Dr. Nathan Stephenson is a Scientist Emeritus at the Western Ecological Research Center.
Earth’s vast forests provide human communities with irreplaceable goods and services such as carbon sequestration, hydrologic regulation, clean water, biodiversity, critical wildlife habitat, wood products, and recreational and spiritual opportunities. An overarching goal of Dr. Nathan Stephenson's research is to improve scientists' and land managers' ability to understand, forecast, and adapt to the effects of ongoing global changes – particularly changing climatic and disturbance regimes – on forests.
Accordingly, most of his research falls in three broad, complementary themes: (1) improving mechanistic understanding of forest and carbon dynamics, (2) detection, attribution, and interpretation of forest changes, and (3) adaptations to rapid global changes. The last theme extends well beyond forests, to natural areas in general.
RESEARCH INTERESTS
- Forest ecology
- Global change biology
- Climate change
- Fire ecology
- Natural areas management
- Adaptation
EDUCATION
- Ph.D., Ecology and Systematics, Cornell University, 1988
- B.S., Biological Sciences, University of California, Irvine, 1979
Science and Products
Forest health and drought response
Post-Fire Conifer Regeneration Under a Warming Climate: Will Severe Fire Be a Catalyst for Forest Loss?
Adaptations to Rapid Change
Improving Understanding of Forest and Carbon Dynamics
Detection, Attribution, and Interpretation of Forest Changes
Sequoia and Kings Canyon Field Station
Leaf to Landscape: Understanding and Mapping the Vulnerability of Forests to Hotter Droughts
Fighting Drought with Fire: A Comparison of Burned and Unburned Forests in Drought-Impacted Areas of the Southwest
Characterizing a link in the terrestrial carbon cycle: a global overview of individual tree mass growth
Sequoia and Yosemite National Parks Mortality and Fire Data (1990-2019) for Competition-Fire-Drought Interaction Analysis
Data for Use in poscrptR Post-fire Conifer Regeneration Prediction Model
Forest Structure Data for Burned and Unburned Sites at Sequoia and Kings Canyon National Parks
Post-fire conifer regeneration observations for National Forest land in California (2009 - 2017)
Seed and Associated Tree Data from Long Term Research Plots in Sequoia and Yosemite national parks
Tree mortality in blue oak woodlands in Sequoia National Park during the extreme drought
Seedling and tree data from Sequoia National Park and Yosemite National Park
Tree mortality in Sequoia National Park from 2004 to 2007 and during severe drought in 2014 to 2017
Sequoia foliage dieback data from Sequoia National Park
Mortality factors for dead trees from a subset of plots from the Sierra Nevada Forest Dynamics Plot Network from 1998 to 2010
Crowding, climate, and the case for social distancing among trees
Forest resistance to extended drought enhanced by prescribed fire in low elevation forests of the Sierra Nevada
Mapping the vulnerability of giant sequoias after extreme drought in California using remote sensing
Continent-wide tree fecundity driven by indirect climate effects
Why is tree drought mortality so hard to predict?
Seed production patterns of surviving Sierra Nevada conifers show minimal change following drought
Effects of postfire climate and seed availability on postfire conifer regeneration
Compounding effects of white pine blister rust, mountain pine beetle, and fire threaten four white pine species
Mortality predispositions of conifers across western USA
Height-related changes in forest composition, not tree vulnerability, explain increasing mortality with height during an extreme drought
The Fire and Tree Mortality Database, for empirical modeling of individual tree mortality after fire
The influence of pre-fire growth patterns on post-fire tree mortality for common conifers in western U.S. parks
Seed source, not drought, determines patterns of seed production in Sierra Nevada conifers
Science and Products
- Science
Forest health and drought response
Forests provide society with economically important and often irreplaceable goods and services, such as wood products, carbon sequestration, clean water, biodiversity, and recreational opportunities. Yet hotter droughts (droughts in which unusually high temperatures exacerbate the effects of low precipitation) are projected to increase in frequency and intensity in coming decades, potentially...Post-Fire Conifer Regeneration Under a Warming Climate: Will Severe Fire Be a Catalyst for Forest Loss?
The Southwest U.S. is experiencing hotter droughts, which are contributing to more frequent, severe wildfires. These droughts also stress vegetation, which can make it more difficult for forests to recover after fire. Forest regeneration in burned areas may be limited because seeds have to travel long distances to recolonize, and when they do arrive, conditions are often unfavorably hot and dry. CAdaptations to Rapid Change
Management decisions are made at the intersection of facts and values, and WERC's role is to assist decision-makers by bringing the best available science to the table. Dr. Nathan Stephenson seeks to help managers and policy makers reassess their missions in light of rapid and unprecedented changes, develop broad concepts relevant to adapting to such changes, and provide hands-on assistance during...Improving Understanding of Forest and Carbon Dynamics
The community's ability to understand and predict changes in forests and their feedbacks to the global carbon cycle increasingly relies on models spanning several scales of biological organization – from tree leaves to entire forested landscapes. Yet many model assumptions about key processes – such as tree growth and mortality – require long-term data that are sometimes difficult and time...Detection, Attribution, and Interpretation of Forest Changes
Dr. Nathan Stephenson and colleagues seek to determine what changes are occurring in forests, why they are occurring, and what they mean. For example, they have documented a long-term, apparently climatically-induced increase of tree mortality rates in otherwise undisturbed old forests across the western U.S., implying that these forests could become net sources of atmospheric carbon dioxide.Sequoia and Kings Canyon Field Station
The Sequoia and Kings Canyon Field Station is home to research programs that focus on wildfire patterns in Southern California, and the effects of drought on Sierra Nevada forests. Select the "Science" tab for a more comprehensive summary.Leaf to Landscape: Understanding and Mapping the Vulnerability of Forests to Hotter Droughts
Forests across the southwestern U.S. are crucial components of recreation and play an important role in state and local economies. Healthy forests also provide needed habitat for many wildlife species and contribute many other important services to our planet. “Hotter droughts” (otherwise normal droughts whose effects on ecosystems are exacerbated by higher temperatures) are an emerging climate ch...Fighting Drought with Fire: A Comparison of Burned and Unburned Forests in Drought-Impacted Areas of the Southwest
Drought is one of the biggest threats facing our forests today. In the western U.S., severe drought and rising temperatures have caused increased tree mortality and complete forest diebacks. Forests are changing rapidly, and while land managers are working to develop long-term climate change adaptation plans, they require tools that can enhance forest resistance to drought now. To address this immCharacterizing a link in the terrestrial carbon cycle: a global overview of individual tree mass growth
Forests sequester the majority of the terrestrial biosphere’s carbon and are key components of the global carbon cycle, potentially contributing substantial feedbacks to ongoing climatic changes. It is therefore remarkable that no consensus yet exists about the fundamental nature of tree mass growth (and thus carbon sequestration rate). Specifically, does tree mass growth rate increase, decrease, - Data
Sequoia and Yosemite National Parks Mortality and Fire Data (1990-2019) for Competition-Fire-Drought Interaction Analysis
This dataset records mortality-- including involvement of bark beetles-- and burn severity information for trees in long term forest dynamics plots in Sequoia National Park and Yosemite National Park that experienced fire. These data support the following publication: Furniss, T.J., Das, A.J., van Mantgem, P.J., Stephenson, N.L. and Lutz, J.A., 2021. Crowding, climate, and the case for sociData for Use in poscrptR Post-fire Conifer Regeneration Prediction Model
These data support poscrptR (Wright et al. 2021). poscrptR is a shiny app that predicts the probability of post-fire conifer regeneration for fire data supplied by the user. The predictive model was fit using presence/absence data collected in 4.4m radius plots (60 square meters). Please refer to Stewart et al. (2020) for more details concerning field data collection, the model fitting process, anForest Structure Data for Burned and Unburned Sites at Sequoia and Kings Canyon National Parks
These data represent tree diameter, species, mortality status, and plot attributes from 164 plots in Sequoia and Kings Canyon national parks, California. Plots were matched with local records of recent fire history to determine burn status and year burned if applicable. These data support the following publication: van Mantgem, P.J., Caprio, A.C., Stephenson, N.L. and Das, A.J., 2021. Forest RePost-fire conifer regeneration observations for National Forest land in California (2009 - 2017)
This data consists of presence/absence observations for post-fire conifer regeneration. The data also includes estimates of plot-level topography (slope, aspect), relativized differenced normalized burn ratio (RdNBR), post-fire climate, live basal area, and seed rain. These data support the following publication:Seed and Associated Tree Data from Long Term Research Plots in Sequoia and Yosemite national parks
This dataset was used as part of a continent-wide analysis of tree fecundity and its association with climate and tree size. This dataset consists of: plotinfo.csv, which contains basic attribute information for the field plots where the data were collected; seeddata.csv, which contains the data for seeds collected (how many, what type, etc.); trapxycoord.csv, which contains location and identificTree mortality in blue oak woodlands in Sequoia National Park during the extreme drought
These data were collected to quantify oak mortality during the 2014-2017 drought. 30 tenth hectare drought plots were established throughout the blue oak woodland of Sequoia National Park. Information on species, size, and mortality were collected for all standing trees in each plot. In addition, two long-term 2.25 ha plots were established, in which the same data were collected.Seedling and tree data from Sequoia National Park and Yosemite National Park
These tables include data from 25 long-term forest plots located in either Sequoia or Yosemite national park. Trees in these plots (established between 1982 and 2001) are censused annually for mortality and measured for diameter every 4 to 6 years. Plots were mostly 1 hectare (ha) in size (range 0.9 - 2.5 ha) and contained at least two 25 by 25 meter seedling sub-plots to monitor natural seedlingTree mortality in Sequoia National Park from 2004 to 2007 and during severe drought in 2014 to 2017
These data describe tree mortality and the factors associated with tree mortality for a variety of plots in Sequoia National Park. Most of the data were collected between 2014 and 2017 (during an extremely severe drought), along with some comparison data from 2004 to 2007. These data support the following publication: Stephenson, N.L., Das, A.J., Ampersee, N.J., Bulaon, B.M., and Yee, J.L.,Sequoia foliage dieback data from Sequoia National Park
These datasets provide spatially-explicit estimates of the magnitude of giant sequoia foliage dieback along selected trail corridors in Sequoia and Kings Canyon national parks, California, from 2014 through 2017. They additionally provide giant sequoia tree-ring measurements, through the year 1989, for two locations in the Giant Forest grove, Sequoia National Park, California.Mortality factors for dead trees from a subset of plots from the Sierra Nevada Forest Dynamics Plot Network from 1998 to 2010
This dataset was used to summarize and analyze the mortality factors recorded on dead trees in the Sierra Nevada Forest Dynamics Plot Network, which is managed by the Sequoia and Kings Canyon Field station of the U.S. Geological Survey's Western Ecological Research Center. Each row of the dataset represents an individual dead tree. These are dead trees that were recorded in the network from 1998 t - Publications
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Crowding, climate, and the case for social distancing among trees
In an emerging era of megadisturbance, bolstering forest resilience to wildfire, insects, and drought has become a central objective in many western forests. Climate has received considerable attention as a driver of these disturbances, but few studies have examined the complexities of climate–vegetation–disturbance interactions. Current strategies for creating resilient forests often rely on retrForest resistance to extended drought enhanced by prescribed fire in low elevation forests of the Sierra Nevada
Prescribed fire reduces fire hazards by removing dead and live fuels (small trees and shrubs). Reductions in forest density following prescribed fire treatments (often in concert with mechanical treatments) may also lessen competition so that residual trees might be more likely to survive when confronted with additional stressors, such as drought. The current evidence for these effects is mixed anMapping the vulnerability of giant sequoias after extreme drought in California using remote sensing
Between 2012 and 2016, California suffered one of the most severe droughts on record. During this period Sequoiadendron giganteum (giant sequoias) in the Sequoia and Kings Canyon National Parks (SEKI), California, USA experienced canopy water content (CWC) loss, unprecedented foliage senescence, and, in a few cases, death. We present an assessment of the vulnerability of giant sequoia populationsContinent-wide tree fecundity driven by indirect climate effects
Indirect climate effects on tree fecundity that come through variation in size and growth (climate-condition interactions) are not currently part of models used to predict future forests. Trends in species abundances predicted from meta-analyses and species distribution models will be misleading if they depend on the conditions of individuals. Here we find from a synthesis of tree species in NorthWhy is tree drought mortality so hard to predict?
Widespread tree mortality following droughts has emerged as an environmentally and economically devastating ‘ecological surprise’. It is well established that tree physiology is important in understanding drought-driven mortality; however, the accuracy of predictions based on physiology alone has been limited. We propose that complicating factors at two levels stymie predictions of drought-drivenSeed production patterns of surviving Sierra Nevada conifers show minimal change following drought
Reproduction is a key component of ecological resilience in forest ecosystems, so understanding how seed production is influenced by extreme drought is key to understanding forest recovery trajectories. If trees respond to mortality-inducing drought by preferentially allocating resources for reproduction, the recovery of the stand to pre-drought conditions may be enhanced accordingly. We used a 20Effects of postfire climate and seed availability on postfire conifer regeneration
Large, severe fires are becoming more frequent in many forest types across the western United States and have resulted in tree mortality across tens of thousands of hectares. Conifer regeneration in these areas is limited because seeds must travel long distances to reach the interior of large burned patches and establishment is jeopardized by increasingly hot and dry conditions. To better inform pCompounding effects of white pine blister rust, mountain pine beetle, and fire threaten four white pine species
Invasive pathogens and bark beetles have caused precipitous declines of various tree species around the globe. Here, we characterized long‐term patterns of mountain pine beetle (Dendroctonus ponderosae; MPB) attacks and white pine blister rust, an infectious tree disease caused by the pathogen, Cronartium ribicola. We focused on four dominant white pine host species in Sequoia and Kings Canyon NatMortality predispositions of conifers across western USA
Conifer mortality rates are increasing in western North America, but the physiological mechanisms underlying this trend are not well understood.We examined tree‐ring‐based radial growth along with stable carbon (C) and oxygen (O) isotope composition (δ13C and δ18O, respectively) of dying and surviving conifers at eight old‐growth forest sites across a strong moisture gradient in the western USA toHeight-related changes in forest composition, not tree vulnerability, explain increasing mortality with height during an extreme drought
Recently, Stovall et al.1 (hereafter SSY) showed that during an extreme drought, remotely sensed mortality of tall trees was more than double that of short trees. They interpreted this to be a consequence of inherently greater hydraulic vulnerability of tall trees, and suggested that tall-tree vulnerability should thus generalize more broadly. Here we reassess their conclusions using contemporanThe Fire and Tree Mortality Database, for empirical modeling of individual tree mortality after fire
Wildland fires have a multitude of ecological effects in forests, woodlands, and savannas across the globe. A major focus of past research has been on tree mortality from fire, as trees provide a vast range of biological services. We assembled a database of individual-tree records from prescribed fires and wildfires in the United States. The Fire and Tree Mortality (FTM) database includes recordsThe influence of pre-fire growth patterns on post-fire tree mortality for common conifers in western U.S. parks
Fire severity in forests is often defined in terms of post-fire tree mortality, yet the influences on tree mortality following fire are not fully understood. For trees that are not killed immediately by severe fire injury, pre-fire growth may partially predict post-fire mortality probabilities for conifers of the western U.S. Here, we consider the influence of multiple growth patterns on post-fire - News
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- Software
Seed source, not drought, determines patterns of seed production in Sierra Nevada conifers
This release consists of data collected from 26 plots in two national parks over a 19-year period. The data consists of plot-level seed counts for three genera, number of seed traps, live tree basal area, plot area, and climate metrics from the gridmet gridded data set, the daymet gridded data set, the PRISM gridded data set, and two nearby COOP stations.