Mike Ryan, Rocky Mountain Research Station; Richard Birdsey, Northern Research Station; Sarah Hines, Rocky Mountain Research Station.
An archived version of this topic paper is available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.