INTRODUCTION:

Dr. Jerry Sesco, Deputy Chief of Forest Service for Research, U.S. Department of Agriculture (USDA), Washington, DC.

SPEAKERS:

Dr. Richard Birdsey, Program Manager, Northern Global Change Program, USDA Forest Service, Radnor, PA.

Dr. J.G. Isebrands, Project Leader, North Central Forest Experiment Station, USDA Forest Service, Rhinelander, WI.

Carbon dioxide (CO2) is the most important of the greenhouse gases that are influenced directly by human activities. The rising atmospheric concentration of CO2 is predicted to enhance the photosynthesis of some plants and to warm the climate. The CO2 concentration in the atmosphere is determined by the emissions from combustion of fossil fuels and by CO2 uptake and release by the Earth's oceans, vegetation, and soils. CO2 is taken up by land plants, which annually consume about 12 times the world's fossil fuel emissions. The biosphere also gives off about the same amount of CO2 through respiration and plant decay. This makes land plants a critical part of the global carbon cycle.

Global observations clearly indicate atmospheric concentrations of CO2 are increasing. However, the rate of increase in atmospheric CO2 is not as high as expected based on the increase in fossil fuel emissions. This "missing" carbon is being taken up by either the oceans or the terrestrial biosphere (plants and soils), or both. Understanding these carbon transfers is critical to predicting the importance of global climate change and its consequences. Recent studies have pointed to carbon accumulation in temperate forests of the Northern Hemisphere as a likely explanation for the imbalance in the global carbon budget. It is thought that several factors may be accounting for the increase in the carbon uptake by the terrestrial biosphere -- -the regrowth of forests in regions where farming is being reduced, increased plant growth due to the increase in the CO2 concentration of the atmosphere, and increased forest productivity as a result of nitrogen deposition.

Atmospheric Changes and Forest Responses

Trees and ecosystems are affected by an array of environmental factors that vary in space and time, and so a particularly important area of research has been the simultaneous effects of multiple factors on forests. While simple experiments may show the effects of a single factor, it is the timing and intensity of interactions between multiple factors that determine how a tree responds to environmental change. There are also genetic factors that determine the sensitivity of individual trees to stress, and their adaptability to a new environment.

It is generally accepted that scientists need to study natural systems because of significant difficulties with exposure chamber techniques. Under the auspices of the U.S. Global Change Research Program, a multi-agency terrestrial ecology program, augmented by funding from private sources, a new chamberless experimental facility is under development at a U.S. Forest Service site in Rhinelander, Wisconsin. This will be the largest "free air CO2 enrichment" (FACE) experimental facility in the U.S., consisting of 12 exposure rings, several different tree species, and the capability to simulate exposures to CO2 and ozone, singly and in combination.

In multi-factor, multi-year experiments using exposure chambers, different tree species have shown very different responses to elevated ozone levels, alone and in combination with elevated CO2 levels. For example, aspen is highly sensitive to ozone and there are strong genotypic differences. Ozone reduces biomass production and root growth in aspen, and an increase in the CO2 concentration does not compensate for the reduction. This negative interaction between CO2 and ozone decreases aspen photosynthesis rates more than ozone stress alone. In contrast, white pine and yellow popular show no significant detectable adverse effects of exposure to ozone, and growth has been stimulated with the simultaneous addition of CO2.

Integrated models of physical, biological, and social systems are being used to address the effects of different scenarios of climate change on forest productivity, carbon storage, and the timber economy. Initial projections from these models suggest that increases in productivity are likely for northern forest types, while southern forest types may show small increases or decreases in productivity. In one extreme scenario for the southeastern U.S., much of the dense pine forest would be replaced by a pine savanna of low productivity. Forests in the western U.S. may be highly sensitive to small climate changes because they often grow at or near the limits of climate tolerated by tree species.

When the ecosystem models are linked with economic models, results show that projected increases in productivity may not lead to increases in harvest at the national scale, because the market responds to many different factors besides timber growth. The models project some redistribution of harvest among regions, which would in turn lead to some changes in fiber types and the ownership of lands from which wood is harvested. Harvested timber will add to a growing pool of carbon in wood products. Byproducts from timber production, which are burned for energy, reduce the combustion of fossil fuels that adds long-stored carbon back into the atmosphere.

Retrospective applications of the models have explored uptake and release of carbon by forests and forest products. Results suggest that U.S. forests are currently a net sink for carbon. Increases in biomass on U.S. forest lands over the last 40 years are estimated to have added 281 million metric tons per year of stored carbon, enough to offset 25 percent of U.S. emissions for the period. Most of this additional carbon is found in re-growing forests on abandoned agricultural land in the eastern U.S. These estimates indeed suggest that some of the "missing" carbon can be accounted for by storage in northern temperate forests.

The integrated models are currently undergoing extensive revisions in preparation for another round of projections based on updated forest inventory data, new climate projections, and developments in modeling techniques. An international model intercomparison study known as VEMAP (Vegetation Ecosystem Model Analysis Project) has compared the results of three biogeochemistry models for simulating the response of 21 different U.S. vegetation types to climate change scenarios. This exercise has helped the model developers understand the strengths and weaknesses in representing key ecosystems processes, and will facilitate analytical review of uncertainty in projections of vegetation change.

Opportunities to increase carbon storage above the expected baseline have been identified and are beginning to influence landowner decisions. Studies have shown that CO2 emissions can be effectively offset by sequestering additional carbon at various steps in the life cycle of wood growth, harvest, use, and disposal. Typical practices to offset carbon emissions include (1) tree planting on marginal agricultural land, (2) increasing timber growth on forests now used for timber production, (3) increasing the use of wood in place of fossil fuels, and (4) improving wood utilization.

Several U.S. agencies in partnership with American Forests, a nonprofit organization that represents many land owners have been quantifying how various management practices used in different regions and forest types may affect carbon storage over long periods of time. This information has been used by utility companies to design carbon offset projects to compensate for CO2 emissions that are a byproduct of energy generation from fossil fuels. Other landowners have begun to use estimates of carbon storage under different forest conditions to quantify accomplishments under the Department of Energy's "Voluntary Reporting of Greenhouse Gas Reductions" Program.

Dr. Richard Birdsey is Program Manager of the Northern Global Change Research Program within the U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station. He is a specialist in quantitative methods for large-scale forest inventories and has pioneered the development of methods to estimate national carbon budgets for forest lands from forest inventory data. He spent 10 years as team leader for forest inventory research in the Midsouth States. He was a principal contributor to several regional and national assessments of future timber supply in the South and the Nation. He designed the first comprehensive inventories of forest resources in Puerto Rico and St. Vincent, West Indies. He is a contributor to the ongoing inventory of U.S. greenhouse gases and sinks compiled by USDA, EPA, and DOE. He also cooperates with the Sukachev Institute of the Russian Academy of Sciences in a project to estimate the Russian carbon budget. He was a major contributor to the most recent assessment of climate change impacts on America's forests conducted as part of the decadal Resources Planning Act Assessment. In his current role as Program Manager, Dr. Birdsey is coordinating a national effort to link biological and economic models with atmospheric models to assess the impacts of global change on U.S. forests, and to analyze mitigation and adaptation strategies. He manages a large basic research program involving a dozen U.S. Forest Service Laboratories and Experimental Forests in the Northeast and North Central States, with research emphases on basic plant processes, ecosystem nutrient cycling, and measurement and modeling techniques. Dr. Birdsey has degrees in quantitative methods and world forestry from the State University of New York, College of Environmental Science and Forestry.