Executive Summary 2002

Introduction

The Sierra Nevada Global Change Research Program began in 1991 as a peer-reviewed, competitively-funded component of the National Park Service's (now USGS-BRD's) Global Change Research Program. While Sequoia, Kings Canyon, and Yosemite national parks form the core study areas of the program, the full study region encompasses adjacent public lands. 

    The goal of the Sierra Nevada Global Change Research Program is to understand and predict the effects of global changes on montane forests. By far the greatest limitation to understanding and predicting the effects of future global changes is the lack of a precise mechanistic understanding of how contemporary forest structure and function are controlled by the physical environment, disturbances, and biotic processes. Our research program therefore places landscape patterns within the context of the physical template (abiotic factors such as climate and soils), disturbances (such as fire), and biotic processes (demography, dispersal, growth, and competition). Our program focusses on developing a mechanistic understanding of this simple model as it applies to Sierra Nevada forests in particular, but also for the montane forests of western North America in general. 

    Our program consists of integrated studies organized around three themes: paleoecology, contemporary ecology, and modeling. The paleoecological theme takes advantage of the Sierra Nevada’s rich endowment of tree-ring and palynological resources to develop an understanding of past climatic changes and the consequent responses of fire regimes and forests. The contemporary ecology theme takes advantage of the Sierra Nevada's substantive climatic gradients as “natural experiments,” allowing us to evaluate climatic mechanisms controlling forest composition, structure, and dynamics. The modeling theme integrates findings from the paleoecological and contemporary studies, and is the indispensable vehicle for scaling up our mechanistic findings to regional landscapes, and predicting which parts of montane landscapes may be most sensitive to future environmental changes. The Sierra Nevada Global Change Research Program currently focuses on addressing nine central sets of questions:

• What is the relative importance of topography and soil on site water balance in the Sierra Nevada, and how well does this compare with model predictions?
• What is the role and importance of reproduction in determining forest pattern and forest sensitivity to climatic change? By what mechanisms does climate control reproduction, and therefore forest sensitivity to climatic change? 
• How do seed dispersal, seedling dynamics, and fine-scale variations in topography and soils interact with climatic change to affect forest sensitivity and change at local scales?
• How does climatic change affect the spatial extent, landscape pattern, and severity of fires?
• What are the relative importances of tree recruitment, death, and growth rates, and their interannual variabilities, in determining forest response to climatic variation in space and time?
• What portions of Sierra Nevada landscapes are most sensitive to climatic changes (temperature, precipitation, and seasonality), what are the implications of this for a greenhouse world, and what are the implications for land managers?
• Does climate synchronize fire regimes at subcontinental scales? If so, what large-scale climatic phenomena drive the synchrony?
• Can agents of pattern formation and mechanisms of forest change be generalized at subcontinental scales?
• How do the relative importances of agents of pattern formation vary among different climates? Is our understanding of mechanisms of forest change sufficient for a single model to explain forest dynamics at several different sites across the continent? 

OVERVIEW OF PROGRESS AND RESULTS

    During 2002 the Sierra Nevada Global Change Research Program produced or contributed to 23 scientific manuscripts (published or accepted for publication, including one Ph.D. dissertation), with another eight manuscripts currently in review, and several in progress. Examples of some of our significant findings are as follows. We continued our work that calls into question some of the basic demographic assumptions common to most forest dynamics models meant to predict the effects of climatic change, and have pointed out how those models might be improved. However, initial analyses of tree growth rates seem to corroborate other model assumptions about the way in which trade-offs in growth rates versus environmental tolerances generate patterns in species replacement along climatic gradients. We also provided the first demonstration that pre-fire growth rate, in addition to fire-induced crown scorch, influences probability of tree death following fire, suggesting that resource managers will need to consider the effects of stressors (such as climatic change) interacting with fire. We developed a conceptual model describing where and when upper treeline might be most sensitive to climatic change, with implications for interactions between forest cover, hydrology, and species habitat. Our detailed records of fire-climate interactions continue to inform projections of the severity of upcoming fire seasons for the fire management community. 

    Investigators of the Sierra Nevada Global Change Research Program continued to leverage funds in support of the program. Swetnam and colleagues used their Sierra Nevada work to help leverage funds for their workshop on "Fire, Climate, and Vegetation in the Americas," held March 23-28, 2002, Tucson, Arizona. This workshop furthered collaborations relevant to the Sierra Nevada and Western Mountain Initiative. Urban's NSF funding continued, leveraging his Sierra Nevada work with a cross-site comparison among forests of the Sierra Nevada, Oregon Cascades, and southern Appalachians. Keeley's Joint Fire Science Program (JFSP) funding continued, allowing us to better determine the effects of fire on forest dynamics (particularly seedling dynamics). Urban's JFSP funding also continued, supporting his student Monique Rocca in her exploration of patterns of fine-scale spatial heterogeneity in microenvironment, fire effects, and herbaceous layer seedling response (including tree seedlings). Ruth Kern (Calif. State Univ., Fresno) captured matching funds to determine how variation in tree seedling dynamics might be driven by fine-scale variations in soil moisture -- information that complements our efforts to understand climatic controls of forest dynamics.

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This page last updated: Thursday, March 22, 2007