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.
- Forest ecology
- Global change biology
- Climate change
- Fire ecology
- Natural areas management
- Ph.D., Ecology and Systematics, Cornell University, 1988
- B.S., Biological Sciences, University of California, Irvine, 1979
Science and Products
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 adaptation planning.
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-consuming to get. Dr. Nathan Stephenson partners with multiple Federal and state agencies, universities, and non-profit organizations to provide this information to scientists and the public.
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.
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.
Patterns and correlates of giant sequoia foliage dieback during California’s 2012–2016 hotter drought
Hotter droughts – droughts in which unusually high temperatures exacerbate the effects of low precipitation – are expected to increase in frequency and severity in coming decades, challenging scientists and managers to identify which parts of forested landscapes may be most vulnerable. In 2014, in the middle of California’s historically...Stephenson, Nathan L.; Das, Adrian J.; Ampersee, Nicholas J.; Cahill, Kathleen G.; Caprio, Anthony C.; Sanders, John E.; Williams, A. Park
What mediates tree mortality during drought in the southern Sierra Nevada?
Severe drought has the potential to cause selective mortality within a forest, thereby inducing shifts in forest species composition. The southern Sierra Nevada foothills and mountains of California have experienced extensive forest dieback due to drought stress and insect outbreak. We used high-fidelity imaging spectroscopy (HiFIS) and light...Paz-Kagan, Tarin; Brodrick, Philip; Vaughn, Nicholas R; Das, Adrian J.; Stephenson, Nathan L.; Nydick, Koren R.; Asner, Gregory P.
Seasonal and diel environmental conditions predict western pond turtle (Emys marmorata) behavior at a perennial and an ephemeral stream in Sequoia National Park, California
Managers making decisions may benefit from a well-informed understanding of a species' population size and trends. Given the cryptic nature and habitat characteristics of the western pond turtle (Emys marmorata), however, imperfect detection may be high and population estimates are frequently varied and unreliable. As a case study to investigate...Ruso, Gabrielle; Meyer, Erik; Das, Adrian J.
Why do trees die? Characterizing the drivers of background tree mortality
The drivers of background tree mortality rates—the typical low rates of tree mortality found in forests in the absence of acute stresses like drought—are central to our understanding of forest dynamics, the effects of ongoing environmental changes on forests, and the causes and consequences of geographical gradients in the nature and strength of...Das, Adrian J.; Stephenson, Nathan L.; Davis, Kristin P.
Does prescribed fire promote resistance to drought in low elevation forests of the Sierra Nevada, California, USA?
Prescribed fire is a primary tool used to restore western forests following more than a century of fire exclusion, reducing fire hazard by removing dead and live fuels (small trees and shrubs). It is commonly assumed that the reduced forest density following prescribed fire also reduces competition for resources among the remaining trees, so...van Mantgem, Phillip J.; Caprio, Anthony C.; Stephenson, Nathan L.; Das, Adrian J.
This long-anticipated reference and sourcebook for California’s remarkable ecological abundance provides an integrated assessment of each major ecosystem type—its distribution, structure, function, and management. A comprehensive synthesis of our knowledge about this biologically diverse state, Ecosystems of California covers the state...North, Malcolm P.; Collins, Brandon M.; Safford, Hugh D.; Stephenson, Nathan L.
Tree mortality from drought, insects, and their interactions in a changing climate
Climate change is expected to drive increased tree mortality through drought, heat stress, and insect attacks, with manifold impacts on forest ecosystems. Yet, climate-induced tree mortality and biotic disturbance agents are largely absent from process-based ecosystem models. Using data sets from the western USA and associated studies, we present...Anderegg, William R. L.; Hicke, Jeffrey A.; Fisher, Rosie A.; Allen, Craig D.; Aukema, Juliann E.; Bentz, Barbara; Hood, Sharon; Lichstein, Jeremy W.; Macalady, Alison K.; McDowell, Nate G.; Pan, Yude; Raffa, Kenneth; Sala, Anna; Shaw, John D.; Stephenson, Nathan L.; Tague, Christina L.; Zeppel, Melanie
Temperate forest health in an era of emerging megadisturbance
Although disturbances such as fire and native insects can contribute to natural dynamics of forest health, exceptional droughts, directly and in combination with other disturbance factors, are pushing some temperate forests beyond thresholds of sustainability. Interactions from increasing temperatures, drought, native insects and pathogens, and...Millar, Constance I.; Stephenson, Nathan L.
Increasing elevation of fire in the Sierra Nevada and implications for forest change
Fire in high-elevation forest ecosystems can have severe impacts on forest structure, function and biodiversity. Using a 105-year data set, we found increasing elevation extent of fires in the Sierra Nevada, and pose five hypotheses to explain this pattern. Beyond the recognized pattern of increasing fire frequency in the Sierra Nevada since the...Schwartz, Mark W.; Butt, Nathalie; Dolanc, Christopher R.; Holguin, Andrew; Moritz, Max A.; North, Malcolm P.; Safford, Hugh D.; Stephenson, Nathan L.; Thorne, James H.; van Mantgem, Phillip J.
Improving estimates of tree mortality probability using potential growth rate
Tree growth rate is frequently used to estimate mortality probability. Yet, growth metrics can vary in form, and the justification for using one over another is rarely clear. We tested whether a growth index (GI) that scales the realized diameter growth rate against the potential diameter growth rate (PDGR) would give better estimates of mortality...Das, Adrian J.; Stephenson, Nathan L.
Making the transition to the third era of natural resources management
We are entering the third era of National Park Service (NPS) natural resources management— an era defined by rapid and unprecedented global changes. This third era promises to overturn not only some of our most fundamental assumptions about parks and protected areas, but also many of the ideals we currently hold dear. A common initial...Stephenson, Nathan L.
Beyond reducing fire hazard: fuel treatment impacts on overstory tree survival
Fuel treatment implementation in dry forest types throughout the western United States is likely to increase in pace and scale in response to increasing incidence of large wildfires. While it is clear that properly implemented fuel treatments are effective at reducing hazardous fire potential, there are ancillary ecological effects that can impact...Collins, Brandon M.; Das, Adrian J.; Battles, John J.; Fry, Danny L.; Krasnow, Kevin D.; Stephens, Scott L.
SEQUOIA NATIONAL PARK, California — Nate Stephenson was a guest on NPR's "All Things Considered." The segment featured collaborative research on the drought's effects on giant sequoias.
California's hotter droughts are a preview of a warmer future world.