Sierra Nevada Forest Dynamics: Pattern, Pace, and Mechanisms of Change

Duration:  February 1999 - September 2003

Global climatic change may have far-reaching effects on forests, and hence on society.  Forests provide us with economically important and often irreplaceable products and services, such as watersheds, wood, fiber, biodiversity, and recreation.  Additionally, forests sequester the majority of the terrestrial biosphere's carbon, making them key components of the global carbon cycle and key contributors of biological feedbacks to global climatic change.

The program focuses not just on the direct effects of global changes, but also on indirect effects, predominately those that occur through altered fire regimes.  The program is organized around three themes: contemporary ecology, paleoecology, and modeling.

Contemporary Ecology

The contemporary ecology theme takes advantage of the Sierra Nevada's strong climatic gradients as "natural experiments," allowing researchers to evaluate climatic mechanisms controlling forest composition, structure, and dynamics.

Elevation in the Sierra Nevada rises from near sea level to 4,418 meters in less than 100 kilometers horizontal distance. A steep temperature gradient parallels the elevational gradient, and is overlain by a gradient of decreasing precipitation from west to east. These climatic gradients combine with highly variable soils and topography to create landscapes with an extraordinary variety of local environments.

Team leaders for contemporary ecology are Dr. Nate Stephenson, Dr. Jon Keeley, and Dr. Jan van Wagtendonk of the U.S. Geological Survey.  Specific questions to be addressed include:

• What is the role and importance of reproduction in determining forest pattern and forest sensitivity to climatic change?
• What is the relative importance of tree recruitment, death, and growth rates in determining forest response to climatic variation?
• Can agents of pattern formation and mechanisms of forest change be generalized at sub-continental scales?

Paleoecology

Paleoecology is the study of past environments, species assemblages, and populations through the examination and interpretation of natural records such as tree-rings, charcoal layers in the soil, and pollen trapped in sediment deposits.  The Sierra Nevada region is unique in having at least four tree species from which multi-millennial tree-ring chronologies of climatic change can be derived.  Additionally, fire scars within giant sequoia tree-rings contain annual- and seasonal-resolution fire histories spanning the last several millennia, and charcoal trapped in meadow sediment documents changes in fire regimes spanning the last 10,000 years.  Forest responses to changing climate and fire regimes are recorded in the age structures of existing forests and in the woody remnants of past forests (such as "ghost forests" of dead trees above the present treeline), and in multi-millennial records of pollen trapped in meadow sediments.

Team leaders for paleoecology are Dr. Tom Swetnam, University of Arizona, and Dr. Lisa Graumlich, Montana State University.  Specific questions to be addressed include:

• How does climatic change affect the spatial extent, landscape pattern, and severity of wildland fires?
• Does climate synchronize fire regimes at subcontinental scales?  If so, how?
• Can agents of pattern formation and mechanisms of forest change be generalized at subcontinental scales?

Modeling

Computer models serve as an integrative framework for research findings from the contemporary and paleoecological themes.  Additionally, modeling is an indispensable tool for scaling up site-specific research results to entire landscapes.

The team leader for modeling is Dr. Dean Urban, Duke University.  Specific questions include:

• What is the relative importance of topography and soils in determining conditions suitable for different tree species in the Sierra Nevada?
• How do seed dispersal, seedling dynamics, and fine-scale variations in topography and soils interact with climatic change to affect forest sensitivity to change on a local scale?
• What portions of Sierra Nevada landscapes are most sensitive to climatic changes, and what are the implications for land managers?
• How does the relative importance of different agents of forest pattern formation vary among different climates at continental scales?

Products

Our work to date has demonstrated that the last 50 years in California have been among the wettest of the last millennium.  Multi-decadal droughts of much greater length and severity than any experienced in California during the last century have occurred regularly in the past.  These findings have served as an abrupt wake-up call for California water resource planners. 

The program's fire history reconstructions are now used by land managers up and down the Sierra Nevada as a target for restoring pre-Euroamerican fire regimes to forests suffering the effects of a century of fire exclusion.  Investigations into the effects of fire regimes on forest pattern and dynamics have led to modifications in both prescribed fire and timber harvesting approaches in the Sierra Nevada. 

The FARSITE fire behavior and spread model, initiated as part of the program, has become the most widely-used fire model by North American land managers, giving managers a valuable tool for planning prescribed fires and for predicting the spread of wildfires.

The program has also supplied resource managers with a means of using simple tree measurements to predict forest fuel accumulation in the fire-prone forests of the Sierra Nevada.

	Primary Contact: Dr. Nathan L. Stephenson, USGS, Western Ecological Research Center E-mail Dr. Stephenson