ISSUES IN ECOLOGY NUMBER THIRTEEN SPRING 2010 ure 250 ndof carbor 200 □Tees wood and t ar Products-L rest lower emissions from fossil fuel use.Once 150 the carbon leaves the forest,it becomes in the to track a ure thar imports and exports must then be tracked (eq/6W) 50 Biophysical effects may cause 0 warming or cooling 250 200 150 100 50 and 0 winter and bumed forests absorb more than 20406080100 Years can ergy ab by forest bon (10 grams;sce Box 1 for units)to the atmosphere,and two-thirds of this release in a oughly the amount o .S.annual fossil fuel ons.I warming.Generally,biophysical effects on cli could be released to the atmosphere from n the by the ation because the he网 e land use change will cause large differences. Untortunately,curren both deforestation and afforestation:About Strategies for increasing carbon 6,000km stores in forests The net incre e in forestlands results from 1.Avoiding deforestation d use and possibly from reduced the of forest land to other uscs,has a significant impact on carbon sink bene arhon global CO emissions.Globally,defore benefits need to be weighed dagainst the global converts approximar tely 90,000 within forests)to other land in the U.S.pushes crop and cattle production annually releases 1,400-2,000 teragrams of car- to other countries,it can lead to deforestation The Ecological Society of Americaesahq@esa.org esa 5
© 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 ■Reflected sunlight occurs,substantial carbon is lost to the atmos. Evaporation ■Transmitted heat Tree plantingwo There are not many risks associated with z.Afforestation We define afforestation as both reestablishing forest 石26 rno with such deforestation-esaly in he move substantial from the atmosphere tropics-is greater than carbon gain associated tree growtl rom afforestation in the ease of 150,00 tera earch Letters atce044006 Forest retention in the western U.S.may be nges. Th at wth forest fire size andin ensiry.insect outbreak varies with s and storm intensity.If forest regeneration fail use the di ide ion cois g8 are per yea tial turbances can convert forests to meadows or rares are found in the Pacific Nort shrublands.When this type of deforestation al Box 1.UNITS FOR CARBON prests in many western forests ors may use the provide a e be For sta tree r of aff ation (outlined in grams).Our report uses carbon mass,not,becau Box 2)are enhanced where forests include a carbon is a stan to CO,mass,multiply by 3.67 to account for the mass of theO teragrams (Tg) 1petagram (Pg) sity an 1。 1000 teragrams tonnes etric tonnes tric(Mg) 1 megatonne oration 04U.S.lo han other forest reestablishment practices. 6 esa The Ecological Society of America esahq@esa.org
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