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2 Introduction

The sustainability of southern forests could be threatened by the interactions of biotic and abiotic stressors (McLaughlin and Percy 1999). Environmental factors such as temperature, precipitation, atmospheric carbon dioxide (CO₂) and ozone (O₃) concentrations, and acid deposition affect forest processes such as carbon, water, and nutrient fluxes. These processes are the foundation of forest ecosystems, and abnormally large variability in their size, timing, or location may influence forest sustainability. From an ecosystem perspective, therefore, changes in forest processes may be indicators of long-term forest function and health.

Sulfur and nitrogen deposition have been indicted as contributors to forest degradation, especially in the high-elevation red spruce and Fraser fir forests that occupy the ridges of the Appalachian Mountains (McLaughlin and Kohut 1992). In an effort to manage and sustain spruce-fir and hardwood forests in a way that does not compromise the ability of future generations to meet their needs, the current and future impacts of sulfur and nitrogen deposition on overall forest health in the Southern United States must be addressed.

Ground level (tropospheric) O₃ is an air pollutant that affects U.S. forests (U.S. EPA 1996). At current ambient levels, O₃ can decrease tree growth, increase the probability of mortality, cause visible foliar damage, and alter forest successional patterns (Flagler and Chappelka 1996, McLaughlin and Downing 1995, Teskey 1996). For these reasons, current and projected O₃ impacts on southern forests are addressed in this Assessment.

Climate influences the establishment and growth of forest trees, affecting the extent and quality of forest ecosystems. The spatial and temporal distribution of air temperature and precipitation are the primary climatic factors shaping forests. Human activities contribute significantly to current global climate change (Dale and others 2000), predominantly due to the increasing concentration of greenhouse gases such as CO₂. Since the beginning of the industrial revolution, CO₂ levels have been steadily increased by fossil fuel burning and land-use changes (U.S. DOE 1999, Sarmiento and Wofsy 1999). Even if changes in CO₂ concentration did not effect climate changes, they would affect plant growth.

Independently developed climate-change scenarios are generated with transient general circulation models (GCMs) that simulate atmospheric dynamics under a gradual doubling in greenhouse gas concentrations from about 1895 to 2100. Emissions of CO₂ to the atmosphere are predicted to increase from 7.4 gigatons/year (Gt/yr) in 1997 to 26 Gt/yr by 2100 (U.S. DOE 1999). For this Assessment, these scenarios are used with ecological process models to investigate the potential effects of climate change on forest ecosystems.

Forest carbon sequestration, the ability of forests to store and release carbon, is currently an important issue debated in the policy arena. Carbon stored in forests affects the amount of carbon contributing to the increasing atmospheric CO₂ concentration. Reductions in carbon emissions have been proposed as a mitigation strategy for rising atmospheric CO₂, which may be causing global warming. Rising atmospheric CO₂ levels could also be mitigated by increasing carbon sequestration through forestry and other land management activities. Terrestrial ecosystems have enormous potential to capture CO₂ and store carbon.

Climate change also could generate forest stress, and extreme weather events cause disturbances that shape forest systems by influencing their composition, structure, and functional processes. We discuss the effects of these disturbances and their relationship to changing temperature and precipitation patterns.

Biotic stressors such as insects and pathogens have major negative impacts on forest ecosystems; in the United States, they cause severe damage on an average of more than 50 million acres per year, costing $2 billion a year (Dale and others 2000). Biotic stressors are the focus of Chapter HLTH-2.

For each of the abiotic stressors, methods, data sources, results, discussion, and conclusions are discussed separately. Current abiotic stressors have been described for different coarse-scale studies. Attempts at regional-scale characterizations and future predictions are underway and are highlighted when feasible.

It is important to recognize the integrated nature of these abiotic stressors and their cumulative effects on forest ecosystems. This integration is referenced throughout the Chapter. It is imperative that readers consider cumulative integrated effects when interpreting the results and conclusions from this Chapter.