Maria Janowiak, Northern Institute of Applied Climate Science, US Forest Service, Houghton, MI.
Chris Swanston, Northern Institute of Applied Climate Science, US Forest Service, Houghton, MI.
Todd Ontl, Northern Institute of Applied Climate Science, US Forest Service, Houghton, MI.
This topic page was developed using information from the report Considering Forest and Grassland Carbon in Land Management (WO-GTR-95).
Carbon storage is typically greater within forest ecosystems when compared to lands that are used for settlements or agriculture (1). Natural ecosystems themselves also have a great degree of variation in how much and for how long carbon is stored based on the interactions among climate, soils, vegetation, and past disturbance in a particular location (2). For this reason, actions to maintain the integrity of forest ecosystems or increase their extent will generally have positive benefits for greenhouse gas mitigation (3, 4).
Figure: Carbon stocks within different ecosystems in the eastern and western U.S. Data from Liu et al. (5) and Liu et al. (6).
Avoided Conversion of Forest to Non-Forest Use
Although the conversion of forest to non-forest use (i.e., deforestation) is often discussed as an international issue, a substantial amount of forested land within the United States is converted to other uses each year. Between 1982 and 2012, more than 1 million acres of U.S. forest land were converted each year to development, agriculture, or other purposes (7). Although this loss is more than accounted for by a gain of more than 1.3 million acres of non-forest area that is converted to or reverts back to forest each year (resulting in a net gain of forest acres) (8), converting land to a non-forest use removes a very large amount of carbon at one time. Because mature forest stands are more likely to be carbon rich from the high volume of tree biomass, recovery through afforestation takes a very long time (9). Forest harvesting can quickly remove much of that accumulated biomass carbon. Further, soil carbon generally declines after deforestation from accelerated decomposition of organic matter such as litter and tree roots (9). Efforts to maintain forest cover and prevent conversion to non-forest uses help to maintain the ability of that land to sequester carbon into the future, thereby preventing emissions and also increasing the potential for additional sequestration.
Afforestation of Non-Forest Lands
Just as avoiding forest losses through deforestation and conversion to other land uses helps maintain both carbon stored in forests and the capacity to continue sequestering additional carbon, afforestation increases the potential for land to store carbon by converting non-forest land to forest. For many decades, the trend in the United States has been toward increasing coverage of forest land as ecosystems recover from past clearing and disturbance and marginal agricultural lands are taken out of production (10). Afforestation can increase sequestration within the United States at an average of about 2.2 to 9.5 metric tons of carbon per acre per year for 120 years (11, 12). Afforestation may be more feasible on lower-value lands that are marginal for agriculture or other activities (13), with benefits for both biomass and soil carbon stocks (14).
Urban Forests Lands
While reductions in forestland conversion and increases in afforestation on previously converted lands play an important role in increasing carbon sequestration, we can also look to non-forested lands, such as urban areas, as a resource for not only sequestering carbon but also mitigating carbon outputs. Urban areas in the continental United States covered approximately 68 million acres in 2010, nearly 3.6 percent of the land area. Tree cover in urban areas averages 35 percent (15), making urban forests important stores of carbon in biomass. Between 597 and 690 million tons of carbon is stored within urban trees, with an annual sequestration rate of 18.9 million tons (16). Overall carbon sequestration of urban forests is proportional to existing canopy cover and tree density within a city (17). Although rural forests typically sequester twice as much CO2 as urban forests due to higher tree densities, urban trees can sequester more carbon per tree from higher growth rates (18). Enhanced carbon sequestration rates in urban trees may be explained by a combination of greater foliar biomass and reduced competition from lower tree densities, in addition to irrigation and fertilization. Urban forests also have additional benefits for carbon outside of sequestration. Trees in urban zones can have an important influence on carbon mitigation by reducing the energy requirements for building heating in winter due to wind protection and summer cooling from tree shading (19). For example, three mature trees spaced around an energy efficient home can reduce annual air conditioning demand by 25 to 43 percent (20).
Managing urban forests for carbon capture often focuses on allocating resources to tree species that are most effective at long-term carbon storage. While growth rate is important for carbon benefits, tree species that are long-lived, large in size, and have dense wood will store the greatest amounts of carbon, particularly relative to many short-lived, fast-growing species (21). Proper site selection for individual species is also important for maximizing the carbon benefits of urban trees. Trees that are well-adapted to their site will have higher growth rates and lower mortality rates, particularly in the initial years following establishment. Proper siting of trees in relation to buildings also optimizes the energy-saving benefits from derived from summer shading or wind protection. For example, trees typically provide the greatest summer cooling benefits when placed on the west side of buildings.
Janowiak, M.; Swanston, C.; Ontl, T. 2017. Importance of Forest Cover. (June, 2017). U.S. Department of Agriculture, Forest Service, Climate Change Resource Center. https://www.fs.usda.gov/ccrc/topics/forest-mgmt-carbon-benefits/forest-cover
Martin, K.L.; Hurteau, M.D.; Hungate, B.A.; Koch, G.W.; North, M.P. 2015. Carbon tradeoffs of restoration and provision of endangered species habitat in a fire-maintained forest. Ecosystems. 18(1): 76-88.
McKinley, D.C.; Ryan, M.G.; Birdsey, R.A.; Giardina, C.P.; Harmon, M.E.; Heath, L.S.; Houghton, R.A.; Jackson, R.B.; Morrison, J.F.;Murray, B.C.; Pataki, D.E.; Skog, K.E. 2011. A synthesis of current knowledge on forests and carbon storage in the United States. Ecological Applications. 21(6): 1902-1924.
Millar, C.I.; Skog, K.E.; McKinley, D.C.C.; Birdsey, R.A.; Swanston, C.W.; Hines, S.J.; Woodall, C.W.; Reinhardt, E.D.; Peterson, D.L.; Vose, J.M. 2012. Adaptation and mitigation. In: J. M. Vose, D. L. Peterson and T. Patel-Weynand, eds. Effects of climatic variability and change on forest ecosystems: a comprehensive science synthesis for the U.S. forest sector. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station: 125-192.
Ryan, M.G.; Harmon, M.E.; Birdsey, R.A.; Giardina, C.P.; Heath, L.S.; Houghton, R.A.; Jackson, R.B.; McKinley, D.C.; Morrison, J.F. 2010. A synthesis of the science on forests and carbon for U.S. forests. Issues in Ecology. 13: 1-16.
U.S. Climate Change Science Program. 2007. The first state of the carbon cycle report (SOCCR): the North American carbon budget and implications for the global carbon cycle. A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. Asheville, NC: National Oceanic and Atmospheric Administration, National Climatic Data Center. 242 pp.
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3. Birdsey, R.; Alig, R.; Adams, D. 2000. Chapter 8: Mitigation Activities in the Forest Sector to Reduce Emissions and Enhance Sinks of Greenhouse Gases. In: L. A. Joyce, R. Birdsey and (eds.), eds. The impact of climate change on America's forests: a technical document supporting the 2000 USDA Forest Service RPA Assessment. Fort Collins, CO: Rocky Mountain Research Station, USDA Forest Service.
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