronous wildfires driven by climatic variation, such as those that swept western North America during 1996, 2000, and 2002, can result in major environmental and societal impacts. Understanding relationships between continental-scale patterns of drought and modes of sea surface temperatures (SSTs) such as El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) may explain how interannual to multidecadal variability in SSTs drives fire at continental scales. We used local wildfire chronologies reconstructed from fire scars on tree rings across western North America and independent reconstructions of SST developed from tree-ring widths at other sites to examine the relationships of multicentury patterns of climate and fire synchrony. From 33,039 annually resolved fire-scar dates at 238 sites (the largest paleofire record yet assembled), we examined forest fires at regional and subcontinental scales. Since 1550 CE, drought and forest fires covaried across the West, but in a manner contingent on SST modes. During certain phases of ENSO and PDO, fire was synchronous within broad subregions and sometimes asynchronous among those regions. In contrast, fires were most commonly synchronous across the West during warm phases of the AMO. ENSO and PDO were the main drivers of high-frequency variation in fire (interannual to decadal), whereas the AMO conditionally changed the strength and spatial influence of ENSO and PDO on wildfire occurrence at multidecadal scales. A current warming trend in AMO suggests that we may expect an increase in widespread, synchronous fires across the western U.S. in coming decades. © 2006 by The National Academy of Sciences of the USA.
• Margolis, E. Q., Swetnam, T. W., & Allen, C. D. (2007). A stand-replacing fire history in upper montane forests of the southern Rocky Mountains. Canadian Journal of Forest Research, 37(11), 2227-2241.
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  Abstract: Dendroecological techniques were applied to reconstruct stand-replacing fire history in upper montane forests in northern New Mexico and southern Colorado. Fourteen stand-replacing fires were dated to 8 unique fire years (1842-1901) using four lines of evidence at each of 12 sites within the upper Rio Grande Basin. The four lines of evidence were (i) quaking aspen (Populus tremuloides Michx.) inner-ring dates, (ii) fire-killed conifer bark-ring dates, (iii) tree-ring width changes or other morphological indicators of injury, and (iv) fire scars. The annual precision of dating allowed the identification of synchronous stand-replacing fire years among the sites, and co-occurrence with regional surface fire events previously reconstructed from a network of fire scar collections in lower elevation pine forests across the southwestern United States. Nearly all of the synchronous stand-replacing and surface fire years coincided with severe droughts, because climate variability created regional conditions where stand-replacing fires and surface fires burned across ecosystems. Reconstructed stand-replacing fires that predate substantial Anglo-American settlement in this region provide direct evidence that stand-replacing fires were a feature of high-elevation forests before extensive and intensive land-use practices (e.g., logging, railroad, and mining) began in the late 19th century. © 2007 NRC.
• Swetnam, T. (2007). A stand-replacing fire history in upper montane forests of the southern Rocky Mountains.
• Morehouse, B., Christopherson, G., Crimmins, M., Orr, B., Overpeck, J., Swetnam, T., & Yool, S. (2006). Modeling Interactions Among Wildland Fire, Climate and Society in the Context of Climatic Variability and Change in the Southwest US. Regional Climate Change and Variability: Impacts and Responses, 58-78.
• Westerling, A. L., Hidalgo, H. G., Cayan, D. R., & Swetnam, T. W. (2006). Warming and earlier spring increase Western U.S. forest wildfire activity. Science, 313(5789), 940-943.
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  PMID: 16825536;Abstract: Western United States forest wildfire activity is widely thought to have increased in recent decades, yet neither the extent of recent changes nor the degree to which climate may be driving regional changes in wildfire has been systematically documented. Much of the public and scientific discussion of changes in western United States wildfire has focused instead on the effects of 19th- and 20th-century land-use history. We compiled a comprehensive database of large wildfires in western United States forests since 1970 and compared it with hydroclimatic and land-surface data. Here, we show that large wildfire activity increased suddenly and markedly in the mid-1980s, with higher large-wildfire frequency, longer wildfire durations, and longer wildfire seasons. The greatest increases occurred in mid-elevation, Northern Rockies forests, where land-use histories have relatively little effect on fire risks and are strongly associated with increased spring and summer temperatures and an earlier spring snowmelt.
• Hessburg, P. F., Kuhlmann, E. E., & Swetnam, T. W. (2005). Examining the recent climate through the lens of ecology: Inferences from temporal pattern analysis. Ecological Applications, 15(2), 440-457.
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  Abstract: Ecological theory asserts that the climate of a region exerts top-down controls on regional ecosystem patterns and processes, across space and time. To provide empirical evidence of climatic controls, it would be helpful to define climatic regions that minimized variance in key climate attributes, within climatic regions - define the periods and features of climatic regimes, and then look for concordance between regional climate and ecosystem patterns or processes. In the past, these steps have not been emphasized. Before we evaluated the recent climate of the northwestern United States, we established a Northwest climatic region by clustering time series of the Palmer Drought Severity Index (PDSI) for the period of 1675-1978, for the western United States. The background climatic regime and anomalies of the recent northwestern U.S. climate were then identified through temporal pattern analysis involving application of correspondence analysis to the same PDSI time series. Our analysis distinguished 10 distinct periods and four unique types of regimes (climatic signals). Five of the 10 periods (79% of the ∼300-year record) were marked by mild and equitable moisture conditions (Pacific regime), the "background" climate of the Northwest. The remaining periods were anomalies. Two periods displayed a high-variance, mixed signal marked by switching between severe to extreme annual to interannual dry and wet episodes (High/Mixed regime; 9% of the record). Two more periods displayed a moderate-variance, mixed signal marked by switching between moderate to severe annual to interannual dry and wet episodes (Moderate/Mixed regime; 5%). Only one period was unidirectional and relatively low variance, marked by persistent yet mild to moderate drought (Low/Dry regime, 7%). Our method distinguished decadal- to interdecadal-scale regimes, defined regime periods, and detected both mixed and unidirectional anomalies from the background climate. The ability to distinguish the variance, direction, and period of sequential climatic regimes provides a plausible basis for examining the role of past climate within terrestrial ecosystems of the Northwest. For example, we found concordance between the period of the Low/Dry anomaly and a period of tree establishment in the Olympic Mountains of Washington, close alignment between tree growth with the Moderate/Mixed and High/Mixed signals in Oregon, and a mixed fire response to mixed climatic signals in northeastern Oregon. Linking historical climatic regimes to particular ecosystem patterns and processes also aids in the prediction of future ecosystem changes by providing evidence of the kinds of interactions that may be anticipated. © 2005 by the Ecological Society of America.
• Swetnam, T. (2005). EXAMINING THE RECENT CLIMATE THROUGH THE LENS OF ECOLOGY: INFERENCES FROM TEMPORAL PATTERN ANALYSIS.
• Swetnam, T. (2004). Tree rings and climate: Sharpening the focus.
• Kaye, M., & Swetnam, T. (2003). An assessment of fire, climate, and Apache history in the Sacramento Mountains, New Mexico. PHYSICAL GEOGRAPHY, 20(4), 305-330.
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  Fire historians typically attribute the causes of temporal change in past fire regimes to climatic variation, human land use, or some combination of the two. Most long-term historical reconstructions, however, lack time and place-specific chronologies for all three variables of fire, climate, and people. To test the hypothesis that Mescalero Apache of southeastern New Mexico influenced fire regimes of the Sacramento Mountains, we reconstructed and compared chronologies of key variables for the period A.D. 1700 to the present. Fire-scarred trees were used to reconstruct fire frequencies and culturally modified (peeled) trees, and written histories were used to identify places and times of Mescalero presence. Independent precipitation reconstructions from tree rings were compared with fire and human histories. We found that Mescalero frequently visited the western escarpment of the Sacramento Mountains during the late 1700s through the late 1800s, especially the Dog Canyon area. Fire frequency was higher and seasonal timing of fires was different in sites near Dog Canyon compared to relatively distant sites. Interannual to decadal-scale drought might explain some temporal variability in fire and peeling activity, but these relationships were not consistent. We conclude that people,increased fire occurrence during certain time periods in localized areas, but broad-scale and persistent human impacts did not occur until the end of the 19th century with the rise of livestock grazing by European settlers and fire suppression by government agencies.
• Ryerson, D. E., Swetnam, T. W., & Lynch, A. M. (2003). A tree-ring reconstruction of western spruce budworm outbreaks in the San Juan Mountains, Colorado, U.S.A.. Canadian Journal of Forest Research, 33(6), 1010-1028.
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  Abstract: Tree-ring records were used to reconstruct spatial and temporal patterns of western spruce budworm (Choristoneura occidentalis Freeman) outbreaks in mixed conifer forests of southern Colorado. Reconstructions in 11 host stands showed a regionally synchronous pattern of at least 14 outbreaks during the past 350 years. Intervals between outbreaks were highly variable within stands, but at the regional scale outbreak intervals were more consistent. Spectral analyses of regional outbreak time series confirmed periodicities at about 25, 37, and 83 years. Comparison with an independent drought reconstruction indicated that outbreaks typically corresponded to increased moisture, while relatively little budworm activity occurred during dry periods. In contrast to other published reconstructions in Colorado and New Mexico, reconstructions from this study area did not exhibit significant 20th-century changes in the frequency of outbreak occurrence or magnitude of growth reduction. Sharply reduced growth during outbreaks was not clearly visible on the increment core samples, and budworm-induced reductions in tree-ring growth were usually detectable only after comparison with nonhost tree-ring series. This finding emphasizes that defoliation effects on ring growth can be highly relativistic. Hence, caution should be exercised in reconstructing insect outbreak histories based only on visual detection approaches, or without comparison with nonhost or nondefoliated tree-ring control series.
• Swetnam, T. (2003). A tree-ring reconstruction of western spruce budworm outbreaks in the San Juan Mountains, Colorado, U.S.A..
• Swetnam, T. (2003). Interannual to decadal drought and wildfire in the western United States.
• Swetnam, T. (2003). Reply to Baker and Genty’s comments on “A test of annual resolution in stalagmite using tree rings”.
• Allen, C. D., Savage, M., Falk, D. A., Suckling, K. F., Swetnam, T. W., Schulke, T., Stacey, P. B., Morgan, P., Hoffman, M., & Klingel, J. T. (2002). Ecological restoration of Southwestern ponderosa pine ecosystems: A broad perspective. Ecological Applications, 12(5), 1418-1433.
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  Abstract: The purpose of this paper is to promote a broad and flexible perspective on ecological restoration of Southwestern (U.S.) ponderosa pine forests. Ponderosa pine forests in the region have been radically altered by Euro-American land uses, including livestock grazing, fire suppression, and logging. Dense thickets of young trees now abound, old-growth and biodiversity have declined, and human and ecological communities are increasingly vulnerable to destructive crown fires. A consensus has emerged that it is urgent to restore more natural conditions to these forests. Efforts to restore Southwestern forests will require extensive projects employing varying combinations of young-tree thinning and reintroduction of low-intensity fires. Treatments must be flexible enough to recognize and accommodate: high levels of natural heterogeneity; dynamic ecosystems; wildlife and other biodiversity considerations; scientific uncertainty; and the challenges of on-the-ground implementation. Ecological restoration should reset ecosystem trends toward an envelope of "natural variability," including the reestablishment of natural processes. Reconstructed historic reference conditions are best used as general guides rather than rigid restoration prescriptions. In the long term, the best way to align forest conditions to track ongoing climate changes is to restore fire, which naturally correlates with current climate. Some stands need substantial structural manipulation (thinning) before fire can safely be reintroduced. In other areas, such as large wilderness and roadless areas, fire alone may suffice as the main tool of ecological restoration, recreating the natural interaction of structure and process. Impatience, overreaction to crown fire risks, extractive economics, or hubris could lead to widespread application of highly intrusive treatments that may further damage forest ecosystems. Investments in research and monitoring of restoration treatments are essential to refine restoration methods. We support the development and implementation of a diverse range of scientifically viable restoration approaches in these forests, suggest principles for ecologically sound restoration that immediately reduce crown fire risk and incrementally return natural variability and resilience to Southwestern forests, and present ecological perspectives on several forest restoration approaches.
• Betancourt, J. L., Grissino-Mayer, H. D., Salzer, M. W., & Swetnam, T. W. (2002). A test of "Annual resolution" in stalagmites using tree rings. Quaternary Research, 58(2), 197-199.
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  Abstract: So-called annual banding has been identified in a number of speleothems in which the number of bands approximates the time interval between successive U-series dates. The apparent annual resolution of speleothem records, however, remains largely untested. Here we statistically compare variations in band thickness from a late Holocene stalagmite in Carlsbad Cavern, Southern New Mexico, USA, with three independent tree-ring chronologies form the same region. We found no correspondence. Although there may be various explanations for the discordance, this limited exercise suggests that banded stalagmites should be held to the same rigorous standards in chronology building and climatic inference as annually resolved tree rings, corals, and ice cores. © 2002 University of Washington.
• Rollins, M. G., Morgan, P., & Swetnam, T. (2002). Landscape-scale controls over 20^th century fire occurrence in two large Rocky Mountain (USA) wilderness areas. Landscape Ecology, 17(6), 539-557.
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  Abstract: Topography, vegetation, and climate act together to determine the spatial patterns of fires at landscape scales. Knowledge of landscape-fire-climate relations at these broad scales (1,000s ha to 100,000s ha) is limited and is largely based on inferences and extrapolations from fire histories reconstructed from finer scales. In this study, we used long time series of fire perimeter data (fire atlases) and data for topography, vegetation, and climate to evaluate relationships between large 20thcentury fires and landscape characteristics in two contrasting areas: the 486,673-ha Gila/Aldo Leopold Wilderness Complex (GALWC) in New Mexico, USA, and the 785,090-ha Selway-Bitterroot Wilderness Complex (SBWC) in Idaho and Montana, USA. There were important similarities and differences in gradients of topography, vegetation, and climate for areas with different fire frequencies, both within and between study areas. These unique and general relationships, when compared between study areas, highlight important characteristics of fire regimes in the Northern and Southern Rocky Mountains of the Western United States. Results suggest that amount and horizontal continuity of herbaceous fuels limit the frequency and spread of surface fires in the GALWC, while the moisture status of large fuels and crown fuels limits the frequency of moderate-to-high severity fires in the SBWC. These empirically described spatial and temporal relationships between fire, landscape attributes, and climate increase understanding of interactions among broad-scale ecosystem processes. Results also provide a historical baseline for fire management planning over broad spatial and temporal scales in each wilderness complex.
• Rollins, M., Swetnam, T., & Morgan, P. (2002). Evaluating a century of fire patterns in two Rocky Mountain wilderness areas using digital fire atlases. CANADIAN JOURNAL OF FOREST RESEARCH-REVUE CANADIENNE DE RECHERCHE FORESTIERE, 31(12), 2107-2123.
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  Changes in fire size, shape, and frequency under different fire-management strategies were evaluated using time series of fire perimeter data (fire atlases) and mapped potential vegetation types (PVTs) in the Gila - Aldo Leopold Wilderness Complex (GALWC) in New Mexico and the Selway-Bitterroot Wilderness Complex (SBWC) in Idaho and Montana. Relative to pre-Euro-American estimates, fire rotations in the GALWC were short during the recent wildfire-use period (1975-1993) and long during the pre-modern suppression period (1909-1946). In contrast, fire rotations in the SBWC were short during the pre-modern suppression period (1880-1934) and long during the modern suppression period (1935-1975). In general, fire-rotation periods were shorter in mid-elevation, shade-intolerant PVTs. Fire intervals in the GALWC and SBWC are currently longer than fire intervals prior to Euro-American settlement. Proactive fire and fuels management are needed to restore fire regimes in each wilderness complex to within natural ranges of variability and to reduce the risk of catastrophic wildfire in upper elevations of the GALWC and nearly the entire SBWC. Analyses of fire atlases provide baseline information for evaluating landscape patterns across broad landscapes.
• Swetnam, T. (2002). A Test of “Annual Resolution” in Stalagmites Using Tree Rings.
• Barton, A. M., Swetnam, T. W., & Baisan, C. H. (2001). Arizona pine (Pinus arizonica) stand dynamics: Local and regional factors in a fire-prone madrean gallery forest of Southeast Arizona, USA. Landscape Ecology, 16(4), 351-369.
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  Abstract: In southwestern North America, large-scale climate patterns appear to exert control on moisture availability, fire occurrence, and tree demography, raising the compelling possibility of regional synchronization of forest dynamics. Such regional signals may be obscured, however, by local, site-specific factors, such as disturbance history and land use. Contiguous sites with similar physical environments, lower and middle Rhyolite Canyon, Arizona, USA, shared nearly the same fire history from 1660-1801, but then diverged. For the next 50 years, fires continued to occur frequently in lower Rhyolite, but, probably as result of flood-induced debris deposition, largely ceased in middle Rhyolite. We related stand dynamics of Arizona pine (Pinus arizonica) to fire history and drought severity and compared the dynamics in the two sites before and after the divergence in fire frequency. Fires occurred during unusually dry years, and possibly following unusually moist years. Arizona pine exhibited three age structure peaks: two (1810-1830 and 1870-1900) shared by the two sites and one (1610-1640) only in middle Rhyolite. The latter two peaks occurred during periods of unusually low fire frequency, suggesting that fire-induced mortality shapes age structure. Evidence was mixed for the role of favorable moisture availability in age structure. As expected, moisture availability had a prominent positive effect on radial growth, but the effect of fire was largely neutral. The two sites differed only moderately in stand dynamics during the period of divergence, exhibiting subtle age structure contrasts and, in middle Rhyolite only, reduced growth during a 50-year fire hiatus followed by fire-induced release. These results suggest that, despite local differences in disturbance history, forest responses to regional fire and climate processes can persist.
• Díaz, S. C., Touchan, R., & Swetnam, T. W. (2001). A Tree-Ring reconstruction of past precipitation for Baja California Sur, Mexico. International Journal of Climatology, 21(8), 1007-1019.
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  Abstract: There is great interest in the climatic variability of Baja California and the Sea of Cortes, but long-term information is limited because instrumental climate records begin in the 1940s or 1960s. The first tree-ring chronology of Pinus lagunae was developed from the southern part of the Baja California Peninsula and the chronology is used to reconstruct the history of precipitation variations. A September-July precipitation reconstruction is developed for the period AD 1862-1996 (R = 0.71, p < 0.0001, n = 56, cross-validation = 0.68). This reconstruction is used to assess precipitation variability over the past two centuries, including the relationship with ENSO events. The reconstructed precipitation series indicates a long drought period from 1939 to 1958. It also shows that 1983, one of the strongest El Niño events of the 20th century, is the wettest year. El Niño events during the 20th century are associated with above-normal precipitation, whereas La Niña events are characterized by below-normal precipitation. Four of the most extreme wet years occurred in association with these warm events (1905, 1912, 1919 and 1983). Seventy-one percent of La Niña events are characterized by below-normal precipitation. Sixty-two percent of El Niño events are characterized by above-normal precipitation. Tree-ring growth of P. lagunae is most strongly correlated with winter precipitation in Sonora, Sinaloa and southern Baja California Sur. Precipitation data from meteorological stations in northern Baja California do not correlate well with the tree-ring chronology because this zone has a Mediterranean climate, which differs from the rest of northwest Mexico. Copyright © 2001 Royal Meteorological Society.
• Grissino-Mayer, H., & Swetnam, T. (2001). Century-scale climate forcing of fire regimes in the American Southwest. HOLOCENE, 10(2), 213-220.
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  Interannual time-scale associations between fire occurrence and drought indices, the Southern Oscillation, and other synoptic patterns demonstrate that large-scale, long-term atmospheric features are precursors to regional fire activity. However, our knowledge of fire-climate relations over longer (century) timescales is fragmentary because of the rarity of comparable climate and fire time-series with sufficient resolution, length and regional extent. In this study, we develop reconstructions of wildfire occurrence from tree-ring data collected from northwestern New Mexico to compare with a millennium-length dendroclimatic reconstruction of precipitation. Reconstructions of both wildfires and climate show simultaneous changes since AD 1700 that indicate climate forcing of wildfire regimes on interannual to century timescales. Following a centuries-long dry period with high fire frequency (c. AD 1400-1790), annual precipitation increased, fire frequency decreased, and the season of fire shifted from predominantly midsummer to late spring. We hypothesize that these shifts in fire regimes reflect long-term changes in rainfall patterns associated with changes in synoptic-scale atmospheric circulation patterns and the Southern Oscillation. Our evidence supports century-scale climate forcing of fire regimes in the American Southwest, providing a useful analogue of future wildfire regimes expected under changing global climate conditions.
• Hessburg, P., Kuhlmann, E., & Swetnam, T. (2001). Examining the recent climate through the lens of ecology: Inferences from temporal pattern analysis. ECOLOGICAL APPLICATIONS, 15(2), 440-457.
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  Ecological theory asserts that the climate of a region exerts top-down controls on regional ecosystem patterns and processes, across space and time. To provide empirical evidence of climatic controls, it would be helpful to define climatic regions that minimized variance in key climate attributes, within climatic regions-define the periods and features of climatic regimes, and then look for concordance between regional climate and ecosystem patterns or processes. In the past, these steps have not been emphasized. Before we evaluated the recent climate of the northwestern United States, we established a Northwest climatic region by clustering time series of the Palmer Drought Severity Index (PDSI)for the period of 1675-1978, for the western United States. The background climatic regime and anomalies of the recent northwestern U.S. climate were then identified through temporal pattern analysis involving application of correspondence analysis to the same PDSI time series.
• Kitzberger, T., Swetnam, T. W., & Veblen, T. T. (2001). Inter-hemispheric synchrony of forest fires and the El Niño-Southern Oscillation. Global Ecology and Biogeography, 10(3), 315-326.
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  Abstract: Fire histories were compared between the southwestern United States and northern Patagonia, Argentina using both documentary records (1914-87 and 1938-96, respectively) and tree-ring reconstructions over the past several centuries. The two regions share similar fire-climate relationships and similar relationships of climatic anomalies to the El Niño-Southern Oscillation (ENSO). In both regions, El Niño events coincide with above-average cool season precipitation and increased moisture availability to plants during the growing season. Conversely, La Niña events correspond with drought conditions. Monthly patterns of ENSO indicators (southern oscillation indices and tropical Pacific sea surface temperatures) preceding years of exceptionally widespread fires are highly similar in both regions during the 20th century. Major fire years tend to follow the switching from El Niño to La Niña conditions. El Niño conditions enhance the production of fine fuels, which when desiccated by La Niña conditions create conditions for widespread wildfires. Decadal-scale patterns of fire occurrence since the mid-17th century are highly similar in both regions. A period of decreased fire occurrence in both regions from c. 1780-1830 coincides with decreased amplitude and/or frequency of ENSO events. The interhemispheric synchrony of fire regimes in these two distant regions is tentatively interpreted to be a response to decadal-scale changes in ENSO activity. The ENSO-fire relationships of the south-western USA and northern Patagonia document the importance of high-frequency climatic variation to fire hazard. Thus, in addition to long-term trends in mean climatic conditions, multi-decadal scale changes in year-to-year variability need to be considered in assessments of the potential influence of climatic change on fire regimes.
• Morgan, P., Hardy, C. C., Swetnam, T. W., Rollins, M. G., & Long, D. G. (2001). Mapping fire regimes across time and space: Understanding coarse and fine-scale fire patterns. International Journal of Wildland Fire, 10(3-4), 329-342.
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  Abstract: Maps of fire frequency, severity, size, and pattern are useful for strategically planning fire and natural resource management, assessing risk and ecological conditions, illustrating change in disturbance regimes through time, identifying knowledge gaps, and learning how climate, topography, vegetation, and land use influence fire regimes. We review and compare alternative data sources and approaches for mapping fire regimes at national, regional, and local spatial scales. Fire regimes, defined here as the nature of fires occurring over an extended period of time, are closely related to local site productivity and topography, but climate variability entrains fire regimes at regional to national scales. In response to fire exclusion policies, land use, and invasion of exotic plants over the last century, fire regimes have changed greatly, especially in dry forests, woodlands, and grasslands. Comparing among and within geographic regions, and across time, is a powerful way to understand the factors determining and constraining fire patterns. Assembling spatial databases of fire information using consistent protocols and standards will aid comparison between studies, and speed and strengthen analyses. Combining multiple types of data will increase the power and reliability of interpretations. Testing hypotheses about relationships between fire, climate, vegetation, land use, and topography will help to identify what determines fire regimes at multiple scales.
• Morgan, P., Hardy, C., Swetnam, T., Rollins, M., & Long, D. (2001). Mapping fire regimes across time and space: Understanding coarse and fine-scale fire patterns. INTERNATIONAL JOURNAL OF WILDLAND FIRE, 10(3-4), 329-342.
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  Maps of fire frequency, severity, size, and pattern are useful for strategically planning fire and natural resource management, assessing risk and ecological conditions, illustrating change in disturbance regimes through time, identifying knowledge gaps, and learning how climate, topography, vegetation, and land use influence fire regimes. We review and compare alternative data sources and approaches for mapping fire regimes at national, regional, and local spatial scales. Fire regimes, defined here as the nature of fires occurring over an extended period of time, are closely related to local site productivity and topography, but climate variability entrains fire regimes at regional to national scales. In response to fire exclusion policies, land use, and invasion of exotic plants over the last century, fire regimes have changed greatly, especially in dry forests, woodlands, and grasslands. Comparing among and within geographic regions, and across time, is a powerful way to understand the factors determining and constraining fire patterns. Assembling spatial databases of fire information using consistent protocols and standards will aid comparison between studies, and speed and strengthen analyses. Combining multiple types of data will increase the power and reliability of interpretations. Testing hypotheses about relationships between fire, climate, vegetation, land use, and topography will help to identify what determines fire regimes at multiple scales.
• Perkins, D., & Swetnam, T. (2001). A dendroecological assessment of whitebark pine in the Sawtooth-Salmon River region, Idaho. CANADIAN JOURNAL OF FOREST RESEARCH-REVUE CANADIENNE DE RECHERCHE FORESTIERE, 26(12), 2123-2133.
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  Whitebark pine (Pinus albicaulis Engelm.) tree-ring chronologies of 700 to greater than 1000 years in length were developed for four sites in the Sawtooth - Salmon River region, central Idaho. These ring-width chronologies were used to (i) assess the dendrochronological characteristics of this species, (ii) detect annual mortality dates of whitebark pine attributed to a widespread mountain pine beetle (Dendroctonus ponderosae Hopkins (Coleoptera: Scolytidae)) epidemic during the 1909-1940 period, and (iii) establish the response of whitebark pine ring-width growth to climate variables. Cross-dating of whitebark pine tree-ring patterns was verified. Ring-width indices had low mean sensitivity (0.123-0.174), typical of high-elevation conifers in western North America, and variable first-order autocorrelation (0.206-0.551). Mountain pine beetle caused mortality of dominant whitebark pine peaked in 1930 on all four sites. Response functions and correlation analyses with state divisional weather records indicate that above-average radial growth is positively correlated with winter and spring precipitation and inversely correlated with May temperature. These correlations appear to be a response to seasonal snowpack. Whitebark pine is a promising species for dendroclimatic studies.
• Rollins, M. G., Swetnam, T. W., & Morgan, P. (2001). Evaluating a century of fire patterns in two Rocky Mountain wilderness areas using digital fire atlases. Canadian Journal of Forest Research, 31(12), 2107-2123.
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  Abstract: Changes in fire size, shape, and frequency under different fire-management strategies were evaluated using time series of fire perimeter data (fire atlases) and mapped potential vegetation types (PVTs) in the Gila - Aldo Leopold Wilderness Complex (GALWC) in New Mexico and the Selway-Bitterroot Wilderness Complex (SBWC) in Idaho and Montana. Relative to pre-Euro-American estimates, fire rotations in the GALWC were short during the recent wildfire-use period (1975-1993) and long during the pre-modern suppression period (1909-1946). In contrast, fire rotations in the SBWC were short during the pre-modern suppression period (1880-1934) and long during the modern suppression period (1935-1975). In general, fire-rotation periods were shorter in mid-elevation, shade-intolerant PVTs. Fire intervals in the GALWC and SBWC are currently longer than fire intervals prior to Euro-American settlement. Proactive fire and fuels management are needed to restore fire regimes in each wilderness complex to within natural ranges of variability and to reduce the risk of catastrophic wildfire in upper elevations of the GALWC and nearly the entire SBWC. Analyses of fire atlases provide baseline information for evaluating landscape patterns across broad landscapes.
• Speer, J. H., Swetnam, T. W., Wickman, B. E., & Youngblood, A. (2001). Changes in pandora moth outbreak dynamics during the past 622 years. Ecology, 82(3), 679-697.
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  Abstract: Episodic outbreaks of pandora moth (Coloradia pandora Blake), a forest insect that defoliates ponderosa pine (Pinus ponderosa Dougl. ex Laws.) and other pine species in the western United States, have recurred several times during the 20th century in forests of south-central Oregon. We collected and analyzed tree-ring samples from stands affected by recent outbreaks of pandora moth to develop a long-term record of outbreaks. Outbreaks were evident in tree-ring series as a characteristic "signature" of sharply reduced latewood width within a ring, followed by reduced ring widths lasting 4-20 yr. We verified that this tree-ring signature was unrelated to drought or other climatic fluctuations by comparing the timing of known and inferred outbreaks with independent climatic data. Using the pandora moth tree-ring signature, we reconstructed a 622-year record of 22 individual outbreaks in 14 old-growth ponderosa pine stands. This is currently the longest regional reconstruction of forest insect outbreak history in North America. Intervals between pandora moth outbreaks were highly variable within individual forest stands, ranging from 9 yr to 156 yr. Spectral analyses of a composite time series from all stands, however, showed more consistent intervals between outbreaks, suggesting quasicyclical population dynamics at regional and decadal scales. Waveforms extracted from the regional outbreak time series had periods ranging over ∼18-24 yr (39.7% variance explained) and ∼37-41 yr (37.3% variance explained). The periods and strengths of these cycles varied across the centuries, with the largest outbreaks occurring when relatively high-amplitude periods of the dominant cycles were in phase. Twentieth-century outbreaks were not more synchronous (extensive), severe, or longer in duration than outbreaks in previous centuries, but there was an unusual 60-yr reduction in regional activity during ∼1920-1980. The changing dynamical behavior of pandora moth populations highlights the need to evaluate historical factors that may have influenced this system, such as climatic variations, forest fires, and human land uses. Although cyclical dynamics in animal populations have most commonly been attributed to endogenous, ecological processes (e.g., "delayed density dependence," predators, pathogens, and parasites) our findings suggest that exogenous processes (e.g., climatic oscillations) may also be involved.
• Swetnam, T. (2001). A tree-ring reconstruction of past precipitation for Baja California Sur, Mexico.
• Swetnam, T. (2001). Changes in Pandora Moth Outbreak Dynamics during the Past 622 Years.
• Swetnam, T. (2001). Evaluating a century of fire patterns in two Rocky Mountain wilderness areas using digital fire atlases.
• Swetnam, T. (2001). Inter-hemispheric synchrony of forest fires and the El Nino-Southern Oscillation.
• Bigio, E., & Swetnam, T. W. (2000). A comparison and integration of tree-ring and alluvial records of fire history at the Missionary Ridge Fire, Durango, Colorado, USA. HOLOCENE, 20(7), 1047-1061.
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  We used tree-ring and alluvial sediment methods to reconstruct past fire regimes for a mixed conifer forest within a 1 km(2) drainage basin which was severely burned by a wildfire near Durango, Colorado. Post-fire debris flow events incised the valley-filling alluvial sediments in the lower basin, and created exposures of fire-related of deposits of late-Holocene age. Tree-ring and alluvial sediment fire history records were created separately, and then compared and integrated to create a similar to 3000 year record of past fire activity. The tree-ring record showed that from AD 1679 to 1879, there were frequent surface fires, while patches of high-severity fire occurred during widespread fire years. The alluvial record showed that a low- to moderate- and mixed-severity fire regime has likely been dominant over the past similar to 2600 calibrated calendar years before present, as shown by locally episodic deposition of charcoal-rich, fine-grained sediments. Radiocarbon dating suggested that in two stratigraphic sections, there was rapid deposition of several fine-grained sediment layers. One of these episodes occurred during the Medieval Climatic Anomaly (AD 900-1300). A charcoal-rich debris flow deposit in the oldest exposed part of the stratigraphic record dated to similar to 2600 calibrated calendar years before present. This event was potentially equivalent in magnitude to the debris-flow events following the recent wildfire in the study area, and is evidence of a high-severity fire that burned a large proportion of the study basin. The timing of this event coincides with a period of less frequent, yet more severe wildfires in a nearby lake sediment record, and is associated with the end of a Neoglacial period of cooler and wetter temperatures.
• Grissino-Mayer, H. D., & Swetnam, T. W. (2000). Century-scale climate forcing of fire regimes in the American Southwest. Holocene, 10(2), 213-220.
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  Abstract: Interannual time-scale associations between fire occurrence and drought indices, the Southern Oscillation, and other synoptic patterns demonstrate that large-scale, long-term atmospheric features are precursors to regional fire activity. However, our knowledge of fire-climate relations over longer (century) timescales is fragmentary because of the rarity of comparable climate and fire time-series with sufficient resolution, length and regional extent. In this study, we develop reconstructions of wildfire occurrence from tree-ring data collected from northwestern New Mexico to compare with a millennium-length dendroclimatic reconstruction of precipitation. Reconstructions of both wildfires and climate show simultaneous changes since AD 1700 that indicate climate forcing of wildfire regimes on interannual to century timescales. Following a centuries-long dry period with high fire frequency (c. AD 1400-1790), annual precipitation increased, fire frequency decreased, and the season of fire shifted from predominantly midsummer to late spring. We hypothesize that these shifts in fire regimes reflect long-term changes in rainfall patterns associated with changes in synoptic-scale atmospheric circulation patterns and the Southern Oscillation. Our evidence supports century-scale climate forcing of fire regimes in the American Southwest, providing a useful analogue of future wildfire regimes expected under changing global climate conditions.
• Margolis, E. Q., Swetnam, T. W., & Allen, C. D. (2000). HISTORICAL STAND-REPLACING FIRE IN UPPER MONTANE FORESTS OF THE MADREAN SKY ISLANDS AND MOGOLLON PLATEAU, SOUTHWESTERN USA. FIRE ECOLOGY, 7(3), 88-107.
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  The recent occurrence of large fires with a substantial stand-replacing component in the southwestern United States (e. g., Cerro Grande, 2000; Rodeo-Chedeski, 2002; Aspen, 2003; Horseshoe 2, Las Conchas, and Wallow, 2011) has raised questions about the historical role of stand-replacing fire in the region. We reconstructed fire dates and stand-replacing fire patch sizes using four lines of tree-ring evidence at four upper montane forest sites (>2600 m) in the Madrean Sky Islands and Mogollon Plateau of Arizona and New Mexico, USA. The four lines of tree-ring evidence include: (1) quaking aspen (Populus tremuloides) and spruce-fir age structure, (2) conifer death dates, (3) traumatic resin ducts and ring-width changes, and (4) conifer fire scars. Pre-1905 fire regimes in the upper montane forest sites were variable, with drier, south-facing portions of some sites recording frequent, low-severity fire (mean fire interval of all fires ranging from 5 yr to 11 yr among sites), others burning with stand-replacing severity, and others with no evidence of fire for >300 yr. Reconstructed fires at three of the four sites (Pinaleno Mountains, San Francisco Peaks, and Gila Wilderness) had stand-replacing fire patches >200 ha, with maximum patch sizes ranging from 286 ha in mixed conifer-aspen forests to 521 ha in spruce-fir forests. These data suggest that recent stand-replacing fire patches as large as 200 ha to 500 ha burning in upper elevation (>2600 m) mixed conifer-aspen and spruce-fir forests may be within the historical range of variability.
• Markgraf, V., Baumgartner, T., Bradbury, J. P., Diaz, H. F., Dunbar, R. B., Luckman, B. H., Seltzer, G. O., Swetnam, T. W., & Villalba, R. (2000). Paleoclimate reconstruction along the Pole-Equator-Pole transect of the Americas (PEP 1). Quaternary Science Reviews, 19(1-5), 125-140.
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  Abstract: Examples are presented of inter-hemispheric comparison of instrumental climate and paleoclimate proxy records from the Americas for different temporal scales. Despite a certain symmetry of seasonal precipitation patterns along the PEP I transect, decadal variability of winter precipitation shows different characteristics in terms of amplitude and frequency in both the last 100 and last 1000 years. Such differences in variability are also seen in a comparison of time series of different El Nino/Southern Oscillation proxy records from North and South America, however, these differences do not appear to affect the spatial correlation with Pacific sea surface temperature patterns. Local and regional differences in response to climate change are even more pronounced for records with lower temporal resolution, and inter-hemispheric synchroneity may or may not be indicative of the same forcing. This aspect is illustrated in an inter-hemispheric comparison of the last 1000 years of glacier variability, and of the full- and lateglacial lake level history.
• Ryerson, D., Swetnam, T., & Lynch, A. (2000). A tree-ring reconstruction of western spruce budworm outbreaks in the San Juan Mountains, Colorado, USA. CANADIAN JOURNAL OF FOREST RESEARCH-REVUE CANADIENNE DE RECHERCHE FORESTIERE, 33(6), 1010-1028.
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  Tree-ring records were used to reconstruct spatial and temporal patterns of western spruce budworm (Choristoneura occidentalis Freeman) outbreaks in mixed conifer forests of southern Colorado. Reconstructions in 11 host stands showed a regionally synchronous pattern of at least 14 outbreaks during the past 350 years. Intervals between outbreaks were highly variable within stands, but at the regional scale outbreak intervals were more consistent. Spectral analyses of regional outbreak time series confirmed periodicities at about 25, 37, and 83 years. Comparison with an independent drought reconstruction indicated that outbreaks typically corresponded to increased moisture, while relatively little budworm activity occurred during dry periods. In contrast to other published reconstructions in Colorado and New Mexico, reconstructions from this study area did not exhibit significant 20th-century changes in the frequency of outbreak occurrence or magnitude of growth reduction. Sharply reduced growth during outbreaks was not clearly visible on the increment core samples, and budworm-induced reductions in tree-ring growth were usually detectable only after comparison with nonhost tree-ring series. This finding emphasizes that defoliation effects on ring growth can be highly relativistic. Hence, caution should be exercised in reconstructing insect outbreak histories based only on visual detection approaches, or without comparison with nonhost or nondefoliated tree-ring control series.
• Kaufmann, M. R., Binkley, D., Fule, P. Z., Johnson, M., Stephens, S. L., & Swetnam, T. W. (1999). Defining old growth for fire-adapted forests of the Western United States. ECOLOGY AND SOCIETY, 12(2).
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  There are varying definitions of old-growth forests because of differences in environment and differing fire influence across the Intermountain West. Two general types of forests reflect the role of fire: 1) forests shaped by natural changes in structure and species makeup-plant succession-that are driven by competitive differences among species and individual trees and by small-scale disturbances, and 2) forests where plant succession processes are disrupted by major biological disturbances (fire, insects, wind, or drought) extending across larger areas. Some case examples of old-growth forests where fire was historically frequent are used. The examples sketch out the typical biophysical settings, fire regime, natural disturbance factors, spatial features of patches, and the processes and conditions that produce spatial changes of the landscape over time. These examples confirm the complexity of describing or defining old growth in frequent-fire forests. We define fire-adapted forests at three spatial scales, whereas the standard definition of old growth refers to a patch or stand condition. Our definition is based on ecological principles rather than on the cultural aspects of old growth. It focuses on central tendencies, given all the possible combinations of conditions and processes, that move forests toward old growth in the fire-adapted forests of the Intermountain West.
• Kaye, M. W., & Swetnam, T. W. (1999). An assessment of fire, climate, and apache history in the Sacramento mountains, New Mexico. Physical Geography, 20(4), 305-330.
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  Abstract: Fire historians typically attribute the causes of temporal change in past fire regimes to climatic variation, human land use, or some combination of the two. Most long-term historical reconstructions, however, lack time and place-specific chronologies for all three variables of fire, climate, and people. To test the hypothesis that Mescalero Apache of southeastern New Mexico influenced fire regimes of the Sacramento Mountains, we reconstructed and compared chronologies of key variables for the period A.D. 1700 to the present. Fire-scarred trees were used to reconstruct fire frequencies and culturally modified (peeled) trees, and written histories were used to identify places and times of Mescalero presence. Independent precipitation reconstructions from tree rings were compared with fire and human histories. We found that Mescalero frequently visited the western escarpment of the Sacramento Mountains during the late 1700s through the late 1800s, especially the Dog Canyon area. Fire frequency was higher and seasonal timing of fires was different in sites near Dog Canyon compared to relatively distant sites. Inter-annual to decadal-scale drought might explain some temporal variability in fire and peeling activity, but these relationships were not consistent. We conclude that people increased fire occurrence during certain time periods in localized areas, but broad-scale and persistent human impacts did not occur until the end of the 19th century with the rise of livestock grazing by European settlers and fire suppression by government agencies.
• Parsons, D. J., Swetnam, T. W., & Christensen, N. L. (1999). Uses and limitations of historical variability concepts in managing ecosystems. Ecological Applications, 9(4), 1177-1178.
• Pfeifer, M., Burgess, N. D., Swetnam, R. D., Platts, P. J., Willcock, S., & Marchant, R. (1999). Protected Areas: Mixed Success in Conserving East Africa's Evergreen Forests. PLOS ONE, 7(6).
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  In East Africa, human population growth and demands for natural resources cause forest loss contributing to increased carbon emissions and reduced biodiversity. Protected Areas (PAs) are intended to conserve habitats and species. Variability in PA effectiveness and 'leakage' (here defined as displacement of deforestation) may lead to different trends in forest loss within, and adjacent to, existing PAs. Here, we quantify spatial variation in trends of evergreen forest coverage in East Africa between 2001 and 2009, and test for correlations with forest accessibility and environmental drivers. We investigate PA effectiveness at local, landscape and national scales, comparing rates of deforestation within park boundaries with those detected in park buffer zones and in unprotected land more generally. Background forest loss (BFL) was estimated at -9.3% (17,167 km(2)), but varied between countries (range: -0.9% to -85.7%; note: no BFL in South Sudan). We document high variability in PA effectiveness within and between PA categories. The most successful PAs were National Parks, although only 26 out of 48 parks increased or maintained their forest area (i.e. Effective parks). Forest Reserves (Ineffective parks, i.e. parks that lose forest from within boundaries: 204 out of 337), Nature Reserves (six out of 12) and Game Parks (24 out of 26) were more likely to lose forest cover. Forest loss in buffer zones around PAs exceeded background forest loss, in some areas indicating leakage driven by Effective National Parks. Human pressure, forest accessibility, protection status, distance to fires and long-term annual rainfall were highly significant drivers of forest loss in East Africa. Some of these factors can be addressed by adjusting park management. However, addressing close links between livelihoods, natural capital and poverty remains a fundamental challenge in East Africa's forest conservation efforts.
• Swetnam, T. W., & Anderson, R. S. (1999). Fire Climatology in the western United States: introduction to special issue. INTERNATIONAL JOURNAL OF WILDLAND FIRE, 17(1), 1-7.
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  Advances in fire climatology have derived from recent studies of modern and paleoecological records. We convened a series of workshops and a conference session to report and review regional-scale findings, and these meetings led to the 10 papers in this special issue. Two papers focus on fire and climate patterns in the modern era using documentary records, four papers utilise tree rings to evaluate recent centuries of change, and four papers evaluate charcoal and pollen in lake, bog, and alluvial sediments over the Holocene. Here we summarise some of the key findings from these papers in the context of other recent fire climatology literature. These studies illustrate the value of long-term perspectives and spatial networks of fire and climate data in discovering the patterns and modes of past fire regime and climate variations.
• Swetnam, T. W., Allen, C. D., & Betancourt, J. L. (1999). Applied historical ecology: Using the past to manage for the future. Ecological Applications, 9(4), 1189-1206.
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  Abstract: Applied historical ecology is the use of historical knowledge in the management of ecosystems. Historical perspectives increase our understanding of the dynamic nature of landscapes and provide a frame of reference for assessing modern patterns and processes. Historical records, however, are often too brief or fragmentary to be useful, or they are not obtainable for the process or structure of interest. Even where long historical time series can be assembled, selection of appropriate reference conditions may be complicated by the past influence of humans and the many potential reference conditions encompassed by nonequilibrium dynamics. These complications, however, do not lessen the value of history; rather they underscore the need for multiple, comparative histories from many locations for evaluating both cultural and natural causes of variability, as well as for characterizing the overall dynamical properties of ecosystems. Historical knowledge may not simplify the task of setting management goals and making decisions, but 20th century trends, such as increasingly severe wildfires, suggest that disregarding history can be perilous. We describe examples from our research in the southwestern United States to illustrate some of the values and limitations of applied historical ecology. Paleoecological data from packrat middens and other natural archives have been useful for defining baseline conditions of vegetation communities, determining histories and rates of species range expansions and contractions, and discriminating between natural and cultural causes of environmental change. We describe a montane grassland restoration project in northern New Mexico that was justified and guided by an historical sequence of aerial photographs showing progressive tree invasion during the 20th century. Likewise, fire scar chronologies have been widely used to justify and guide fuel reduction and natural five reintroduction in forests. A southwestern network of fire histories illustrates the power of aggregating historical time series across spatial scales. Regional fire patterns evident in these aggregations point to the key role of interannual lags in responses of fuels and fire regimes to the El Nino-Southern Oscillation (wet/dry cycles), with important implications for long-range fire hazard forecasting. These examples of applied historical ecology emphasize that detection and explanation of historical trends and variability are essential to informed management.
• Margolis, E. Q., Swetnam, T. W., & Allen, C. D. (1998). A stand-replacing fire history in upper montane forests of the southern Rocky Mountains. CANADIAN JOURNAL OF FOREST RESEARCH-REVUE CANADIENNE DE RECHERCHE FORESTIERE, 37(11), 2227-2241.
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  Dendroecological techniques were applied to reconstruct stand-replacing fire history in upper montane forests in northern New Mexico and southern Colorado. Fourteen stand-replacing fires were dated to 8 unique fire years (18421901) using four lines of evidence at each of 12 sites within the upper Rio Grande Basin. The four lines of evidence were (i) quaking aspen (Populus tremuloides Michx.) inner-ring dates, (h) fire-killed conifer bark-ring dates, (iii) tree-ring width changes or other morphological indicators of injury, and (iv) fire scars. The annual precision of dating allowed the identification of synchronous stand-replacing fire years among the sites, and co-occurrence with regional surface fire events previously reconstructed from a network of fire scar collections in lower elevation pine forests across the southwestern United States. Nearly all of the synchronous stand-replacing and surface fire years coincided with severe droughts, because climate variability created regional conditions where stand-replacing fires and surface fires burned across ecosystems. Reconstructed stand-replacing fires that predate substantial Anglo-American settlement in this region provide direct evidence that stand-replacing fires were a feature of high-elevation forests before extensive and intensive land-use practices (e.g., logging, railroad, and mining) began in the late 19th century.
• Swetnam, T. (1998). Mesoscale Disturbance and Ecological Response to Decadal Climatic Variability in the American Southwest.
• Swetnam, T. W., & Betancourt, J. L. (1998). Mesoscale disturbance and ecological response to decadal climatic variability in the American Southwest. Journal of Climate, 11(12), 3128-3147.
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  Abstract: Ecological responses to climatic variability in the Southwest include regionally synchronized fires, insect outbreaks, and pulses in tree demography (births and deaths). Multicentury, tree-ring reconstructions of drought, disturbance history, and tree demography reveal climatic effects across scales, from annual to decadal, and from local (
• Baisan, C. H., & Swetnam, T. W. (1997). Interactions of fire regimes and land use in the central Rio Grande Valley. Research Paper - US Department of Agriculture, Forest Service, RM-RP-330.
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  Abstract: Four centuries of land use history were compared to fire regime characteristics along a use-intensity gradient. Changes in intensity and type of utilization varied directly with changes in fire regime characteristics near population centers, while remote areas shows little effect. Changes in fire frequency and fore-climate relationships during some periods suggest that humans augmented 'natural' fire associated with lightning ignitions. Our results show that human alterations in fire regime characteristics can be documented and, in some cases, can be distinguished from pre-existing conditions dominated by physical and biological processes that operate independently of human cultural effects. results also support the view that pre-20th century human impacts on landscapes were localized and episodic, rather than regional and constant.
• Kitzberger, T., Brown, P. M., Heyerdahl, E. K., Swetnam, T. W., & Veblen, T. T. (1996). Contingent Pacific-Atlantic Ocean influence on multicentury wildfire synchrony over western North America. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 104(2), 543-548.
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  Widespread synchronous wildfires driven by climatic variation, such as those that swept western North America during 1996, 2000, and 2002, can result in major environmental and societal impacts. Understanding relationships between continental-scale patterns of drought and modes of sea surface temperatures (SSTs) such as El Ni (n) over tildeo-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) may explain how interannual to multiclecadal variability in SSTs drives fire at continental scales. We used local wildfire chronologies reconstructed from fire scars on tree rings across western North America and independent reconstructions of SST developed from tree-ring widths at other sites to examine the relationships of multicentury patterns of climate and fire synchrony. From 33,039 annually resolved fire-scar dates at 238 sites (the largest paleofire record yet assembled), we examined forest fires at regional and subcontinental scales. Since 1550 CE, drought and forest fires covaried across the West, but in a manner contingent on SST modes. During certain phases of ENSO and PDO, fire was synchronous within broad subregions and sometimes asynchronous among those regions. In contrast, fires were most commonly synchronous across the West during warm phases of the AMO. ENSO and PDO were the main drivers of high-frequency variation in fire (interannual to decadal), whereas the AMO conditionally changed the strength and spatial influence of ENSO and PDO on wildfire occurrence at multiclecadal scales. A current warming trend in AMO suggests that we may expect an increase in widespread, synchronous fires across the western U.S. in coming decades.
• Markgraf, ., Baumgartner, T., Bradbury, J., Diaz, H., Dunbar, R., Luckman, B., Seltzer, G., Swetnam, T., & Villalba, R. (1996). Paleoclimate reconstruction along the Pole-Equator-Pole transect of the Americas (PEP 1). QUATERNARY SCIENCE REVIEWS, 19(1-5), 125-140.
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  Examples are presented of inter-hemispheric comparison of instrumental climate and paleoclimate proxy records from the Americas for different temporal scales. Despite a certain symmetry of seasonal precipitation patterns along the PEP 1 transect, decadal variability of winter precipitation shows different characteristics in terms of amplitude and frequency in both the last 100 and last 1000 years. Such differences in variability are also seen in a comparison of time series of different El Nino/Southern Oscillation proxy records from North and South America, however, these differences do not appear to affect the spatial correlation with Pacific sea surface temperature patterns. Local and regional differences in response to climate change are even more pronounced for records with lower temporal resolution, and inter-hemispheric synchroneity may or may not be indicative of the same forcing. This aspect is illustrated in an inter-hemispheric comparison of the last 1000 years of glacier variability, and of the full- and lateglacial lake level history. (C) 1999 Elsevier Science Ltd. All rights reserved.
• Perkins, D. L., & Swetnam, T. W. (1996). A dendroecological assessment of whitebark pine in the Sawtooth - Salmon River region, Idaho. Canadian Journal of Forest Research, 26(12), 2123-2133.
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  Abstract: Whitebark pine (Pinus albicaulis Engelm.) tree-ring chronologies of 700 to greater than 1000 years in length were developed for four sites in the Sawtooth - Salmon River region, central Idaho. These ring-width chronologies were used to (i) assess the dendrochronological characteristics of this species, (ii) detect annual mortality dates of whitebark pine attributed to a widespread mountain pine beetle (Dendroctonus ponderosae Hopkins (Coleoptera: Scolytidae)) epidemic during the 1909-1940 period, and (iii) establish the response of whitebark pine ring-width growth to climate variables. Cross-dating of whitebark pine tree-ring patterns was verified. Ring-width indices had low mean sensitivity (0.123-0.174), typical of high-elevation conifers in western North America, and variable first-order autocorrelation (0.206-0.551). Mountain pine beetle caused mortality of dominant whitebark pine peaked in 1930 on all four sites. Response functions and correlation analyses with state divisional weather records indicate that above-average radial growth is positively correlated with winter and spring precipitation and inversely correlated with May temperature. These correlations appear to be a response to seasonal snowpack. Whitebark pine is a promising species for dendroclimatic studies.
• Swetnam, T. (1996). A dendroecological assessment of whitebark pine in the Sawtooth–Salmon River region, Idaho.
• Williams, A. P., Allen, C. D., Millar, C. I., Swetnam, T. W., Michaelsen, J., Still, C. J., & Leavitt, S. W. (1996). 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.
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  In recent decades, intense droughts, insect outbreaks, and wildfires have led to decreasing tree growth and increasing mortality in many 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 approximate to 2.7% of southwestern forest and woodland area experienced substantial mortality due to wildfires from 1984 to 2006, and approximate to 7.6% experienced mortality associated with bark beetles from 1997 to 2008. We estimate that up to approximate 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.
• Betancourt, J., Grissino-Mayer, H., Salzer, M., & Swetnam, T. (1995). A test of "annual resolution" in stalagmites using tree rings. QUATERNARY RESEARCH, 58(2), 197-199.
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  So-called annual banding has been identified in a number of speleothems in which the number of bands approximates the time interval between successive U-series dates. The apparent annual resolution of speleothem records, however, remains largely untested. Here we statistically compare variations in band thickness from a late Holocene stalagmite in Carlsbad Cavern, Southern New Mexico, USA, with three independent tree-ring chronologies form the same region. We found no correspondence. Although there may be various explanations for the discordance, this limited exercise suggests that banded stalagmites should be held to the same rigorous standards in chronology building and climatic inference as annually resolved tree rings, corals, and ice cores. (C) 2002 University of Washington.
• MADANY, M., SWETNAM, T., & WEST, N. (1995). COMPARISON OF 2 APPROACHES FOR DETERMINING FIRE DATES FROM TREE SCARS. FOREST SCIENCE, 28(4), 856-861.
• SWETNAM, T. (1995). FIRE HISTORY AND CLIMATE-CHANGE IN GIANT SEQUOIA GROVES. SCIENCE, 262(5135), 885-889.
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  Fire scars in giant sequoia [Sequoiadendron giganteum (Lindley) Buchholz] were used to reconstruct the spatial and temporal pattern of surface fires that burned episodically through five groves during the past 2000 years. Comparisons with independent dendroclimatic reconstructions indicate that regionally synchronous fire occurrence was inversely related to yearly fluctuations in precipitation and directly related to decadal-to-centennial variations in temperature. Frequent small fires occurred during a warm period from about A. D. 1 000 to 1300, and less frequent but more widespread fires occurred during cooler periods from about A.D. 500 to 1000 and after A.D. 1300. Regionally synchronous fire histories demonstrate the importance of climate in maintaining nonequilibrium conditions.
• SWETNAM, T., & LYNCH, A. (1995). A TREE-RING RECONSTRUCTION OF WESTERN SPRUCE BUDWORM HISTORY IN THE SOUTHERN ROCKY MOUNTAINS. FOREST SCIENCE, 35(4), 962-986.
• Sanchez-Salguero, R., Navarro-Cerrillo, R. M., Swetnam, T. W., & Zavala, M. A. (1995). Is drought the main decline factor at the rear edge of Europe? The case of southern Iberian pine plantations. FOREST ECOLOGY AND MANAGEMENT, 271, 158-169.
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  Drought has been frequently discussed as a trigger for forest decline. Today, large-scale forest decline is observed at the rear edge of Mediterranean forests, with drought identified as the most likely driver. The vulnerability of Mediterranean mountain plantations to regional climatic variations; however, is poorly understood. In this paper, we analyze the impact of biotic and abiotic factors on the growth and vigor of two pine species in drought-prone areas. We assess the main factors influencing crown defoliation and radial growth to develop a predictive model of forest decline risk for pine plantations at the dry edge of the species range. Dendrochronological data were collected on 50 plots for Pinus nigra subsp. salzmanii and 40 plots for Pinus sylvestris. We examined tree size, competition, site characteristics and climate variables related to decline of pine plantations. Correlation and principal component analysis (PCA) were used to identify the correlates of decline with crown condition and growth, separately. Logistic regression and generalized linear models were used to study the relationship between canopy defoliation and growth, respectively, for P. nigra and P. sylvestris. Explanatory variables were introduced in a stepwise selection. The PCA revealed three main axes, associated with climate, competition and physiographic variables. Those three axes were associated with crown damage and basal area growth, respectively. Probability of crown damage was associated with tree size, competition and climate conditions. P. sylvestris was more sensitive than P. nigra to summer potential evapotranspiration effects. Also, climate and competition were the two main drivers affecting basal area growth. Unlike crown defoliation, physiography had an important effect. Within each species, there was a divergence between healthy and damaged trees, mainly related to competition factors. On the other hand, growth trends as an independent variable were not included in the crown damage probability model in a stepwise selection. However, declining growth found in trees at higher crown defoliation and drier sites may imply a greater vulnerability to decline, suggesting an enhanced die-off risk. The sharp growth reduction and widespread defoliation in declining pine plantations make their future persistence in xeric sites subject to frequent and severe droughts unlikely under expected warmer and drier conditions in the future. (C) 2012 Elsevier B.V. All rights reserved.
• Swetnam, T. W., Wickman, B. E., Gene, P. H., & Baisan, C. H. (1995). Historical patterns of western spruce budworm and Douglas-fir tussock moth outbreaks in the northern Blue Mountains, Oregon, since AD 1700. Research Paper - US Department of Agriculture, Forest Service.
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  Abstract: Dendroecology methods were used to reconstruct a three-century history of western spruce budworm Choristoneura occidentalis and Douglas-fir tussock moth Orgyia pseudotsugata outbreaks in the Blue Mountains of NE Oregon. Budworm outbreaks were more confidently reconstructed than were tussock moth outbreaks. Since AD 1700, at least eight regional budworm outbreaks have occurred at intervals of about 21 to 53 yrs. Reduced radial growth caused by defoliation lasted from about 13 to 17 yrs. Two regional budworm outbreaks occurred during the 19th century, and at least three and possibly four regional outbreaks have occurred during the 20th century. -from Authors
• Brown, P. M., & Swetnam, T. W. (1994). A cross-dated fire history from coast redwood near Redwood National Park, California. Canadian Journal of Forest Research, 24(1), 21-31.
• Swetnam, T. (1994). A cross-dated fire history from coast redwood near Redwood National Park, California.
• Swetnam, T. (1993). Multicentury, Regional-Scale Patterns of Western Spruce Budworm Outbreaks.
• Swetnam, T. W. (1993). Fire history and climate change in giant sequoia groves. Science, 262(5135), 885-889.
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  PMID: 17757357;Abstract: Fire scars in giant sequoia [Sequoiadendron giganteum (Lindley) Buchholz] were used to reconstruct the spatial and temporal pattern of surface fires that burned episodically through five groves during the past 2000 years. Comparisons with independent dendro-climatic reconstructions indicate that regionally synchronous fire occurrence was inversely related to yearly fluctuations in precipitation and directly related to decadal-to-centennial variations in temperature. Frequent small fires occurred during a warm period from about A.D. 1000 to 1300, and less frequent but more widespread fires occurred during cooler periods from about A.D. 500 to 1000 and after A.D. 1300. Regionally synchronous fire histories demonstrate the importance of climate in maintaining nonequilibrium conditions.
• Swetnam, T. W., & Batancourt, T. L. (1993). Temporal patterns of El Nino/Southern Oscillation - wildfire teleconnections in the southwestern United States. El Nino: historical and paleoclimatic aspects of the Southern Oscillation, 259-270.
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  Abstract: Wildland fire occurrence in the SW USA is correlated with winter-spring precipitation, tree-ring growth, and the Southern Oscillation. Twentieth century (1905-1985) records from all National Forest lands in Arizona and New Mexico show reduced/increased annual area burned during low/high (respectively) mean December through February Southern Oscillation indices. A 206-yr record of regional fire activity (1700-1905) derived from fire scars in conifer trees shows high correlation with precipitation-responsive tree-ring width chronologies during the 1740s to 1830s and 1870s to 1900s. Synchronized biennial patterns of high/low tree-ring growth and low-high fire activity, respectively, is evident. Temporal changes in southwestern fire regimes over the past three centuries may be indicative of ecological changes in these forests (eg other natural or human disturbances), or changes in the ENSO phenomenon. -from Authors
• Swetnam, T. W., & Lynch, A. M. (1993). Multicentury, regional-scale patterns of western spruce budworm outbreaks. Ecological Monographs, 63(4), 399-424.
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  Abstract: Tree ring chronologies from 24 mixed-conifer stands were used to reconstruct the long-term history of western spruce budworm (Choristoneura occidentalis) in northern New Mexico. Temporal and spatial patterns of budworm infestations (within-stand occurrences) and outbreaks (more-or-less synchronous infestations across many stands) were investigated to identify local-scale to regional-scale forest disturbance patterns. Nine regional-scale outbreaks were identified from 1690 to 1989. One ancient stand of Douglasfir trees (Pseudotsuga menziesii) exceeding 700 yr in age revealed that budworms and overstory trees can coexist for extraordinary lengths of time. Using spectral analysis we found that the regional outbreak record contained important cyclical components with periods varying from ≈ 20 to 33 yr. The statistically significant (P < .05) but variable periodicity of regional outbreaks suggests the forest-budworm dynamic is pseudoperiodic (i.e., a stable limit cycle or damped oscillator perturbed by noise). Duration of infestations within stands was ≈ 11 yr and has not obviously changed in the 20th century; however, infestations tended to be more synchronous among stands in this century than during earlier centuries. Regional budworm activity was low from the mid-1920s to late 1930s and mid-1960s to late 1970s, and the most recent outbreak, beginning in the late 1970s, was unusually severe. These results, and contrasting infestation patterns in mountain ranges with different land use histories, generally support a hypothesis that human-induced changes in Southwestern forests have led to more widespread and intense budworm outbreaks in the late 20th century. Despite human-induced changes in the 20th century, climate variation also appears to have been important to budworm regimes in this century as well as in earlier times. Regional outbreaks in the 20th century tended to occur during years of increased spring precipitation, and decreased budworm activity coincided with decreased spring precipitation. No clear association with temperature was identified. Comparisons of regional outbreak history since AD 1600 with a reconstruction of spring precipitation from limber pine (Pinus flexilis) ring width chronologies also shows that periods of increased and decreased budworm activity coincided with wetter and drier periods, respectively. This finding contrasts with results from shorter time-scale studies conducted in northwestern U.S. and Canada (western spruce budworm) and eastern Canada (spruce budworm C. fumiferana), where low precipitation and/or warmer temperatures were generally associated with outbreaks. Different patterns of budworm population response to changing moisture regimes might be due to differences in regional forest-budworm systems, or to differences in the spatial and temporal scales of observation. We conclude that changes in forest structure in the southwestern U.S. may have shifted the spatial and temporal pattern of budworm outbreaks. The dynamic behavior and statistically significant association between multicentury, regional budworm and climate time series also suggest that complex budworm dynamics are driven by a combination of internal and external factors.
• Barton, A., Swetnam, T., & Baisan, C. (1992). Arizona pine (Pinus arizonica) stand dynamics: local and regional factors in a fire-prone madrean gallery forest of Southeast Arizona, USA. LANDSCAPE ECOLOGY, 16(4), 351-369.
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  In southwestern North America, large-scale climate patterns appear to exert control on moisture availability, fire occurrence, and tree demography, raising the compelling possibility of regional synchronization of forest dynamics. Such regional signals may be obscured, however, by local, site-specific factors, such as disturbance history and land use. Contiguous sites with similar physical environments, lower and middle Rhyolite Canyon, Arizona, USA, shared nearly the same fire history from 1660-1801, but then diverged. For the next 50 years, fires continued to occur frequently in lower Rhyolite, but, probably as result of flood-induced debris deposition, largely ceased in middle Rhyolite. We related stand dynamics of Arizona pine (Pinus arizonica) to fire history and drought severity and compared the dynamics in the two sites before and after the divergence in fire frequency. Fires occurred during unusually dry years, and possibly following unusually moist years. Arizona pine exhibited three age structure peaks: two (1810-1830 and 1870-1900) shared by the two sites and one (1610-1640) only in middle Rhyolite. The latter two peaks occurred during periods of unusually low fire frequency, suggesting that fire-induced mortality shapes age structure. Evidence was mixed for the role of favorable moisture availability in age structure. As expected, moisture availability had a prominent positive effect on radial growth, but the effect of fire was largely neutral. The two sites differed only moderately in stand dynamics during the period of divergence, exhibiting subtle age structure contrasts and, in middle Rhyolite only, reduced growth during a 50-year fire hiatus followed by fire-induced release. These results suggest that, despite local differences in disturbance history, forest responses to regional fire and climate processes can persist.
• Baisan, C. H., & Swetnam, T. W. (1990). Fire history on a desert mountain range: Rincon Mountain Wilderness, Arizona, USA. Canadian Journal of Forest Research, 20(10), 1559-1569.
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  Abstract: Modern fire records and fire-scarred remnant material collected from logs, snags, and stumps were used to reconstruct and analyze fire history in the mixed-conifer and pine forest above 2300 m within the Rincon Mountain Wilderness of Saguaro National Monument. The fire regime was dominated by large scale (>200 ha), early-season (May-July) surface fires. Mean fire interval over the Mica Mountain study area for 1657-1893 was 6.1 yr with a range of 1-13 yr for larger fires. Mean fire interval for the mixed-conifer forest type (1748-1886) was 9.9 yr with a range of 3-19 yr. Thirty-five major fire years between 1700-1900 were compared with a tree-ring reconstruction of the Palmer drought severity index (PDSI). Mean July PDSI for 2 yr prior to fires was higher (wetter) than average, while mean fire year PDSI was near average. -from Authors
• SWETNAM, T., & BETANCOURT, J. (1990). FIRE - SOUTHERN OSCILLATION RELATIONS IN THE SOUTHWESTERN UNITED-STATES. SCIENCE, 249(4972), 1017-1020.
• Swetnam, T. (1990). Early 19th-Century Fire Decline Following Sheep Pasturing in a Navajo Ponderosa Pine Forest.
• Swetnam, T. (1990). Fire history on a desert mountain range: Rincon Mountain Wilderness, Arizona, U.S.A..
• Swetnam, T. W. (1990). Fire history and climate in the southwestern United States. General Technical Report - US Department of Agriculture, Forest Service, 6-17.
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  Abstract: Forest fire occurrence during the past three centuries was examined using historic records from documents and fire-scarred trees. The influence of climate on fire regimes was apparent in regional synchroneity of large fires and association of reduced fire activity and El Nino-Southern Oscillation (ENSO) events. The latter association may have forecasting value. -Author
• Swetnam, T. W., & Betancourt, J. L. (1990). Fire - Southern Oscillation relations in the southwestern United States. Science, 249(4972), 1017-1020.
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  PMID: 17789609;Abstract: Fire scar and tree growth chronologies (1700 to 1905) and fire statistics (since 1905) from Arizona and New Mexico show that small areas burn after wet springs associated with the low phase of the Southern Oscillation (SO), whereas large areas burn after dry springs associated with the high phase of the SO. Through its synergistic influence on spring weather and fuel conditions, climatic variability in the tropical Pacific significantly influences vegetation dynamics in the southwestern United States. Synchrony of fire-free and severe fire years across diverse southwestern forests implies that climate forces fire regimes on a subcontinental scale; it also underscores the importance of exogenous factors in ecosystem dynamics.
• Bowman, D. M., Balch, J., Artaxo, P., Bond, W. J., Cochrane, M. A., D'Antonio, C. M., DeFries, R., Johnston, F. H., Keeley, J. E., Krawchuk, M. A., Kull, C. A., Mack, M., Moritz, M. A., Pyne, S., Roos, C. I., Scott, A. C., Sodhi, N. S., & Swetnam, T. W. (1989). The human dimension of fire regimes on Earth. JOURNAL OF BIOGEOGRAPHY, 38(12), 2223-2236.
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  Humans and their ancestors are unique in being a fire-making species, but natural (i.e. independent of humans) fires have an ancient, geological history on Earth. Natural fires have influenced biological evolution and global biogeochemical cycles, making fire integral to the functioning of some biomes. Globally, debate rages about the impact on ecosystems of prehistoric human-set fires, with views ranging from catastrophic to negligible. Understanding of the diversity of human fire regimes on Earth in the past, present and future remains rudimentary. It remains uncertain how humans have caused a departure from natural background levels that vary with climate change. Available evidence shows that modern humans can increase or decrease background levels of natural fire activity by clearing forests, promoting grazing, dispersing plants, altering ignition patterns and actively suppressing fires, thereby causing substantial ecosystem changes and loss of biodiversity. Some of these contemporary fire regimes cause substantial economic disruptions owing to the destruction of infrastructure, degradation of ecosystem services, loss of life, and smoke-related health effects. These episodic disasters help frame negative public attitudes towards landscape fires, despite the need for burning to sustain some ecosystems. Greenhouse gas-induced warming and changes in the hydrological cycle may increase the occurrence of large, severe fires, with potentially significant feedbacks to the Earth system. Improved understanding of human fire regimes demands: (1) better data on past and current human influences on fire regimes to enable global comparative analyses, (2) a greater understanding of different cultural traditions of landscape burning and their positive and negative social, economic and ecological effects, and (3) more realistic representations of anthropogenic fire in global vegetation and climate change models. We provide an historical framework to promote understanding of the development and diversification of fire regimes, covering the pre-human period, human domestication of fire, and the subsequent transition from subsistence agriculture to industrial economies. All of these phases still occur on Earth, providing opportunities for comparative research.
• Fritts, H. C., & Swetnam, T. W. (1989). Dendroecology: A Tool for Evaluating Variations in Past and Present Forest Environments. Advances in Ecological Research, 19(C), 111-188.
• Swetnam, T. W., & Lynch, A. M. (1989). A tree-ring reconstruction of western spruce budworm history in the southern Rocky Mountains. Forest Science, 35(4), 962-986.
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  Abstract: Tree-ring width chronologies from 10 mixed-conifer stands in the Colorado Front Range and New Mexico Sangre de Cristo Mountains were used to reconstruct the timing, duration, and radial growth impacts of past outbreaks of Choristoneura occidentalis. At least 9 outbreaks were identified from 1700-1983. Severity and timing of outbreaks was high variable. Average duration of reduced growth periods caused by budworms was 12.9 yr and ranged from 5-26 yr. Average interval between initial years of successive outbreaks was 34.9 yr and ranged from 14-58 years. Average maximum and periodic radial growth reductions were 50% and 21.7%, respectively. There was a relatively long period of reduced budworm activity in the first few decades of the 20th century, and since that time outbreaks have been markedly more synchronous among the sampled stands, possibly due to changes in age structure and species composition following harvesting and fire suppression in the late 19th and early 20th centuries. -from Authors
• Dieterich, J. H., & Swetnam, T. W. (1984). Dendrochronology of a fire-scarred ponderosa pine.. Forest Science, 30(1), 238-247.
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  Abstract: Mean fire interval for the study area was c2 yr; mean fire interval for the individual Pinus ponderosa specimen was 4 yr for the 178-year period, 1722-1900. -from Authors
• Swetnam, T. (1983). Comment on dating forest disturbances.
• Swetnam, T. W., Sutherland, E. K., & Thompson, M. A. (1983). Comment on dating forest disturbances. Quaternary Research, 19(3), 400-401.
• Madany, M. H., Swetnam, T. W., & West, N. E. (1982). Comparison of two approaches for determining fire dates from tree scars ( ponderosa pine Pinus ponderosa) .. Forest Science, 28(4), 856-861.
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  Abstract: Occurrence of missing or false rings presents an obstacle to accurate dating of individual fires. Two different techniques of dating partial cross sections of fire-scarred ponderosa pine Pinus ponderosa were compared using the same samples. Method I involves the tabulation of 'raw' dates followed by a subsequent adjustment based on synchronization with fire dates from adjacent trees. Method II uses correlations with master chronologies based on analysis of increment cores taken in the same region as the sampled fire scar sections. Method I is prone to error since the assumption that fire dates differing by a few years may actually be from the same year cannot be independently verified. Method II provides a higher degree of certainty with regard to the accuracy of individual dates because the cross dating of local patterns of ring widths circumvents the problem of absent or false rings. -from Authors

Proceedings Publications

• Westerling, A., Schoennagel, T., Swetnam, T. W., Turner, M., & Veblen, T. (2014, Spring). Briefing: Climate and wildfire in western U.S. forests. In Forest conservation and management in the Anthropocene, In: Sample, V. Alaric; Bixler, R. Patrick, eds. Forest conservation and management in the Anthropocene: Conference proceedings. Proceedings. RMRS-P-71. Fort Collins, CO: US Department of Agriculture, Forest Service. Rocky Mountain Research Station. p. 81-102..
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  Wildfire in western U.S. federally managed forests has increased substantially in recent decades, with large (>1000 acre) fires in the decade through 2012 over five times as frequent (450 percent increase) and burned area over ten times as great (930 percent increase) as the 1970s and early 1980s. These changes are closely linked to increased temperatures and a greater frequency and intensity of drought. Projected additional future warming implies that wildfire activity may continue to increase in western forests. However, the interaction of changes in climate, fire and other disturbances, vegetation and land management may eventually transform some forest ecosystems and fire regimes, with changes in the spatial extent of forest and fire regime types. In particular, forests characterized by infrequent, high-severity stand replacing fire may be highly sensitive to warming. Increased wildfire combined with warming may transform these ecosystems such that fuel availability, rather than flammability, becomes the dominant constraint on fire activity. Climate will continue to warm for some time regardless of future greenhouse gas emissions, requiring adaptation to warmer temperatures. Changes in forest location, extent and type will result in substantial changes in ecosystem services.
• Baisan, C., Swetnam, T., Brown, J., Mutch, R., Spoon, C., & Wakimoto, R. (2008). Historical fire occurrence in remote mountains of southwestern New Mexico and northern Mexico. In PROCEEDINGS: SYMPOSIUM ON FIRE IN WILDERNESS AND PARK MANAGEMENT, 320, 153-156.
• Caprio, A., Swetnam, T., Brown, J., Mutch, R., Spoon, C., & Wakimoto, R. (2007). Historic fire regimes along an elevational gradient on the west slope of the Sierra Nevada, California. In PROCEEDINGS: SYMPOSIUM ON FIRE IN WILDERNESS AND PARK MANAGEMENT, 320, 173-179.
• Mutch, L., Swetnam, T., Brown, J., Mutch, R., Spoon, C., & Wakimoto, R. (2007). Effects of fire severity and climate on ring-width growth of giant sequoia after burning. In PROCEEDINGS: SYMPOSIUM ON FIRE IN WILDERNESS AND PARK MANAGEMENT, 320, 241-246.
• SWETNAM, T., & KRAMMES, J. (2007). FIRE HISTORY AND CLIMATE IN THE SOUTHWESTERN UNITED-STATES. In EFFECTS OF FIRE MANAGEMENT OF SOUTHWESTERN NATURAL RESOURCES, 191, 6-17.
• Swetnam, T., Baisan, C., Ffolliott, P., DeBano, L., Baker, M., Gottfried, G., SolisGarza, G., Edminster, C., Neary, D., Allen, L., & Hamre, R. (2005). Fire histories of Montane forests in the Madrean borderlands. In EFFECTS OF FIRE ON MADREAN PROVINCE ECOSYSTEMS - A SYMPOSIUM PROCEEDINGS, 289, 15-36.
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  ln this paper we summarize historical fire regime patterns reconstructed using fire-scarred tree-ring specimens from seventeen montane forest sites in the Madrean Borderlands. In addition to a brief description of general patterns we also illustrate, with examples, several unique fire occurrence patterns influenced by land-use history and landscape configurations. Mean fire intervals and other statistical descriptors of fire interval distributions show that widespread surface fires were frequent in nearly all forests before ca. 1900, with fires occurring at least once per decade. Most fires occurred during the arid foresummer and lightning fire season from April through June. High spatial and temporal variability of fire frequency and other fire regime properties point to the importance of unique site and time-specific factors in controlling fire occurrence. These factors include the continuity of fuels and topography, livestock grazing, and possibly, the burning of some areas by Apaches during certain time periods. Ponderosa pine and mixed-conifer stands in rugged mountain ranges, such as the Animas in New Mexico, sustained mixed-fire regimes of both surface and crown fires. Frequent, widespread, surface fires ceased to occur in most U.S. Borderland sites at about the time intensive livestock grazing began. In contrast, montane forests on the Mexican side of the border sustained continuous surface fire regimes throughout the 20th century. With the elimination of frequent, widespread surface fires on the U.S. side, woody fuels have greatly increased in amount and continuity, and as a consequence, the size and intensity of recent crown fires were probably historically and ecologically anomalous.
• SWETNAM, T., BROWN, P., & , . (2002). OLDEST KNOWN CONIFERS IN THE SOUTHWESTERN UNITED-STATES - TEMPORAL AND SPATIAL PATTERNS OF MAXIMUM AGE. In OLD-GROWTH FORESTS IN THE SOUTHWEST AND ROCKY MOUNTAIN REGIONS : PROCEEDINGS OF A WORKSHOP, 213, 24-38.
• GrissinoMayer, H., Baisan, C., Swetnam, T., DeBano, L., Gottfried, G., Hamre, R., Edminster, C., Ffolliott, P., & OrtegaRubio, A. (1994). Fire history in the Pinaleno Mountains of southeastern Arizona: Effects of human-related disturbances. In BIODIVERSITY AND MANAGEMENT OF THE MADREAN ARCHIPELAGO: THE SKY ISLANDS OF SOUTHWESTERN UNITED STATES AND NORTHWESTERN MEXICO, 264, 399-407.
• Touchan, R., Swetnam, T., GrissinoMayer, H., Brown, J., Mutch, R., Spoon, C., & Wakimoto, R. (1993). Effects of livestock grazing on pre-settlement fire regimes in New Mexico. In PROCEEDINGS: SYMPOSIUM ON FIRE IN WILDERNESS AND PARK MANAGEMENT, 320, 268-272.
• Kipfmueller, K., Swetnam, T., Cole, D., McCool, S., Borrie, W., & OLoughlin, J. (1990). Fire-climate interactions in the Selway-Bitterroot Wilderness Area. In WILDERNESS SCIENCE IN A TIME OF CHANGE CONFERENCE, VOL 5, 5, 270-275.
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  Tree-ring reconstructed summer drought was examined in relation to the occurrence of 15 fires in the Selway-Bitterroot Wilderness Area (SBW). The ten largest fire years between 1880 and 1995 were selected from historical fire atlas data; five additional fire years were selected from a fire history completed in a subalpine forest within the SEW. Results of the analysis indicate summers during the fire year were significantly (p0.05). A significant (p
• Danzer, ., Baisan, C., Swetnam, T., Ffolliott, P., DeBano, L., Baker, M., Gottfried, G., SolisGarza, G., Edminster, C., Neary, D., Allen, L., & Hamre, R. (1984). The influence of fire and land-use history on stand dynamics in the Huachuca mountains of southeastern Arizona. In EFFECTS OF FIRE ON MADREAN PROVINCE ECOSYSTEMS - A SYMPOSIUM PROCEEDINGS, 289, 265-270.
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  Dendrochronological methods were used to reconstruct fire regimes and stand age structures in the Huachuca Mountains of Southeastern Arizona. Pre-settlement (i.e., before ca. 1870) fire intervals ranged from 4 to 8 years, with many fires spreading over the entire sample area. Stand age distributions show an increase in more shade-tolerant tree species. Although ponderosa pine still dominates stands, recent recruitment is predominantly southwestern white pine and Douglas-fir. Establishment of Ft. Huachuca in 1877 was a precursor to extensive use of timber, mineral, range and water resources in the Huachuca Mountains. The. fire regime was clearly altered at this time, with only one subsequent widespread surface fire recorded in 1899. Settlement era land-use practices may be responsible for changes in stand structure and composition.
• Swetnam, T., Goldammer, J., & Furyeav, V. (1982). Fire and climate history in the central Yenisey region, Siberia. In FIRE IN ECOSYSTEMS OF BOREAL EURASIA, 48, 90-104.

Presentations

• Swetnam, T., Falk, D. A., Guiterman, C. H., Baisan, C. H., Margolis, E., & Farris, C. A. (2019, November). Modern area burned in a historical perspective in two Southwest wilderness areas.. Assocation for Fire Ecology, Eighth International Fire Congress. Tucson, AZ: Assocation for Fire Ecology.
• Swetnam, T., Falk, D. A., Margolis, E., & Guiterman, C. H. (2019, November). A synthesis of historical fire regimes in the southwestern United States.. Assocation for Fire Ecology, Eighth International Fire Congress. Tucson, AZ: Assocation for Fire Ecology.
• Arizpe, A., Falk, D. A., Swetnam, T., & Woodhouse, C. A. (2016, November). Widespread Fire Years in Conifer Forests are Contingent on Both Winter and Monsoon Precipitation in the US-Mexico Sky Islands. AFE Southwest Conference. Tucson, AZ: Association for Fire Ecology (AFE).
• Falk, D. A., Bigio, E. R., Swetnam, T., Hall, G., Sutherland, E. K., Kitzberger, T., Brown, P. M., & Velasquez, E. (2016, March). Historical pyrogeography of western North America 1600-1900 CE.. Third American Dendrochronology Conference (AmeriDendro 2016). Mendoza, Argentina: Tree Ring Society.
• Guiterman, C. H., Margolis, E. Q., Allen, C. D., Falk, D. A., & Swetnam, T. W. (2016, November). Persistence and fire regimes of oak shrubfields suggest increasing dominance with climate change. AFE Southwest Conference. Tucson, AZ: Association for Fire Ecology (AFE).
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  Christopher Guiterman:
• Guiterman, C. H., Margolis, E. Q., Baisan, C. H., Falk, D. A., Towner, R. D., & Swetnam, T. W. (2016, November). Navajo settlement, pastoralism, and the interruption of frequent fires in northeastern Arizona. AFE Southwest Conference. Tucson, AZ: Association for Fire Ecology (AFE).
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  Christopher Guiterman:
• Kitzberger, T., Falk, D. A., Westerling, L., Swetnam, T. W., Bigio, E., & Hall, M. H. (2016, January). Historical pyrogeography of western North America, 1600-1900 AD. International AmeriDendro, Mendoza, Argentina.
• Falk, D. A., O'Connor, C. D., Lynch, A. M., & Swetnam, T. W. (2013, May). Spatial and temporal interactions between Spruce beetle outbreaks, fire, and climate in a remnant isolated spruce-fir forest. AmeriDendro: Second American Dendrochronology Conference. Tucson: Tree-Ring Society.
• Falk, D. A., Swetnam, T. W., Bigio, E., Hall, M. A., Velasquez, E., Kitzberger, T., Brown, P. M., & Sutherland, E. K. (2013, May). A new North American fire scar network for reconstructing historical pyrogeography. AmeriDendro: Second American Dendrochronology Conference. Tucson, AZ: Tree-Ring Society.
• Lynch, A. M., O'Connor, C. D., Falk, D. A., Wilcox, C. P., & Swetnam, T. W. (2013, February). 1660 to 2010: Origin and nature of the Pinaleno high elevation forests.. Regional Meeting, Southwest Society of American Foresters,. Willcox, AZ: Southwest Society of American Foresters.
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  .
• Lynch, A. M., O'Connor, C. D., Falk, D. A., Wilcox, C. P., & Swetnam, T. W. (2013, June). Unstable times: fire, insect, climate, and human interactions in Arizona's Sky Island forests.. Rocky Mountain Research Station, Leadership Team. Flagstaff, AZ: USDA Forest Service, Rocky Mountain Research Station.

Poster Presentations

• Lynch, A. M., O'Connor, C. D., Swetnam, T. W., & Falk, D. A. (2013, March). Using dendrochronology to characterize forest insect outbreak regimes.. Bryant Bannister Tree-Ring Building Dedication and Public Open House. University of Arizona: Laboratory of Tree-Ring Research.
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  Poster presentation and laboratory presentations at the Bryant Bannister Tree-Ring Building Dedication and Public Open House, Laboratory of Tree-Ring Research, to approx. 1200 visitors.
• Dewar, J. J., Falk, D. A., Allen, C. D., Parmenter, R., Swetnam, T. W., & Baisan, C. H. (2011, April). Fire History of Montane Grasslands and Ecotones of the Valles Caldera, New Mexico, USA.. Annual Meeting, Association of American Geographers,. Seattle, WA: Association of American Geographers.

Reviews

• Allen, C., Savage, M., Falk, D., Suckling, K., Swetnam, T., Schulke, T., Stacey, P., Morgan, P., Hoffman, M., & Klingel, J. (2009. Ecological restoration of Southwestern ponderosa pine ecosystems: A broad perspective(pp 1418-1433).
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  The purpose of this paper is to promote a broad and flexible perspective on ecological restoration of Southwestern (U.S.) ponderosa pine forests. Ponderosa pine forests in the region have been radically altered by Euro-American land uses, including livestock grazing, fire suppression, and logging. Dense thickets of young trees now abound, old-growth and biodiversity have declined, and human and ecological communities are increasingly vulnerable to destructive crown fires. A consensus has emerged that it is urgent to restore more natural conditions to these forests. Efforts to restore Southwestern forests will require extensive projects employing varying combinations of young-tree thinning and reintroduction of low-intensity fires. Treatments must be flexible enough to recognize and accommodate: high levels of natural heterogeneity; dynamic ecosystems; wildlife and other biodiversity considerations; scientific uncertainty; and the challenges of on-the-ground implementation. Ecological restoration should reset ecosystem trends toward an envelope of "natural variability," including the reestablishment of natural processes. Reconstructed historic reference conditions are best used as general guides rather than rigid restoration prescriptions. In the long term, the best way to align forest conditions to track ongoing climate changes is to restore fire, which naturally correlates with current climate. Some stands need substantial structural manipulation (thinning) before fire can safely be reintroduced. In other areas, such as large wilderness and roadless areas, fire alone may suffice as the main tool of ecological restoration, recreating the natural interaction of structure and process. Impatience, overreaction to crown fire risks, extractive economics, or hubris could lead to widespread application of highly intrusive treatments that may further damage forest ecosystems. Investments in research and monitoring of restoration treatments are essential to refine restoration methods. We support the development and implementation of a diverse range of scientifically viable restoration approaches in these forests, suggest principles for ecologically sound restoration that immediately reduce crown fire risk and incrementally return natural variability and resilience to Southwestern forests, and present ecological perspectives on several forest restoration approaches.
• Bowman, D. M., Balch, J. K., Artaxo, P., Bond, W. J., Carlson, J. M., Cochrane, M. A., D'Antonio, C. M., DeFries, R. S., Doyle, J. C., Harrison, S. P., Johnston, F. H., Keeley, J. E., Krawchuk, M. A., Kull, C. A., Marston, J. B., Moritz, M. A., Prentice, I. C., Roos, C. I., Scott, A. C., , Swetnam, T. W., et al. (2009. Fire in the Earth System(pp 481-484).
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  Fire is a worldwide phenomenon that appears in the geological record soon after the appearance of terrestrial plants. Fire influences global ecosystem patterns and processes, including vegetation distribution and structure, the carbon cycle, and climate. Although humans and fire have always coexisted, our capacity to manage fire remains imperfect and may become more difficult in the future as climate change alters fire regimes. This risk is difficult to assess, however, because fires are still poorly represented in global models. Here, we discuss some of the most important issues involved in developing a better understanding of the role of fire in the Earth system.
• Falk, D. A., Heyerdahl, E. K., Brown, P. M., Farris, C., Fule, P. Z., McKenzie, D., Swetnam, T. W., Taylor, A. H., & Van Horne, M. L. (2001. Multi-scale controls of historical forest-fire regimes: new insights from fire-scar networks(pp 446-454).
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  Anticipating future forest-fire regimes under changing climate requires that scientists and natural resource managers understand the factors that control fire across space and time. Fire scars - proxy records of fires, formed in the growth rings of long-lived trees - provide an annually accurate window into past low-severity fire regimes. In western North America, networks of the fire-scar records spanning centuries to millennia now include hundreds to thousands of trees sampled across hundreds to many thousands of hectares. Development of these local and regional fire-scar networks has created a new data type for ecologists interested in landscape and climate regulation of ecosystem processes - which, for example, may help to explain why forest fires are widespread during certain years but not others. These data also offer crucial reference information on fire as a dynamic landscape process for use in ecosystem management, especially when managing for forest structure and resilience to climate change.
• Mann, B. R., McMullen, A. R., Swetnam, D. M., & Barrett, A. D. (1993. Molecular Epidemiology and Evolution of West Nile Virus in North America(pp 5111-5129).
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  West Nile virus (WNV) was introduced to New York in 1999 and rapidly spread throughout North America and into parts of Central and South America. Displacement of the original New York (NY99) genotype by the North America/West Nile 2002 (NA/WN02) genotype occurred in 2002 with subsequent identification of a novel genotype in 2003 in isolates collected from the southwestern Unites States region (SW/WN03 genotype). Both genotypes co-circulate to date. Subsequent WNV surveillance studies have confirmed additional genotypes in the United States that have become extinct due to lack of a selective advantage or stochastic effect; however, the dynamic emergence, displacement, and extinction of multiple WNV genotypes in the US from 1999-2012 indicates the continued evolution of WNV in North America.
• FRITTS, H., & SWETNAM, T. (1990. DENDROECOLOGY - A TOOL FOR EVALUATING VARIATIONS IN PAST AND PRESENT FOREST ENVIRONMENTS(pp 111-188).
• Romme, W. H., Allen, C. D., Balley, J. D., Baker, W. L., Bestelmeyer, B. T., Brown, P. M., Eisenhart, K. S., Floyd, M. L., Huffman, D. W., Jacobs, B. F., Miller, R. F., Muldavin, E. H., Swetnam, T. W., Tausch, R. J., & Weisberg, P. J. (1990. Historical and Modern Disturbance Regimes, Stand Structures, and Landscape Dynamics in Pinon-Juniper Vegetation of the Western United States(pp 203-222).
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  Pinon-juniper is a major vegetation type in western North America. Effective management of these ecosystems has been hindered by inadequate understanding of 1) the variability in ecosystem structure and ecological processes that exists among the diverse combinations of pinons, junipers, and associated shrubs, herbs, and soil organisms; 2) the prehistoric and historic disturbance regimes; and 3) the mechanisms driving changes in vegetation structure and composition during the past 150 yr. This article summarizes what we know (and don't know) about three fundamentally different kinds of pinon-juniper vegetation. Persistent woodlands are found where local soils, climate, and disturbance regimes are favorable for pinon, juniper, or a mix of both; fires have always been infrequent in these woodlands. Pinon juniper savannas are found where local soils and climate are suitable for both trees and grasses; it is logical that low-severity fires may have maintained low tree densities before disruption of fire regimes following Euro-American settlement, but information is insufficient to support any confident statements about historical disturbance regimes in these savannas. Wooded shrublands are found where local soils and climate support a shrub community, but trees can increase during moist climatic conditions and periods without disturbance and decrease during droughts and following disturbance. Dramatic increases in tree density have occurred in portions of all three types of pinon-juniper vegetation, although equally dramatic mortality events have also occurred in some areas. The potential mechanisms driving increases in tree density-such as recovery from past disturbance, natural range expansion, livestock grazing, fire exclusion, climatic variability, and CO, fertilization-generally have not received enough empirical or experimental investigation to predict which is most important in any given location. The intent of this synthesis is 1) to provide a Source of information for managers and policy makers; and 2) to stimulate researchers to address the most important unanswered questions.
• Swetnam, T., & Betancourt, J. (1989. Mesoscale disturbance and ecological response to decadal climatic variability in the American Southwest(pp 3128-3147).
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  Ecological responses to climatic variability in the Southwest include regionally synchronized fires, insect outbreaks, and purses in tree demography (births and deaths). Multicentury, tree-ring reconstructions of drought, disturbance history, and tree demography reveal climatic effects across scales, from annual to decadal, and from local (

Others

• Swetnam, T. (2011). Tree Rings and Climate: Sharpening the Focus.
• Swetnam, T. (2010). Mesoscale Disturbance and Ecological Response to Decadal Climatic Variability in the American Southwest.
• Parsons, D., Swetnam, T., & Christensen, N. (2006, NOV). Uses and limitations of historical variability concepts in managing ecosystems. ECOLOGICAL APPLICATIONS.
• Stephens, S. L., Agee, J. K., Fule, P. Z., North, M. P., Romme, W. H., Swetnam, T. W., & Turner, M. G. (2002, OCT 4). Managing Forests and Fire in Changing Climates. SCIENCE.
• SAVAGE, M., & SWETNAM, T. (2001, DEC). EARLY 19TH-CENTURY FIRE DECLINE FOLLOWING SHEEP PASTURING IN A NAVAJO PONDEROSA PINE FOREST. ECOLOGY.
• DIETERICH, J., & SWETNAM, T. (1996). DENDROCHRONOLOGY OF A FIRE-SCARRED PONDEROSA PINE. FOREST SCIENCE.