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Executive Summary

Richard A. Birdsey and John L. Hom - Part 2
USDA Forest Service, NERS, Newtown Square, PA 19073


Monitoring and Predicting Regional Environmental Change

Information about historical climate is used in the analysis of ecological data to develop models relating ecosystem processes and responses to climate, and to develop analogs for future climate scenarios at regional to local scales. NGCRP researchers have developed techniques for modeling historical and current climate at specific locations of interest to land managers, and can produce estimates of precipitation and ionic deposition at any point in the NGCRP region. We have the capability to project regional climate changes by nesting a mesoscale climate model within a specific area covered by much larger scale global climate models that are unable to resolve important local features such as topography and large water bodies.

Responses of Northern Tree Species to Regional Stress

Our understanding of how and why individual trees respond to change is inadequate because multiple interacting stresses may produce responses that are considerably different from reactions to any one factor. Since controlled experiments are limited in scope to evaluation of a few factors and their interactions, we have linked carefully selected experimental studies with models of physiological mechanisms to predict the complicated tree responses more realistically. Our ongoing objective has been to conduct comparative experiments on immature and mature trees, with the eventual goal of conducting experiments at the stand or ecosystem level.

We have featured research on the effects of chronic CO 2 and ozone exposure, interacting with nutrient and moisture limitations. Key responses include changes in carbon allocation, altered ecosystem productivity, and effects on susceptibility to insect defoliation and disease. Extrapolation of these results to the ecosystem level and larger regional scales of interest to land managers and policy makers is based on mechanistic models at different scales.

A sizable body of research indicates that deposition of nitrogen and sulfur compounds via clouds, rain, snow, and wind is impacting the health of some tree species in the northeastern United States. Several NGCRP studies are advancing our understanding of the affects of acid deposition on spruce physiology and cold tolerance. Additional studies examine the effects of acid deposition on ecosystems through changes in nutrient cycling.

Several long-term studies have been initiated or adopted by the NGCRP to improve understanding of the physiological and genetic mechanisms of tree resistance to stresses such as physical and biological damage, disease, and water stress. An understanding of the mechanisms by which trees adapt to rapid change is critical to planning for climate change. Adaptation may be a significant alternate survival strategy to migration for some species, and will surely have an impact on future species composition, biodiversity, and the status of sensitive ecosystems.

Responses of Ecosystem Processes to Regional Stress

To successfully manage forests in a changing environment we must understand the complex dynamics of forest ecosystems as well as the responses of individual trees. Unless we have long-term records of environmental variables such as temperature, soil solution chemistry, and vegetation responses such as tree growth and mortality, we have no yardstick against which to measure our predictions about ecosystem responses to environmental change or to test the results of our models. The NGCRP has continued observations at seven intensive research sites established during the National Acid Precipitation Assessment Program (NAPAP). Measurements of climate variables, atmospheric deposition, throughfall chemistry, and soil solution chemistry have been made for up to six years. Continuing such measurements over a long period of time can provide complete characterization of the variability in deposition and mineral cycling rates, and their relation to tree health.

Northern forest soils are susceptible to increases in the amount of soluble aluminum as a result of acidic deposition. Elevated concentrations of aluminum are toxic to roots and can inhibit the availability of calcium, an important nutrient. Establishing a cause-effect link between acid deposition, soil chemistry, and tree health was one of the major challenges of the NAPAP program; however, there are many factors affecting northern forest ecosystems and it became extremely challenging to establish a direct causal link. Under NGCRP, research has continued on these important soil-mediated effects of acidic deposition. Several experiments have been established to study how a changing climate may affect tree growth through alterations in soil processes.

Forests in the North are dynamic and typically in a state of recovery or succession following the last disturbance, whether timber harvesting, land clearing and reversion to forest, or natural events such as hurricanes. Understanding ecosystem responses to environmental change requires a solid understanding of how disturbance impacts the processes that govern successional change. NGCRP scientists are studying the impacts of harvesting and natural disturbance on microclimate, species succession, nitrogen saturation, cation depletion, and carbon dynamics.

Forest and Landscape Responses to Regional Stress and Management Activities

A significant alteration in the mix of tree species, in the productivity of forest lands, or in the health of existing forests could have a substantial impact on aesthetic and commercial values of forests, as well as wildlife populations and associated forest values. Understanding how basic forest parameters are affected by environmental change is the objective of a series of observational and modeling studies. Where possible, long-term permanent plots have been relocated and remeasured to detect changes associated with disturbance, acid deposition, and climate. These data sets are used to both explain observed vegetation characteristics, and to parameterize models so that predictions about future changes over large areas can be made.

Some tree species and ecosystems are more sensitive to climate and atmospheric chemistry than others, and are more rapidly affected by change. The susceptibility of vegetation to change, and the rate of change, will be influenced by the interaction of weather patterns and such factors as soil chemistry, elevation, and land use. Studies of vegetation history have shown that the occurrence of an individual species or a whole biome is closely related to past climate changes. It is inevitable that species distributions will shift, either from climate change or other natural and human-induced environmental changes.

Changes in disturbance patterns are likely to be one of the more striking features of a changing climate. Studies of individual disturbances and relationships to global change will eventually lead to a more comprehensive understanding of the long-term role of disturbance in shaping forest communities. At the landscape scale, the NGCRP has focused disturbance research on drought and fire regimes, and the possible role of global change in patterns and intensities of these disturbances. The climate changes predicted by Global Climate Change Models (GCMs) would also lead to altered patterns of insect infestation. To anticipate these changes and their implications for forest health and productivity, the NGCRP has undertaken research on the potential effects of climate change on insect populations.

Human-Forest Interactions and Regional Change

A growing number of NGCRP studies address natural resource policy issues and management in a changing environment. Estimating carbon sequestration or release for managed forest ecosystems is crucial to understanding the role of terrestrial ecosystems under changing climates. This estimation requires a reliable assessment of the quantities of carbon stored in various ecosystem components such as the plant biomass, forest floor, and soil, along with a recognition of spatial patterns of ecosystem carbon variability. Regional-scale estimates must consider patterns of carbon variability and interactions with environmental factors and land use change to accurately estimate the effects of change on forests at landscape and regional scales.

The NGCRP includes a series of studies designed to quantify how carbon in forested landscapes changes over time. Attempts to quantify the role of Northern forests in the global carbon cycle, and to understand the effects of alternate management activities on carbon storage, have been hampered by a lack of quantitative information. These studies are intended to fill this knowledge gap.

An Integrated Model of the Effects of Global Change on US Forests

Environmental change could be rapid relative to the ability of a species to adapt or migrate. The capability to project successional change is particularly important because rapid environmental change could induce forest health problems during a transitional change from one vegetation type to another. Significant problems with forest health would in turn affect forest resource use and the people dependent on the multiple resource values of forests, from the timber industry to the subsistence user.

NGCRP scientists are participating in the development of a national integrated model of global change effects on forests. The integrated model combines submodels of the physical, biological, and social systems. Climate models at global and regional scales, and hydrologic models represent major physical systems. Several different models of ecosystem change are being developed and evaluated at different temporal and spatial scales. The human dimensions are partially captured with econometric models of the forest sector, which can project land use change, harvesting activity, and impacts of change on the forest products industry.

Development and application of the integrated model are based on models used to conduct national assessments required by the Resources Planning Act (RPA). The integrated model provides analyses of the effects of scenarios of global change on forests, and can be used as a tool for evaluating alternate policy responses to projected changes. Initial efforts are focused on improving projections of potential forest vegetation distribution, forest ecosystem composition, forest growth, and the national carbon budget. As capability to project vegetation changes improves, the modeling system will be extended to project changes in other forest system attributes such as biodiversity and wildlife habitat.

An ongoing synthesis of basic forest statistics and development of carbon accounting models has highlighted the past and prospective role of US forests in the global carbon cycle and provided input to policy decisions regarding the effects of alternate strategies for offsetting greenhouse gas emissions through forestry actions. The US carbon budget model (FORCARB) predicts carbon in major forest components: trees, understory vegetation, the litter layer and coarse woody debris, and soils. Although still under development, early versions of FORCARB have provided input to national decisions regarding the effects of alternate policies for offsetting greenhouse gas emissions through forestry actions. Using statistics from a nationwide inventory of trees and data from site-specific studies, the model makes continental-scale projections of the carbon that would be released and/or sequestered by various management activities that could result from national policy decisions.

Analyses at the national scale may obscure important regional changes that are likely to occur in specific ecoregions. There is a need to develop integrated models at the regional scale that are optimized for the specific domains under study -- large watersheds, river basins, multi-state areas, economic regions, and the like. At the same time, there is an opportunity to establish feedbacks between national- and regional-scale models so that information developed at the national scale provides the context for regional assessments, and regional studies (that may contradict national studies) can be used to verify national-scale results or investigate possible regional effects in more detail. Model comparisons at different spatial and temporal scales are important to understanding the capabilities and limitations of models that are becoming widely used in assessments.

Boreal Forests and Global Change

Boreal forests play a major economic, social, and ecological role in the global environment. Circling the northern latitudes, boreal forests occur within the borders of Russia, Canada, the United States (Alaska), Finland, Norway, and Sweden. With an area of 920 million hectares they comprise 29 percent of the world's total forest cover. Boreal forests may be the single largest terrestrial carbon sink, with an estimated 40 billion tons of the world's stored carbon in Siberia's forests alone.

Boreal forest range and health are closely tied to prevailing climate conditions. Current projections indicate that global warming will be detected earlier and most strongly at high latitudes. NGCRP scientists are part of a cooperative international effort to study boreal forests and the implications of global change for this critical ecosystem.

Executive Summary: Part 1