Forests and Carbon Storage
U.S. forests currently serve as a carbon 'sink', offsetting approximately 13% of U.S. emissions from burning fossil fuels in 2011, and from 10 to 20% of U.S. emissions each year. Climate change may affect the ability of U.S. forests to continue to store and sequester carbon.
The synthesis discusses the role of forests in carbon sequestration, and management options for helping forests maintain or increase their capacity to store carbon, even under future conditions.
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An archived version of this topic paper is available
Climate change increases the uncertainty of U.S. forests' ability to serve as a "sink" for carbon storage, but management options exist that could buffer the impacts of climate change on forests, and even lead to increased forest carbon storage potential.
Trees take up carbon dioxide (CO2) and release oxygen (O2) through photosynthesis, transferring the carbon (C) to their trunks, limbs, roots, and leaves as they grow. When leaves or branches fall and decompose, or trees die, the stored C will be released by respiration and/or combustion back to the atmosphere or transferred to the soil. Because of these processes, forests and forested landscapes can store considerable carbon and their growth can provide a carbon sink; landscapes that have been recently converted or reconverted to forests (from another land cover) can provide a carbon sink that is considerably larger than other land cover types.
Approximately 33% (303 million hectares) of the U.S. land base is forested (1). This represents roughly 7.5% of the world’s total forestland (2). In 2010, U.S. forests and long-lived wood products accounted for a net sink of 251 million metric tons of carbon (922 million metric tons CO2) (3). Forest growth and afforestation currently offset approximately 16% of U.S. emissions from burning fossil fuels (4). This is an enormous ecosystem service; Jackson and Schlesinger (5) estimated that offsetting another 10 percent of emissions would require the conversion of one-third of our current U.S. cropland to forest plantations.
While individual trees or tracts release some or all of their carbon if harvested, burned, or otherwise disturbed, subsequent forest regrowth will sequester carbon from the atmosphere. Forested landscapes tend to include a mix of disturbed and regrowing forest stands, and have a carbon balance of near zero over the medium and longer term (6,7) (Figure 1).
Our large carbon sink today is a legacy of harvesting and forest conversion that took place in the past. These disturbances released much carbon dioxide (CO2) into the atmosphere decades ago, and the regrowing forest is recovering some of that released CO2 on land that has not been permanently converted to non-forest cover (8) (Figure 2).
The persistence of the current U.S. forest carbon sink is uncertain because the effects of historic land use should taper off, while projected increases in the rates of natural disturbances such as fire may liberate current carbon stocks (4). Atmospheric factors may change forest growth rates, since increased nitrogen deposition and atmospheric CO2 concentrations from fossil fuel emissions can enhance tree growth. These factors may also augment current rates of carbon sequestration by forests (9). However, other global change factors, such as increased transpiration rates and atmospheric pollutants and the likelihood of increased drought, may offset potential increased sequestration rates (10).
While much about the forest carbon cycle is well understood, several key unknowns remain. The scientific community, including foresters, understands the carbon value of keeping forests as forests, planting forests where none existed historically (afforestation), replanting forests where they were located historically (reforestation), using forest biomass as fuel in place of fossil fuel, and storing carbon in long-lived products (which may continue to store carbon for years or decades). However, further research on several topics could improve our ability to design good forest management practices with respect to carbon:
- Understand the biophysical limits on storage over landscapes and over time, and how these limits may change in the future.
- Improve capabilities to predict the frequency and severity of forest disturbances.
- Implement a system of complete accounting of the global warming effects of forest management, which would include their albedo or reflectance (forests are dark and absorb solar energy) and the release of other greenhouse gases such as nitrogen oxides.
- Improve data and accounting of storage in all forest carbon pools and the displacement of carbon loss to other areas.
Over the next 50 years, the United States is expected to become warmer and wetter, which may enhance forest growth in some regions. However, forest carbon stocks are likely to be more vulnerable to disturbances that are exacerbated by climate change, such as insect outbreaks, fire, drought, and storms. This may in turn lower the productivity and storage capacity of some forests while threatening the ability of some forests to remain forests. Because of more frequent/repeat burning, some forests may convert to shrubland. (11,12,13,7). The effects of climate change are likely to affect forested landscapes in the eastern and western U.S. differently. The following are examples of what may occur, but does not represent a comprehensive list:
- In the eastern U.S., elevated temperature and atmospheric CO2 concentrations will likely continue to enhance sequestration by forests, but this sequestration may be offset by forest fragmentation and disturbances by invasive insects.
- In the Southeast, warmer temperatures may increase the rate of decomposition of soil organic matter, thereby increasing CO2 emissions and reducing the potential for sequestration in soils (4, p.12).
- In the western U.S., elevated temperatures and decreased precipitation is expected to lead to drought conditions that will exacerbate stress complexes that include fire and insect disturbance. Insect infestations are expected to affect more land than wildfire on an annual basis (4). Higher tree mortality, slower regeneration, and changes in the mix of tree species may result from these disturbances. While short-term effects will depend upon the amount of area affected, the cumulative impact of disturbances may turn western forests from a carbon sink into a source of atmospheric carbon.
In the decade since 2002, forest fires annually burned 0.9 percent of forested land in the United States, with the largest fire year (2006), burning 1.3 percent of forested land. This corresponds to an overall average return interval of 100 years for U.S. forest fires. Models run with downscaled climate data for the Greater Yellowstone Ecosystem predict substantial increases in fire in this region by mid-century, with fire rotation reduced to less than 30 years from the current 100-300 year return interval (14). If fires become more severe, especially where ecosystems are not adapted to severe fire, the likelihood that fire will change forest to shrublands or grasslands may increase (15). Annual carbon emissions related to fire vary considerably depending upon the year. Circumstances that directly affect fire activity include atmospheric circulation, temperature, and moisture patterns (16). Estimates of fire-related emissions range from 22.6 million metric tons/year (2010) to 84.4 million metric tons (2006), compared to net forest sequestration of 251 million metric tons/year (3).
Options for Management
The most defensible options for managing forests for their carbon storage are (7):
- Keep forests as forests (avoid deforestation), including active regeneration of frequent fire forests subjected to crown fires where natural regeneration may take centuries.
- Manage forests sustainably for a variety of ecosystem services (maximizing carbon stores on a landscape in the near-term may ultimately lead to more uncertain carbon outcomes, due to an increased risk of fire or disturbance in the mid to long-term).
- Reforest areas where forests historically occurred.
- Substitute forest biomass for fossil-fuel use, especially forest biomass generated in normal operations, fuels treatment and forest restoration activities.
- Promote long-lived forest products such as wood-framed buildings. Long-lived forest products continue to act as carbon stores whereas substitute materials, such as concrete, result in significant carbon emissions. (7).
Harvesting old-growth forests for their forest products is not an effective carbon conservation strategy because the carbon remaining in the wood products plus the regrowth are not enough to compensate for the loss of large carbon stocks in the intact forests (17). Harvest and regeneration of young to middle-aged forests for long-lived forest products can help with carbon storage. In forests with an ecological history of surface and mixed-severity fires, managing for maximum carbon storage will lead to an increase in stand density and the probability of more severe fires. In contrast, managing to reduce fuels and the risk of crown fire will reduce the carbon stored in the forest and will likely be a source of atmospheric carbon unless the thinnings are used for biomass fuel.
Selling carbon or other ecosystem service credits may provide a supplementary revenue stream to help reduce costs of forest management in the future, especially if carbon reductions become more valuable in the United States. However, because carbon trading markets typically require long-term encumbrances on the land, participation in these markets may not be a viable option for all landowners. In addition, the specific accounting rules related to carbon offset must recognize that forest carbon stores are reversible and can be affected by economic drivers. Protocols that ensure that any carbon offsets generated will be real, permanent, additional, verifiable, and enforceable are critical to the integrity of any market-based solution. Another option would simply be to pay landowners for maintaining forest biomass, as the current Conservation Reserve Program pays landowners to maintain a certain land use. While perhaps more expensive, it would be much simpler to understand and administrate.
Forest managers must recognize that carbon is only one of many ecosystem services that forests provide and that focusing solely on carbon could lead to non-optimal management decisions. Fuels treatments and forest restoration activities in frequent-fire forests will promote a more adaptable, sustainable forest that tends to experience low intensity fires instead of crown fires. Yet such treatments may move carbon from the forest to the atmosphere. Intensive biomass use could also move carbon from the forest to the atmosphere, at least in the short term. Carbon should be only one of the many factors considered when making forest management decisions.
- Smith, W.B.; Miles, P.D.; Perry, C.H.; Pugh, S.A. 2009. Forest resources of the United States, 2007.
General Technical Report WO-78. Washington, DC: USDA Forest Service, Washington Office.
- Food and Agricultural Organization (FAO) of the United Nations. 2011. State of the World's Forests 2011. Rome, Italy.
- US Environmental Protection Agency. 2012. Inventory of US Greenhouse Gas Emissions and Sinks: 1990-2010. Washington D.C.
- Vose, J. M.; Peterson, D. L.; Patel-Weynand, T., eds. 2012. Effects of climatic variability and change on forest ecosystems: a comprehensive science synthesis for the U.S. forest sector. Gen. Tech. Rep. PNW-GTR-870. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 265 p.
- Jackson, R.B.; Schlesinger, W.H.. 2004. Curbing the U.S. carbon deficit. Proceedings of the National Academy of Sciences. USA 101: 15827-15829.
- Kashian, D. M.; Romme, W. H.; Tinker, D. B.; Turner, M. G.; Ryan, M. G. 2006. Carbon storage on landscapes with stand-replacing ï¬res. BioScience. 56: 598-606.
- 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.
- Birdsey, R.; Pregitzer, K.; Lucier, A. 2006. Forest carbon management in the United States 1600-2100. Journal of Environmental Quality. 35: 1461-1469.
- Canadell, J. G.; Pataki, D. E.; Gifford, R.; Houghton, R. A.; Lou, Y.; Raupach, M. R.; Smith, P.; Steffen, W. 2007. Saturation of the terrestrial carbon sink. In: Canadell, J.G.; Pataki, D. E.; Pitelka, L., eds. Terrestrial ecosystems in a changing world. Berlin Heidelberg, Germany: The IGBP Series , SpringerVerlag: 59-78.
- Felzer, B.; Reilly, J.; Melillo, J.; Kicklighter, D.; Sarofim, M.; Wang, C.; Prinn, R.; Zhuang, Q. 2005. Future effects of ozone on carbon sequestration and climate change policy using a global biogeochemical model. Climatic Change (73): 345-373.
- Dale, V. H.; Joyce, L. A. ; McNulty, S.; Neilson, R. P. 2000. The interplay between climate change, forests, and disturbances. Science of the Total Environment. 262: 201-204.
- Westerling, A. L.; Gershunov, A.; Brown, T.J.; Cayan, D. R.; Dettinger, M.D. 2003. Climate and wildï¬re in the western United States. Bulletin of the American Meteorological Society 84: 595-604.
- Running, S.W. 2006. Is global warming causing more, larger wildï¬res? Science 313: 927-928.
- Westerling, A.L.; Turner, M.G.; Smithwick, E.A.H.; Romme, W.H.; Ryan, M.G. 2011. Continued warming could transform Greater Yellowstone fire regimes by mid-21st century. Proceedings of the National Academy of Sciences. 108(32): 13165-13170.
- Smithwick, E. A. H.; Harmon, M. E.; Domingo, J.B. 2007. Changing temporal patterns of forest carbon stores and net ecosystem carbon balance: the stand to landscape transformation. Landscape Ecology 22: 77-94.
- McKenzie, D.; Heinsch, F.A.; Heilman, W.E. 2011. Wildland Fire and Climate Change. U.S. Department of Agriculture, Forest Service, Climate Change Resource Center.
- Harmon, M.E.; Ferrell, W.K.; Franklin, J.F. 1990. Effects on carbon storage of conversion of old-growth forests to young forests. Science. 247: 699-702.
- 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.
Ryan, M.G.; Birdsey, R.A.; Hines, S.J. (October 2012). Forests and Carbon Storage. U.S. Department of Agriculture, Forest Service, Climate Change Resource Center. www.fs.usda.gov/ccrc/topics/forest-carbon
The following documents have been recommended by the authors of the synthesis paper and by the CCRC Production team.
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.
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.
Metsaranta, J.M.; Kurz, W.A.; Neilson, E.T.; Stinson, G. 2010. Implications of future disturbance regimes on the carbon balance of Canada's managed forest (2010-2100). Tellus Series B. 62: 719-728.
Hines, S.J.; Heath, L.S.; Birdsey, R.A. 2010. An annotated bibliography of scientific literature on managing forests for carbon benefits. Gen. Tech. Rep. NRS-57. Newtown Square, PA: Department of Agriculture, Forest Service, Northern Research Station. 49 p.
Hauser, R.; Archer, S.; Backlund, P.; Hatfield, J.; Janetos, A.; Lettenmaier, D.; Ryan, M.G.; Schimel, D.; Wlash, M. 2009. The Effect of Climate Change on US Ecosystems. US Global Change Research Program. 24 p.
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.
Nabuurs, G.J.; Masera, O.; Andrasko, K.; Benitez-Ponce, P.; Boer, R. ; Dutschke, M.; Elsiddig, E.; Ford-Robertson, J.; Frumhoff, P. ; Karjalainen, T.; Krankina, O.; Kurz, W.A.; Matsumoto, M.; Oyhantcabal, W.; Ravindranath, N.H.; Sanz Sanchez, M.J.; Zhang, X. 2007. Forestry. In: Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (eds)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
There is extensive Forest Service research on carbon storage in forests - some examples are available below via the CCRC Research Roundup.
Pacific Northwest Research Station
Water stress represents a common mechanism for many of the primary disturbances affecting forests, and forest management needs to explicitly address the very large physiological demands that vegetation has for water. This study demonstrates how state-of-science ecohydrologic models can be used to explore how different management strategies might improve forest health.
Pacific Northwest Research Station
Research and policy discussions highlight the role of forests in reducing greenhouse gases by storing carbon. An important factor regarding forests and carbon is simply maintaining the amount of land that is retained in forest cover. Since 1973, Oregon’s statewide land-use planning program has sought to maintain forest and agricultural lands in the face of increasing development by maintaining forest and agricultural zones and to limit growth to within urban growth boundaries. We combine projections of forest and agricultural land development with estimates of average carbon stocks for different land uses to examine what effect land-use planning has had in maintaining forest carbon in western Oregon. In addition to other benefits arising from the conservation of forestland, results indicate that Oregon’s land-use planning system in western Oregon yields significant gains in carbon storage equivalent to a reduction of 1.7 million metric tons of carbon dioxide (CO2) emissions per year.
The U.S.-China Carbon Consortium is a collaborative effort between American and Chinese institutions interested in studying the role of managed ecosystems in global carbon and water cycles. The overall goal is to develop a network of study sites so that data and results can be shared and synthesized at broad spatial scales in order to assess the importance of human influences on carbon and water fluxes in a changing climate.
Scientists are contributing to a four-year National Science Foundation-funded project focused on decadal and regional climate prediction using earth system models. The project's specific goals are to improve upon and extend current modeling capabilities to offer new assessment tools for climate change research and management agencies.
The PINEMAP project integrates research, extension, and education to enable southern pine landowners to manage forests to increase carbon sequestration; increase efficiency of nitrogen and other fertilizer inputs; and adapt forest managment approaches to increase forest resilience and sustainability under variable climates.
Researchers are assessing the causal relationships between management regime or disturbance and the environmental controls of biosphere-atmosphere exchange of carbon and water. The overall objective is to measure and model the coupling effects of forest management and changing climate on carbon dioxide and water fluxes in eastern forests of the United States and China.
Woody production systems and conversion technologies are needed to: maintain healthy forests and ecosystems, create high paying manufacturing jobs, and meet local/regional energy demands. Poplars are dedicated energy crops that can be strategically placed in the landscape to conserve soil and water, recycle nutrients, and sequester carbon. However, key environmental and economic uncertainties preclude broad-scale production of biofuels/bioproducts from poplar wood. Therefore, building on decades of research conducted at our Institute and throughout the region, we are evaluating the fate of carbon in soils and woody biomass, soil greenhouse gas emissions, and conversion efficiency barriers throughout the energy supply chain.
Elena Aguaron-Fuente and Greg McPherson authored a chapter in the book Carbon Sequestration in Urban Ecosystems. They found substantial variability in sequestration estimates produced by four methods-i Tree Streets, i-Tree Eco, the CUFR Tree Carbon Calculator, and Urban General Equations-and concluded that the latter could be used to produce conservative estimates from remotely sensed data compared to urban-based species-specific equations.
Northern Research Station
The Northern Forest Ecosystem Experiment is a large-scale, long-term field experiment in which harvested forests regenerate in atmospheres with enhanced concentrations of carbon dioxide (CO2), ozone (O3) or both gasses combined. This Experiment takes place on the same site as the 11-year Aspen FACE Experiment, following the final data collection for the Aspen FACE project in 2009.
The Water Supply Stress Index (WaSSI) is an integrated model that estimates ecosystem water and carbon balances and the interactions among ecosystem evapotranspiration, productivity, carbon sequestration, and biodiversity at the continental scale by coupling the key processes of the hydrologic and carbon cycles.
COLEv2.0 enables the user to examine forest carbon characteristics of any area of the continental United States.
This Carbon Calculator provides quantitative data on carbon dioxide sequestration and building heating/cooling energy effects provided by individual trees.
The ecoSmart Landscapes tool can be used to calculate carbon dioxide sequestration and building energy savings provided by individual trees.
FOFEM is a model that predicts first-order fire effects including tree mortality, fuel consumption, emissions (smoke) production, and soil heating caused by prescribed burning or wildfire.
The Forest CarbonPlus calculator can be used to calculate carbon emissions that are directly related to day-to-day operations of Forest Service facilities and activities of Forest Service employees.
The Forest Planner enables landowners to visualize alternative forest management scenarios for their properties and their effect on variables including timber stocking and yields, carbon storage, and fire and pest hazard ratings.
The Forest Vegetation Simulator (FVS) is a family of forest growth simulation models that allow a user to explore how silvicultural treatments may affect growth and yield and, therefore, carbon stocks.
FCCS quantifies and classifies the structural and geographical diversity of wildland fuels in the United States and predicts their relative fire hazard. Current versions also predict surface fire behavior and quantify carbon stores for each calculated fuelbed.
The Global Carbon Atlas lets users explore, visualize and interpret national to global carbon emissions from both human activities and natural processes.
i-Tree is a peer-reviewed software suite that allows users to assess the benefits provided by urban trees. Some applications give estimates of the benefits that trees provide related to greenhouse gas mitigation and building energy savings