《森林生态学》课程教学资源（生态学热点）Issues in ecology - 13 美国森林碳管理 A Synthesis of the Science on Forests and Carbon for U.S. Forests.pdf

© The Ecological Society of America • esahq@esa.org esa 1 ISSUES IN ECOLOGY NUMBER THIRTEEN SPRING 2010 A Synthesis of the Science on Forests and Carbon for U.S. Forests SUMMARY Forests play an important role in the U.S. and global carbon cycle, and carbon sequestered by U.S. forest growth and harvested wood products currently offsets 12-19% of U.S. fossil fuel emissions. The cycle of forest growth, death, and regeneration and the use of wood removed from the forest complicate efforts to understand and measure forest carbon pools and flows. Our report explains these processes and examines the science behind mechanisms proposed for increasing the amount of carbon stored in forests and using wood to offset fossil fuel use. We also examine the tradeoffs, costs, and benefits associated with each mechanism and explain how forest carbon is measured. Current forests are recovering from past land use as agriculture, pasture, or harvest, and because this period of recovery will eventually end, the resulting forest carbon sink will not continue indefinitely. Increased fertilization from atmospheric nitrogen deposition and increased atmospheric carbon dioxide may also be contributing to forest growth. Both the magnitude of this growth and the future of the carbon sink over the next hundred years are uncertain. Several strategies can increase forest carbon storage, prevent its loss, and reduce fossil fuel consumption (listed in order of increasing uncertainty or risk): Avoiding deforestation retains forest carbon and has many co-benefits and few risks. Afforestation increases forest carbon and has many co-benefits. Afforesting ecosystems that do not naturally support forests can decrease streamflow and biodiversity. Decreasing harvests can increase species and structural diversity, with the risk of products being harvested elsewhere and carbon loss in disturbance. Increasing the growth rate of existing forests through intensive silviculture can increase both forest carbon storage and wood production, but may reduce stream flow and biodiversity. Use of biomass energy from forests can reduce carbon emissions but will require expansion of forest management and will likely reduce carbon stored in forests. Using wood products for construction in place of concrete or steel releases less fossil fuel in manufacturing. Expansion of this use mostly lies in the non-residential building sector and expansion may reduce forest carbon stores. Urban forestry has a small role in sequestering carbon but may improve energy efficiency of structures. Fuel treatments trade current carbon storage for the potential of avoiding larger carbon losses in wildfire. The carbon savings are highly uncertain. Each strategy has risks, uncertainties, and, importantly, tradeoffs. For example, avoiding deforestation or decreasing harvests in the U.S. may increase wood imports and lower forest carbon elsewhere. Increasing the use of wood or forest biomass energy will likely reduce carbon stores in the forest and require expansion of the area of active forest management. Recognizing these tradeoffs will be vital to any effort to promote forest carbon storage. Climate change may increase disturbance and forest carbon loss, potentially reducing the effectiveness of management intended to increase forest carbon stocks. Finally, most of these strategies currently do not pay enough to make them viable. Forests offer many benefits besides carbon, and these benefits should be considered along with carbon storage potential. Cover photo credit: Old-growth forest in the Valley of the Giants in Oregon. Photo by Mark E. Harmon, Oregon State University. Inset: Logs harvested at Manitou Experimental Forest in Colorado. Photo by Richard Oakes, USDA Forest Service.

2 esa © The Ecological Society of America • esahq@esa.org Introduction The movement of carbon between the earth and its atmosphere controls the concentration of carbon dioxide (CO2) in the air. CO2 is important because it is a greenhouse gas and traps heat radiation given off when the sun warms the earth. Higher concentrations of greenhouse gases in the atmosphere cause the earth to warm. Before the Industrial Revolution, the concentration of CO2 in the atmosphere was less than 280 parts per million. The burning of fossil fuel for energy and the clearing of forests for agriculture, building material, and fuel has led to an increase in the concentration of atmospheric CO2 to its current (2010) level of 388 parts per million. This current level far exceeds the 180-300 parts per million found over the last 650,000 years. As a result of rising CO2 and other greenhouse gases in the atmosphere, global surface temperatures have increased by 0.74˚C (1.3˚F) since the late 1800s, with the rate of warming increasing substantially. As more CO2 is added to the air, temperatures will continue to increase and the warmer earth will have an impact on the earth’s climate, climate variability, and ecosystems. Rain and snowfall patterns will shift, and extreme weather events may become more common. Some regions that currently support forests will no longer do so, and other regions that currently do not support forests may become suitable for forest growth. Forests store large amounts of carbon in their live and dead wood and soil and play an active role in controlling the concentration of CO2 in the atmosphere (Figure 1). In the U.S. in 2003, carbon removed from the atmosphere by forest growth or stored in harvested wood products offset 12-19% of U.S. fossil fuel emissions (the 19% includes a very uncertain estimate of carbon storage rate in forest soil). U.S. forest growth rates are thought to be higher than those before European settlement because of recovery from past land use and disturbance, but the current growth rate will not continue indefinitely. Given the role that U.S. forests play in offsetting CO2 emissions, our report asks: 1) Which human actions influence forest carbon sinks (storage rates) and can these sinks be enhanced for a meaningful period of time through management and use of forest products? and 2) What are some of the major risks, uncertainties, tradeoffs, and co-benefits of using forests and forest products in proposed carbon emission mitigation strategies? The purpose of our report is to answer these questions, or, if answers are not yet available, to present the best current information. We present the state of knowledge on the role of A Synthesis of the Science on Forests and Carbon for U.S. Forests Michael G. Ryan, Mark E. Harmon, Richard A. Birdsey, Christian P. Giardina, Linda S. Heath, Richard A. Houghton, Robert B. Jackson, Duncan C. McKinley, James F. Morrison, Brian C. Murray, Diane E. Pataki, and Kenneth E. Skog Figure 1. Plants and soil play a large role in the global carbon cycle as shown by global stocks (boxes) and flows (arrows) of carbon in petagrams (1000 teragrams). Numbers in light blue and green are the historical fluxes between the oceans and the atmosphere and plants and soil and the atmosphere that would have occurred without human influence. The number in dark blue is the additional ocean absorption of CO2, resulting from increased CO2 in the atmosphere since the Industrial Revolution. The numbers in black are the fluxes to the atmosphere from fossil fuel combustion or deforestation. The number in brown is the flux from the atmosphere to the land, mostly from forest regrowth. The measured atmospheric increase of 4.1 petagrams per year is not equal to the sum of the additions and withdrawals because they are estimated separately and with associated uncertainties. ISSUES IN ECOLOGY NUMBER THIRTEEN SPRING 2010 Courtesy of Richard A. Houghton, Woods Hole Research Institute, 2009. Global Stocks and Flows of Carbon ATMOSPHERE 816 (+4.1/year) OCEANS 37,000 8.7 100 2.3 100 3.0 100 100 PLANTS & SOIL 2,000 SEDIMENTS AND SEDIMENTARY ROCKS 66,000,000 – 100,000,000 COAL, OIL & NATURAL GAS 10,000 1.4

© The Ecological Society of America • esahq@esa.org esa 3 ISSUES IN ECOLOGY NUMBER THIRTEEN SPRING 2010 forests in the carbon cycle in a straightforward manner so that it can be understood by forest managers, policymakers, educators, and the interested public. We begin with a description of the forest carbon cycle and biophysical effects. We then present details on the strategies that have been proposed for using forests to slow the amount of CO2 entering the air. These strategies include: • Avoiding deforestation – Keeping forests intact. • Afforestation – The restoration of forest on land that has been without forest cover for some time, and the establishment of forest on land that has not previously been forested. • Forest management: decreasing carbon loss – Increasing the harvest interval and/or decreasing harvest intensity. • Forest management: increasing forest growth – Use of improved silvicultural practices, genetic improvement, and rapid regeneration. • Forest management: thinning to reduce fire threat. • Urban forestry – Planting trees in urban areas for carbon storage and shading for energy savings. • Biomass energy – Using fuel from wood and biomass in place of fossil fuel. • Carbon storage in forest products and substitution – Storing carbon in long-lived forest products (such as lumber) and substituting forest products for products (such as steel and concrete) whose manufacture releases much more CO2 than does the processing of wood. We then discuss carbon offsets and credits, how forest carbon could be monitored to determine whether changes result in the desired outcomes, and what the costs would need to be for carbon to encourage changes. We also discuss some of the uncertainties inherent in the use of forests for carbon storage, because changes in climate, population, and land use may lower projected carbon storage. We especially note the potential loss of carbon that might occur with increased disturbance in a warmer climate. Finally, we provide conclusions and recommendations. Forests and carbon Carbon in the forest Forest carbon storage differs from many other mechanisms that control atmospheric CO2 because forests have a life cycle during which carbon stocks, gains, and losses vary with forest age. Carbon enters a forest through photosynthesis, where leaves capture the energy in sunlight and convert CO2 from the atmosphere and water into sugars that are used to build new leaves, wood, and roots as trees grow (Figure 2). About half of the CO2 that is converted to sugars is respired by living trees to maintain their metabolism, and the other half produces new leaves, wood, and roots. As they grow, trees shed dead branches, leaves, and roots and some of the trees die. Microorganisms decompose this dead material, releasing CO2 back to the atmosphere, but some of the carbon remains in the soil. Live and dead trees contain about 60% of the carbon in a mature forest, and soil and forest litter contain about 40%. The carbon in live and dead trees (50% of their biomass) varies the most with forest age. Carbon can leave the forest in several ways besides tree and microorganism respiration. Forest fires release stored carbon into the atmosphere from the combustion of leaves and small twigs, the litter layer, and some dead trees and logs, leaving behind a great deal of stored carbon in dead trees and soil. Storms and insect outbreaks also kill trees and increase the amount of material available for decomposition. Harvesting removes carbon from the forest, although some of it is stored in wood products (preventing its immediate release to the atmosphere) and some is available for use as biomass energy (displacing fossil fuel use). In addition, water can remove carbon from a forest either by transporting soil and litter away in streams (especially from erosion after fire) or by transporting soluble carbon molecules created during decomposition. After fire, other disturbance, or harvest, regenerated forests will eventually recover all of the carFigure 2. Flows of carbon from the atmosphere to the forest and back. Carbon is stored mostly in live and dead wood as forests grow. Recent CO2 CO2 Photosynthesis Recent and Older CO2 Dead Wood Microbe Respiration Litter Carbon in leaves, wood, roots Microbes Dead Roots Old, Stable Soil Carbon New, Labile Soil Carbon Plant Respiration

ISSUES IN ECOLOGY NUMBER THIRTEEN SPRING 2010 bon lost so that a complete cycle is carbon neutral regarding storage if the recovery is long enough (Figure 3). But if disturbances increase, as is projected with climate change, a fire, storm, or insect outbreak may occur before the ecosystem recovers the carbon it had prior to the disturbance. In that case, the amount of carbon stored on the landscape will decrease. Forests are biological systems that continually gain and lose carbon via processes such as photosynthesis, respiration, and combustion; whether forests show a net gain or loss of carbon depends on the balance of these processes. The observation that carbon is lost from forests has led to the notion that carbon cannot be permanently stored in forests. However, this view ignores the inevitable increase and eventual recovery of carbon that follows most disturbances. Thus over time, a single forest will vary dramatically in its ability to store carbon; however, when considering many different forests over a large area or landscape, such “boom and bust” cycles may not be apparent because the landscape is composed of forest stands that are in different stages of recovery from disturbance or harvesting (Figure 4). To determine how quickly carbon increases in a forest system, it is important to know the starting point or “baseline.” A forest that already stores a substantial amount of carbon is likely to lose carbon when converted to something else, and a system with the potential to store carbon but that does not currently store much is easier to convert to one that stores more carbon (Figure 5). A forest’s timeline for increasing carbon storage is important because carbon must be removed quickly to reduce CO2 in the atmosphere and thereby slow global warming. While the biological processes of photosynthesis, respiration, and decomposition are similar for all forests, their relative importance differs by forest type and location. Some forests grow more rapidly, but dead trees in fast-growing forests also decompose more rapidly. In addition, disturbances vary regionally: for example, fire disturbance is more common in the western U.S. and hurricanes more common in the East. Forests are managed in different ways with varying harvest intervals and regeneration practices that will influence the optimum strategy for storing more carbon. Each forest has a different potential to store carbon. For example, this potential is particularly high in the Pacific Northwest where forests are relatively productive, trees live a long time, decomposition is relatively slow, and fires are infrequent. The differences between forests must therefore be taken into consideration when determining how they should be managed to store carbon. Carbon from the forest All forest products eventually decompose, but before they do, they store carbon. Some products have a short lifespan (such as fence posts) and some a longer lifespan (for example, houses) – the longer the lifespan, the more carbon is stored. Disposed forest products in landfills can have a very long lifespan; however, the decomposition in landfills Figure 3. If a forest regenerates after a fire, and the recovery is long enough, the forest will recover the carbon lost in the fire and in the decomposition of trees killed by the fire. This figure illustrates this concept by showing carbon stored in forests as live trees, dead wood, and soil and how these pools change after fire. (Adapted from Kashian and others 2006. BioScience 56(7):598-606.) Figure 4. Management actions should be examined for large areas and over long time periods. This figure illustrates how the behavior of carbon stores changes as the area becomes larger and more stands are included in the analysis. As the number of stands increases, the gains in one stand tend to be offset by losses in another and hence the flatter the carbon stores curve becomes. The average carbon store of a large number of stands is controlled by the interval and severity of disturbances, as shown in Figure 7. That is, the more frequent and severe the disturbances, the lower the average becomes. (Courtesy of Mark E. Harmon, Oregon State University, 2009.) Total Carbon Fire Dead Trees Wood Soil –20 0 20 40 60 80 100 120 Year since fire 150 100 50 0 Carbon (Mg C/ha) 1 stand 10 stands 100 stands 0 100 200 300 400 Years 1500 1000 500 0 Carbon Storage (Mg/ha) 4 esa © The Ecological Society of America • esahq@esa.org