Forests provide humans with economically important and often irreplaceable ecosystem products and services, such as clean water, wood, biodiversity, and recreational opportunities.  Additionally, forests sequester the majority of the terrestrial biosphere’s carbon, making them key components of the global carbon cycle.  However, ongoing global changes -- particularly interacting changes in climate, land use, and disturbance regimes -- have the potential to rapidly alter forests in undesirable ways, such as by converting forest to shrubland as a result of catastrophic wildfire.  For society to anticipate and mitigate these increasing threats to forests, we must greatly improve our ability to understand and predict forest responses to environmental changes.  Yet such efforts currently are severely limited by our poor understanding of many of the basic processes that drive forest dynamics.

Project Details

In 2010 we will focus on accomplishing research tasks in five broad areas.  First, and perhaps most important, we will continue our efforts to better pinpoint causes of the rapid, apparently temperature-induced increase in tree mortality rates across the West over the last few decades (which we reported in 2009 in a high-profile article in Science magazine).  Specifically, we will work with Craig Allen (USGS) and Nate McDowell (DOE) to process and analyze stable carbon isotope trends in individual tree rings from living and dead trees, which we sampled in 2009 in five Western states.  Stable isotopes of carbon provide a proxy for determining whether trees across the West have indeed experienced increasing temperature-induced drought stress over the last few decades.

                Second, we will complete our efforts (begun in 2009) to model tree mortality rates in California's Sierra Nevada under a range of future climate-change scenarios.

                Third, we will continue to work with our collaborators and West-wide longitudinal database of nearly 60,000 trees to further explore some of the correlates and causes of the widespread increase in tree mortality rates.  For example, our questions include the following.  In water-limited forests, has the increase in mortality rates been paralleled by a decrease in tree growth rates?  What about energy-limited forests?  Mortality rates have increased more rapidly in some tree taxa than others -- has this led to significant changes in community composition?

        Fourth, we will collect annual forest demographic data in our network of long-term study plots arrayed along a climatic (elevational) gradient in the Sierra Nevada.  In so doing, we will maintain the world's longest annual-resolution data set on forest dynamics (1982 to the present) -- the "seed corn" that has led to the discoveries and research thrusts described above.  Our field work entails revisiting each of about 20,000 trees to determine which have died, and the associated causes of death.

                Finally, we will continue our efforts to help the National Park Service and other land managers plan adaptations to climatic change.  This will include written products, several presentations, and multiple consultations.

USGS Contact For This Project