《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_大学文库

© 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

© The Ecological Society of America • esahq@esa.org esa 5 ISSUES IN ECOLOGY NUMBER THIRTEEN SPRING 2010 generates methane, which is a much more potent greenhouse gas than CO2, reducing the carbon storage benefit. In addition, wood and bark that are burned to run a mill or heat houses, or made into liquid biofuel, lower emissions from fossil fuel use. Once the carbon leaves the forest, it becomes more difficult to track and measure than carbon in the forest, particularly because imports and exports must then be tracked. Biophysical effects may cause warming or cooling Forests have other influences on climate besides that of carbon; these are known as biophysical effects (Figure 6) and include the reflection of solar radiation and transpiration of water vapor. Trees are dark and absorb more radiation than other types of land cover, such as crops or snow-covered tundra. Therefore, converting non-forested land to forest can warm the land and air. Evergreen trees absorb much more energy than deciduous trees in the winter and burned forests absorb more than unburned forests, so species and disturbance can also alter the energy absorbed by forests. In addition, transpiration from forests may have a cooling effect by contributing to the formation of clouds that reflect sunlight. Biophysical effects sometimes act in a direction opposite to that of the effects of storing or releasing CO2. For instance, whereas converting cropland to forest will sequester more CO2, which reduces global warming, it will also increase solar absorption, which increases warming. Generally, biophysical effects on climate are not as strong as the effects of greenhouse gases. Biophysical effects will be most important in evaluating the benefits of afforestation because the land use change will cause large differences. Unfortunately, current estimates of biophysical effects are uncertain because few studies have been done. Strategies for increasing carbon stores in forests 1. Avoiding deforestation Deforestation, or the conversion of forest land to other uses, has a significant impact on global CO2 emissions. Globally, deforestation converts approximately 90,000 km2 (about the size of Indiana) of forests per year (0.2% of all forests) to other land uses. Deforestation annually releases 1,400-2,000 teragrams of carbon (1012 grams; see Box 1 for units) to the atmosphere, and two-thirds of this release occurs in tropical forests. The amount of carbon released by deforestation equals 17-25% of global fossil fuel emissions every year and is roughly the amount of U.S. annual fossil fuel emissions. If current deforestation rates continue, more than 30,000 teragrams of carbon could be released to the atmosphere from deforestation in the Amazon alone by the year 2050. In the U.S., forested area increased 0.1% per year from 2000-2005, and this gain in forested area is partially responsible for the current forest sink of 162 teragrams of carbon per year. The net growth in forested area results from both deforestation and afforestation: About 6,000 km2 are deforested annually, but more than 10,000 km2 of non-forest are afforested. The net increase in forestlands results from changes in land use and possibly from reduced demand for U.S. timber. Although the U.S. forest carbon sink benefits from increased forest area, these carbon benefits need to be weighed against the global consequences of land use change within the U.S. If afforestation or avoided deforestation in the U.S. pushes crop and cattle production to other countries, it can lead to deforestation Figure 5. Projections of carbon storage and fossil fuel displacement if all biomass is used shows considerable storage and offsets for (A) a project that reestablishes forests with periodic harvests. Harvesting a high-biomass old growth forest (B) shows carbon losses, even under the best possible scenario, for several harvests. At each harvest, forest biomass (and thus carbon stock) is removed for use in long- and short-lived wood products (‘Products-L’ and ‘Products-S’, respectively) substituted for more carbon-intensive products, and for biomass energy to displace emissions from fossil fuel use. Because substitution generates more fossil fuel savings than the carbon it contains, substitution would yield a greater carbon benefit after harvest than that which is stored in the biomass. The biomass energy and substitution fossil fuel savings accumulate but represent only hypothetical carbon benefits, as currently little biomass energy use and substitution occurs in the U.S. (Adapted from IPCC 2007.) Soil Litter Trees Products–L Products–S Landfill Substitution Biomass energy (a) (b) 250 200 150 100 50 0 250 200 150 100 50 0 20 40 60 80 100 Years Cumulative carbon (Mg/ha)

ISSUES IN ECOLOGY NUMBER THIRTEEN SPRING 2010 6 esa © The Ecological Society of America • esahq@esa.org and loss of forest carbon elsewhere to create pasture and cropland. Carbon loss associated with such deforestation – especially in the tropics – is greater than carbon gain associated with tree growth from afforestation in the U.S. Forest retention in the western U.S. may be even more important in the future as climate changes. Our warming climate is very likely causing, at least in part, the current increase in forest fire size and intensity, insect outbreaks, and storm intensity. If forest regeneration fails because the disturbances or regeneration conditions are outside of the ecological norms, disturbances can convert forests to meadows or shrublands. When this type of deforestation occurs, substantial carbon is lost to the atmosphere and not recovered by the ecosystem. Tree planting would help recover forest carbon where natural regeneration fails. There are not many risks associated with avoidance of deforestation. Three to note, however, would be risks related to highly fireprone ecosystems near human settlement, economic consequences for not developing agricultural or pasture land, and an increase in forest products harvested elsewhere. On the other hand, avoiding deforestation has many of the co-benefits identified in Box 2. 2. Afforestation We define afforestation as both reestablishing forests on land that has been without forest cover for some time and the establishment of forest on land that has not previously been forested (note that some entities involved in carbon markets and reporting use different definitions for this term). Afforestation can remove substantial CO2 from the atmosphere. Between 1850 and 2000, global land-use change resulted in the release of 156,000 teragrams of carbon to the atmosphere, mostly from deforestation. This amount is equivalent to 21.9 years of global fossil fuel CO2 emissions at the 2003 level. The rate of carbon storage in tree growth varies with species, climate, and management, ranging widely from about 3-20 megagrams (Mg, 106 grams) per hectare per year. In the continental U.S., the highest potential growth rates are found in the Pacific Northwest, the Southeast, and the South Central U.S. Much land currently in pasture and agricultural use in the eastern U.S. and in the Lake States will naturally revert to forests if left fallow, while reestablishing forests in many western forests requires tree planting. The benefits of afforestation (outlined in Box 2) are enhanced where forests include a substantial proportion of native species. Planting native species or allowing natural succession to recreate the forest that historically occupied the site will yield the greatest benefits for species diversity and wildlife habitat and the lowest risk for unintended consequences. Because native species often grow more slowly than exotics or trees selected for improved growth, restoration of the historical ecosystem may yield lower carbon accumulation rates than other forest reestablishment practices. Planting monocultures of non-native or native improved-growth species on historical forest Box 1. UNITS FOR CARBON When discussing regional, national, or global carbon stores and fluxes, the numbers get large quickly. We report carbon in teragrams (1012 grams). Other reports may use other units, so we provide a conversion table below. For standor forest-level stores and fluxes, we use megagrams (Mg) per hectare (106 grams). Our report uses carbon mass, not CO2 mass, because carbon is a standard “currency” and can easily be converted to any other unit. Many reports give stocks and fluxes of the mass of CO2, not carbon. To convert carbon mass to CO2 mass, multiply by 3.67 to account for the mass of the O2. 1000 teragrams (Tg) 1 petagram (Pg) 1000 teragrams 1 billion metric tonnes 1000 teragrams 1 gigatonne 1 teragram 1 million metric tonnes 1 teragram 1 megatonne 1 megagram (Mg) 1 metric tonne 1 metric tonne 0.98 U.S. long ton 1 metric tonne per hectare 0.4 U.S. long tons per acre carbon (C) mass * 3.67 carbon dioxide (CO2) mass Figure 6. Biophysical effects of different land use can have important impacts on climate. Cropland reflects more sunlight than forest, produces less water vapor, and transmits less heat. (From Jackson et al. 2008. Environmental Research Letters 3:article 044006.) Reflected sunlight Evaporation Transmitted heat

© The Ecological Society of America • esahq@esa.org esa 7 ISSUES IN ECOLOGY NUMBER THIRTEEN SPRING 2010 land will likely yield greater carbon accumulation rates but fewer benefits in terms of biodiversity. Afforestation can have negative consequences, too. Planting forests where they were not present historically can have drawbacks such as lower species diversity (if trees are planted in native grassland), changes in water table, and a higher energy absorption compared to the native ecosystem. In addition, afforestation generally reduces streamflow regardless of the ecosystem type because trees use more water than grass or crops. Conversion of agricultural or grazing lands to forest reduces revenue from agricultural products. If afforestation efforts include the addition of nitrogen fertilizer, emissions of nitrous oxide (a greenhouse gas roughly 300 times as powerful as CO2) will increase. 3. Forest management: decreasing carbon loss Lengthening the harvest interval or reducing the amount removed in a harvest will store more carbon in the forest. The greater the increase in harvest interval over the current level, the higher the increase in carbon storage. For example, a five-year increase in the harvest interval would lead to a 15% increase in carbon storage if the harvest interval was changed from 25 to 30 years, but only a 4% increase if the interval was changed from 55 to 60 years (Figure 7). A 50-year increase from 25 to 75 years would increase carbon storage 92% (Figure 7). The carbon impact of reducing the amount of trees removed in a harvest also varies with the harvest interval. For example, reducing the harvest from 100% to 20% of the live trees would increase the average forest carbon stock by 97% for a 25-year harvest interval, but only by 30% for a 100-year harvest interval (Figure 7). Some natural forests are dominated by small disturbances that kill a few trees at a time. Reducing harvest amounts in these systems from complete removal of trees to simply a percentage, for example, could mimic the natural disturbance regime common to the northeastern and midwestern United States. In addition, reducing harvests could be desirable in public forests that are managed for multiple purposes, such as recreation, biodiversity, and water. These strategies would be most suitable in forest regions with active management and a high potential to store carbon, such as those with long-lived species and slowly decomposing dead plant matter, which are common in the Pacific Northwest. The carbon benefit of either of these practices will depend on the temporal and spatial scales at which they are administered – applying these practices over longer timeframes and larger landscapes leads to greater carbon benefits. In addition to an increase in carbon storage, benefits of decreased harvesting also include an increase in structural and species diversity. On the other hand, the costs are an increased risk of carbon loss due to disturbance and the potential for increased harvesting elsewhere to compensate for the reduction in forest products generated. 4. Forest management: increasing forest growth In addition to afforestation, another strategy for increasing carbon storage is to increase the growth rate of existing or new forests. Management practices that can increase forest growth include: regenerating harvested or damaged forests, controlling competing vegetation, fertilizing, planting genetically improved trees, and selecting species for superior productivity. Yield gains from these practices can be impressive. In pine forests in the southern U.S., tree breeding has improved wood growth (and carbon storage rate) by 10-30%, and fertilization can show 100% gains for wood growth. For southern Box 2. CO-BENEFITS OF FORESTS Our report focuses on forests seen through the lens of carbon, and only carbon. However, forests are managed for many purposes, and carbon storage and the growth of wood for products and fuel to offset fossil fuel use are far from the only reasons forests are valuable. Forests also provide many other ecosystem services that are important to the well-being of the U.S. and its inhabitants: protection of watersheds from erosion, nutrient retention, good water quality, reduction of peak streamflow and an increase in base streamflow, wildlife habitat and diversity, recreational opportunities and aesthetic and spiritual fulfillment, and biodiversity conservation. Americans are strongly attached to their forests. In some cases, managing strictly for carbon would conflict with other co-benefits of forests. The option of avoided deforestation retains the co-benefits of forests and the carbon in forest ecosystems, while afforestation adds these co-benefits in addition to increasing carbon storage. Even simple forests, such as plantations, generally reduce erosion, regulate streamflow, and increase wildlife habitat and biodiversity compared to crops or livestock pasture because the frequency of harvest or stand–replacing disturbance is much less for forests. Figure 7. Average carbon stored on a landscape will vary with the time between harvests (harvest interval) and how much biomass is removed each harvest. Lengthening the harvest interval will have a greater effect for harvests where removals are high (blue arrows show an increase in harvest interval from 25 to 75 years). Decreasing harvest intensity from 100% of trees to 20% of trees (black arrows) will have a greater effect for shorter harvest intervals. (Courtesy of Mark E. Harmon, Oregon State University, 2009.) 20% Removal 100% Removal 0 50 100 150 200 Harvest Interval (Years) 200 150 100 50 0 Carbon Storage (Mg/ha)

ISSUES IN ECOLOGY NUMBER THIRTEEN SPRING 2010 8 esa © The Ecological Society of America • esahq@esa.org U.S. pines, operational plantations using improved seedlings, control of competing vegetation, and fertilization grow wood four times faster than naturally regenerated secondgrowth pine forests without competition control. The potential to increase forest growth varies by climate, soil, tree species, and management. Increases in carbon stocks will generally be proportional to increases in growth rates. That is, a 10% increase in growth will result in a 10% increase in carbon stocks, assuming that the harvest interval and amount harvested do not change. As shown in Figure 3, the rate of forest growth will naturally slow down as the forest ages. Management decisions for increasing carbon stocks should take into account forest growth over time, the amount of timber that would end up in wood products if the forest were harvested, and how long the harvested carbon would remain sequestered in the wood products. Knowledge of these variables will help determine when or whether to harvest. The area of forestland in the U.S. that could be managed to increase forest growth includes more than 500 million acres and consists of almost all U.S. public and private forestland, excluding remote and reserved areas such as national parks. However, even reserved areas could potentially be managed to restore damaged ecosystems, which could also lead to increased forest growth. Increasing forest growth through management has benefits and costs. The benefits include increased wood production and the potential for planting species and genotypes adapted to future climates. The costs include reducing the carbon benefit by emissions of nitrous oxide from forest fertilization, reduced water yield (faster growth uses more water), and a loss of biodiversity if faster growth is accomplished by replacing multi-species forests with monocultures. 5. Forest management: fuel management to reduce fire threat Fuel management uses thinning (Box 3) to lower foliage biomass to reduce the risk of crown fire because crown fires are difficult, if not impossible, to control. Fuel management occurs in forests with a variety of historical fire regimes – from forests where historical forest density was lower and the natural fires were mostly surface fires, to forests with standreplacement fire regimes in which crown fires naturally occurred. Fuel management temporarily lowers the carbon stored in forest biomass and dead wood because the thinned trees are typically piled and burned or mulched and then decompose. If a crown fire burns through a forest that was thinned to a low density, the fire may change from a crown to a surface fire in which many of the trees can often survive the fire. In contrast, many or all of the trees in an unthinned stand will be killed by a crown fire. This contrast in survival has led to the notion that fuel treatments offer a carbon benefit: removing some carbon from the forest may protect the remaining carbon. There are two views regarding the science on carbon savings through fuel treatments. Some studies have shown that thinned stands have much higher tree survival and lower carbon losses in a crown fire, or have used modeling to estimate lower carbon losses from thinned stands if they were to burn. However, other stand-level studies have not shown a carbon benefit from fuels treatments, and evidence from landscape-level modeling suggests that fuel treatments in most forests will Box 3. THINNING AND CARBON Thinning is an effective forest management technique used to produce larger stems more quickly, reduce fire risk, and increase tree resistance to insects and disease. Thinning increases the growth of the remaining individual trees, but generally decreases overall forest wood growth until the remaining trees grow enough to re-occupy the site. The carbon stock in a thinned stand is generally lower than that in an unthinned stand. If the harvested trees are used for biomass energy or long-lived forest products, these carbon benefits may compensate for the lower biomass and the wood growth of the thinned stand. Because of lower overall growth of a thinned stand, even 100% use of the harvested trees for products or biomass energy may not produce a total carbon benefit greater than that of the higher storage and storage rate in an unthinned stand. The net carbon consequences of thinning will depend the most on whether the harvested trees are used for long-lasting wood products or biomass energy, but also on the change in risk of a crown fire relative to the probability of fire occurring, the species, the site, the thinning regime, and the length of the harvest interval. Figure 8. A hydro-axe is used to grind up trees to reduce canopy fuel loads and lower the risk of crown fire. Photo by Dan Binkley, Colorado State University

© The Ecological Society of America • esahq@esa.org esa 9 ISSUES IN ECOLOGY NUMBER THIRTEEN SPRING 2010 decrease carbon, even if the thinned trees are used for biomass energy. More research is urgently needed to resolve these different conclusions because thinning to reduce fuel is a widespread forest treatment in the U.S. We recommend that such research focus on the landscape scale because carbon loss in thinning needs to be placed in the context of the expected fire frequency and extent, and the potential for regeneration after fire. Regardless of the outcome of such research, the carbon benefits of fuel treatments can be improved by using the harvested trees for wood or biomass energy. 6. Urban forestry Urban forestry offers very limited potential to store carbon, but we address urban forests here because of the large interest in using them to offset carbon emissions and because urban trees provide many co-benefits, including aesthetic benefits and environmental advantages in addition to carbon sequestration. The potential for carbon offsets of greenhouse gas emissions through urban forestry is very limited for two reasons: 1) urban areas make up only a small fraction of the U.S. landscape and 2) urban forests are intensively managed and may require large energy, water, and fertilizer inputs for planting and maintenance. Urban forests can have important biophysical effects on climate. Trees have a cooling effect on local temperatures due both to shading effects and to evaporative cooling in transpiration. Shading intercepts incoming radiation in the daytime, which can reduce both day and night surface temperatures. When trees are planted very close to buildings, they cool building temperatures and reduce the fossil fuel emissions associated with air conditioning. When urban forests are planted over very large regions, the climate effects are less certain, as trees can have both warming (absorption) and cooling effects. The higher the maintenance required for urban trees, the less likely they will help mitigate climate change. In some regions, cities are located in what would naturally be forested areas; thus, urban forests serve to restore forests to land that was previously deforested. In such regions, trees may have relatively low maintenance requirements. In cities located in grasslands and deserts, urban forests require large amounts of irrigation water for maintenance. Because of these many tradeoffs, the following factors must be taken into account to determine the net climate impact of urban trees: 1) the carbon storage rate of the trees, 2) fossil fuel emissions from energy associated with planting and maintenance, 3) fossil fuel emissions resulting from the irrigation process, 4) nitrous and nitric oxide emissions from fertilizer use, and 5) the net effect of trees on local air temperature and its impact on building energy use. These factors are likely to be highly variable by region and by species. 7. Biomass energy, carbon storage in products, and substitution Biomass energy The use of forest biomass energy prevents carbon emissions from fossil fuel use. In 2003, biomass energy was 28% of the U.S. renewable energy supply and 2% of the total U.S. energy use. Biomass energy is used primarily for electric power in the forest products industry and for residential heating. In the future, biomass may become an important feedstock for liquid biofuels. If cost were not a constraint and the public supported this use of forests, U.S. forests could potentially provide energy production offsetting 190 teragrams of fossil fuel carbon emissions per year, or the equivalent of 12% of U.S. fossil fuel emissions in 2003 (as discussed further in Environmental costs below). It has been estimated that by 2022, forest biomass feedstocks could produce 4 billion gallons of liquid biofuel per year (offsetting 2.6 teragrams of fossil fuel carbon emissions). Figure 9. Sycamores lining Sycamore Street in Los Angeles, California. Photo by Diane E. Pataki, University of California, Irvine