
Thomas William SwetnamUniversity of Arizona | UA · Laboratory of Tree-Ring Research
Thomas William Swetnam
PhD Watershed Management
About
251
Publications
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Introduction
Tom Swetnam studies changes in climate and forest disturbances using dendrochronology. He has worked extensively on wildfire history and ecology in pine and giant sequoia forests of the western U.S., Mexico and South America, and he is currently studying fire, climate and carbon dynamics in central Siberia and the Southwestern U.S. He is Regents’ Professor Emeritus of Dendrochronology at the Laboratory of Tree-Ring Research, University of Arizona.
Additional affiliations
March 2014 - March 2014
August 1980 - present
Education
August 1980 - May 1987
August 1974 - December 1977
Publications
Publications (251)
Prior research suggests that Indigenous fire management buffers climate influences on wildfires, but it is unclear whether these benefits accrue across geographic scales. We use a network of 4824 fire-scarred trees in Southwest United States dry forests to analyze up to 400 years of fire-climate relationships at local, landscape, and regional scale...
Fire regimes in North American forests are diverse and modern fire records are often too short to capture important patterns, trends, feedbacks, and drivers of variability. Tree‐ring fire scars provide valuable perspectives on fire regimes, including centuries‐long records of fire year, season, frequency, severity, and size. Here, we introduce the...
Every year, millions of hectares burn across Siberia, driven by a combination of warming temperatures, regional drought and human-caused ignitions. Dendrochronology provides a long-term context to evaluate recent trends in fire activity and interpret the relative influence of humans and climate drivers on fire regimes. We developed a 400-year recor...
Implementation of wildfire‐ and climate‐adaptation strategies in seasonally dry forests of western North America is impeded by numerous constraints and uncertainties. After more than a century of resource and land use change, some question the need for proactive management, particularly given novel social, ecological, and climatic conditions. To ad...
Context
Montane grasslands and forest-grassland ecotones are unique and dynamic components of many landscapes, but the processes that regulate their dynamics are difficult to observe over ecologically relevant time spans.
Objectives
We aimed to demonstrate the efficacy of using grassland-forest ecotone trees to reconstruct spatial and temporal pro...
Significance
As residential development continues into flammable landscapes, wildfires increasingly threaten homes, lives, and livelihoods in the so-called “wildland–urban interface,” or WUI. Although this problem seems distinctly modern, Native American communities have lived in WUI contexts for centuries. When carefully considered, the past offer...
Supplementary Information for:
Roos, Christopher I., Thomas W. Swetnam, T.J. Ferguson, Matthew J. Liebmann, Rachel A. Loehman, John R. Welch, Ellis Q. Margolis, Christopher H. Guiterman, William C. Hockaday, Michael J. Aiuvalasit, Jenna Battillo, Joshua Farella, and Christopher A. Kiahtipes (2021). Native American Fire Management at an Ancient Wil...
Here, we show that the last century of fire suppression in the western U.S. has resulted in fire intensities that are unique over more than 900 years of record in ponderosa pine forests (Pinus ponderosa). Specifically, we use the heat-sensitive luminescence signal of archaeological ceramics and tree-ring fire histories to show that a recent fire du...
The iconic Plaza Tree of Pueblo Bonito is widely believed to have been a majestic pine standing in the west courtyard of the monumental great house during the peak of the Chaco Phenomenon (AD 850–1140). The ponderosa pine ( Pinus ponderosa ) log was discovered in 1924, and since then, it has been included in “birth” and “life” narratives of Pueblo...
The climate of the south-western United States and northern Mexico borderlands is marked by a bimodal precipitation regime with the majority of moisture arriving during the cool season via Pacific frontal storm systems, and intense convective storms during the North American Monsoon (NAM). The fire season occurs primarily during the arid foresummer...
Unraveling the effects of climate and land use on historical fire regimes provides important insights into broader human–fire–climate dynamics, which are necessary for ecologically based forest management. We developed a spatial human land‐use model for Navajo Nation forests across which we sampled a network of tree‐ring fire history sites to refle...
The primacy of past human activity in triggering change in earth’s ecosystems remains a contested idea. Treating human-environmental dynamics as a dichotomous phenomenon – turning “on” or “off” at some tipping point in the past – misses the broader, longer-term, and varied role humans play in creating lasting ecological legacies. To investigate the...
Extensive high-severity fires are creating large shrubfields in many dry conifer forests of the interior western USA, raising concerns about forest-to-shrub conversion. This study evaluates the role of disturbance in shrubfield formation, maintenance and succession in the Jemez Mountains, New Mexico. We compared the environmental conditions of exta...
Predicting wildfire under future conditions is complicated by complex interrelated drivers operating across large spatial scales. Annual area burned (AAB) is a useful index of global wildfire activity. Current and antecedent seasonal climatic conditions, and the timing of snowpack melt, have been suggested as important drivers of AAB. As climate wa...
Temporal trends in instrumental seasonal precipitation (1972–2006).
Left panels: trend magnitude based on Theil-Sen median slope estimator for JFM, AMJ, JAS respectively. Right panels: trend significance based on Mann-Kendall test. Cool colors indicate increasing precipitation; warm colors indicate decreasing precipitation.
(TIF)
General Circulation Models and runs used for the A1B emission scenario ensemble.
(DOCX)
Spatial correlation of PC5 (preceding year spring temperature, positive correlation with AAB, 11% variance in AAB explained) from PCA analysis of z coefficients of a complete multiple regression model in each grid cell.
Red (blue) colors indicate increases (decreases) in log-transformed AAB.
(TIF)
Wildfire is increasingly a concern in the USA, where 10 million acres burned in 2015. Climate is a primary driver of wildfire, and understanding fire-climate relationships is crucial for informing fire management and modeling the effects of climate change on fire. In the southwestern USA, fire-climate relationships have been informed by tree-ring d...
This data publication includes tree measurements taken from 2008-2013 across a gradient of forest types in the Pinaleño Mountains in southeastern Arizona, USA. Tree data include: species, pith date, and last recorded fire date. These data were collected as part of the USDA Forest Service, Rocky Mountain Research Station (RMRS) Growth and Demography...
In recent decades, warming temperatures and severe drought have contributed to large and severe wildfires in the south-western United States. To put current wildfires in a long-term context, we reconstructed fire events with alluvial stratigraphy methods in south-western Colorado, and compared with paleoclimate records over the late Holocene. The c...
Fire is returning to many conifer dominated forests where species composition and structure have been altered by fire exclusion. Ecological effects of these fires are influenced strongly by the degree of forest change during the fire-free period. Response of fire-adapted species assemblages to extended fire-free intervals is highly variable, even i...
Interannual climate variations have been important drivers of wildfire occurrence in ponderosa pine forests across western North America for at least 400 years, but at finer scales of mountain ranges and landscapes human land uses sometimes over-rode climate influences. We reconstruct and analyse effects of high human population densities in forest...
Quantifying historical fire regimes provides important information for managing contemporary forests. Historical fire frequency and severity can be estimated using several methods; each method has strengths and weaknesses and presents challenges for interpretation and verification. Recent efforts to quantify the timing of historical high-severity f...
Additional age structure plots with fire history Illustration of the range of age structure and possible stand age values associated with frequent fire regimes (Figure A).
Black lines represent fire years and green lines represent tree establishment dates (= sample date–breast height age–correction factor [5 years a-h, 8 years i-l]). Red vertical l...
Significance
Debates about the magnitude, tempo, and ecological effects of Native American depopulation after 1492 CE constitute some of the most contentious issues in American Indian history. Was population decline rapid and catastrophic, with effects extensive enough to change even the earth’s atmosphere? Or was depopulation more moderate, with i...
In ponderosa pine and mixed conifer ecosystems of the Southwestern US, regional-scale climate tends to synchronize fire years among study sites and increase fire extent or severity within a forest. At landscape scales (1-100km2), fire frequency and severity may also be influenced by local-scale differences in elevation and aspect, including fire ba...
Significance
The iconic great houses of Chaco Canyon occupy a nearly treeless landscape and yet were some of the largest pre-Columbian structures in North America. This incongruity has sparked persistent debate over the origins of more than 240,000 trees used in construction. We used tree-ring methods for determining timber origins for the first ti...
Recent high-severity fires in pine-oak forests of the southwestern United States are creating shrubfields that may persist for decades to centuries. Shrubfields embedded in conifer forests that pre-date documentary records are potential evidence of older high-severity fire patches, and may therefore provide insights into the occurrence and extent o...
AimThe purpose of this study was to examine the influence of moisture and fire on historical ponderosa pine (Pinus ponderosa Dougl. ex Laws.) age structure patterns.LocationWe used a natural experiment created over time by the unique desert island geography of southern Arizona.Methods
We sampled tree establishment dates in two sites on Rincon Peak...
We related measurements of annual burned area in the southwest United States during 1984–2013 to records of climate variability. Within forests, annual burned area correlated at least as strongly with spring–summer vapour pressure deficit (VPD) as with 14 other drought-related metrics, including more complex metrics that explicitly represent fuel m...
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 Pinaleño Mou...
In 2011, exceptionally low atmospheric moisture content combined with moderately high temperatures to produce 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...
In our 2011 synthesis (Bowman et al., 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 d...
Forests and communities are now extremely vulnerable to large, severe fires during droughts as a consequence of fire exclusion and other land use practices. The extent to which this vulnerability is influenced by climate and land-use remains unclear. Multi-century case studies reveal that certain types of fire are critical for maintaining the healt...
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 tempe...
With projected climate change, we expect to face much more forest fire in the coming decades. Policymakers are challenged not to categorize all fires as destructive to ecosystems simply because they have long flame lengths and kill most of the trees within the fire boundary. Ecological context matters: In some ecosystems, high-severity regimes are...
During the summer of 1910 large wildfires occurred throughout the western United States, and especially in the Northern Rocky Mountains. The “Great Idaho Fires” of 1910, alone burned about three million acres (~1.2 Mha)—an area that is approximately the size of Connecticut. Multiple fires ignited and coalesced, burning in forests of northern Idaho...
Background/Question/Methods
Drought and insects such as bark beetles frequently interact to produce widespread tree mortality in forest ecosystems. However, our ability to project future forest dynamics is strongly limited because the processes and thresholds leading to tree death are poorly constrained. Tree allocation to defense may be a key el...
Interactions among site conditions, disturbance events, and climate determine the patterns of forest species recruitment and mortality across a landscape. Forests of the American Southwest have undergone significant changes over a century of altered disturbance regimes, human land uses, and changing environmental conditions. Forests located along s...
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 fore...
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 100...
Reconstructions of dry western U.S. forests in the late 19th century in Arizona, Colorado, and Oregon based on General Land Office records were used by Williams and 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 c...
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 frequ...
Climate change causes an increase of extreme weather events such as droughts that can trigger processes of forest decline, especially in drought prone areas such as the southern limit of distribution of tree species. The vulnerability of Mediterranean mountain plantations to drought is conditioned by this climatic variability, which ultimately affe...
A new study has
identified the factors that cause deterioration of Mediterranean
pine plantations, to
develop a model of that can predict the risk of forest decline
to help inform forests management strategies under a changing climate. It
suggests that loss of needles is the most useful predictor of decline for the species
studied.
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 a...
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 futur...
Understanding the role of climate variation in governing fire regimes remains one of the central needs in contemporary fire science and management. Ideally, this understanding should encompass both historical and current fire-climatology, and inform both basic science and ecosystem management. In this project, Fire and Climate Synthesis (FACS) we u...
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-rep...
Droughts in the early 1950s and early 2000s significantly accelerated
tree mortality rates in the Southwestern United States. During the early
2000s, forest inventory data indicate that the proportion of dead
piñon pine, ponderosa pine, and Douglas-fir trees doubled in the
Southwest. The 2000s drought peaked in 2002 and was the most severe
drought...