Global climate change is a widespread and growing concern that has led to extensive international discussions and negotiations. Responses to this concern have focused on reducing emissions of greenhouse gases, especially carbon dioxide, and on measuring carbon absorbed by and stored in forests, soils, and oceans. Numerous issues regarding the carbon cycle in forests, monitoring the levels and changes in forest carbon, and the scientific uncertainties about the relationships among forests, carbon, and climate change are likely to be the subject of ongoing federal research efforts, with funding and oversight by the Congress. Carbon Cycling in Forests

Photosynthesis is the chemical process by which plants use sunlight to convert nutrients into sugars and carbohydrates. Soil carbon is also slowly released to the atmosphere as the vegetation decomposes. Eventually, trees die. Many of these forests are managed to produce commercial wood products, and the management practices used in temperate forests can thus have a significant impact on carbon sequestration. Converting a "working forest" (i.e., one that is managed to produce timber and other values) to a rural residential subdivision would likely reduce forest cover and perhaps undergrowth (thus releasing some vegetative carbon), but the remaining vegetation would likely continue to grow, and thus may continue to sequester carbon for longer in the subdivision than in a working forest (depending on the nature and duration of the wood products derived from the working forest). The proportion of carbon and biomass removed from any particular site varies widely, and depends on the species involved, the density of the stand (which affects both tree form and undergrowth vegetation), the diversity of tree species and tree sizes in the stand, and various environmental factors (e.g., the site's climate and soil fertility). Thus, forestry activities that disturb soils, particularly activities to remove the cut vegetation (such as commercial timber harvesting), will also likely reduce soil carbon levels. Tree planting on sites that have not recently had trees on them, such as pastures, is called afforestation . Other Growth Improvement

Other forestry practices are also intended to increase tree growth rates. Others have calculated that the carbon released by harvesting operations substantially exceeds the additional carbon sequestered by new forest stands. Another study has shown that some old-growth forests continue to accumulate carbon in their soils. Practices to store carbon by reducing or delaying timber harvested domestically can have an effect commonly called leakage —by shifting land uses geographically (e.g., more tropical forest harvests to offset less temperate forest harvests) or by shifting demand to other products that require more carbon to produce (e.g., steel or aluminum studs to replace wood studs in homebuilding). This leakage is undesirable, it is argued, because U.S. forest management protects the environment more than comparable activities in other countries. These rationales are supported by substantial anecdotal evidence, but others counter that such assertions have limited empirical foundation. Federal Assistance for State and Private Forestry

The federal government has numerous programs that provide technical and financial assistance for forest management of nonfederal (mostly state and private) forest lands, though none explicitly includes carbon sequestration as a purpose. Federal Programs Affecting Land Use

Many federal programs, in addition to the programs discussed above, can affect the rate of deforestation and afforestation in the United States. Land use changes and forestry practices alter the level and rates of carbon storage, while "leakage" (shifting production) may offset some of the increases in forest carbon sequestration. Whether and how to account for this carbon sequestration in policies and programs to mitigate climate change has been controversial.
Widespread concern about global climate change has led to interest in reducing emissions of carbon dioxide (CO2) and, under certain circumstances, in counting additional carbon absorbed in soils and vegetation as part of the emissions reductions. Congress may consider options to increase the carbon stored (sequestered) in forests as it debates this and related issues.
Forests are a significant part of the global carbon cycle. Plants use sunlight to convert CO2, water, and nutrients into sugars and carbohydrates, which accumulate in leaves, twigs, stems, and roots. Plants also respire, releasing CO2. Plants eventually die, releasing their stored carbon to the atmosphere quickly or to the soil where it decomposes slowly and increases soil carbon levels. However, little information exists on the processes and diverse rates of soil carbon change.
How to account for changes in forest carbon has been contentious. Land use changes—especially afforestation and deforestation—can have major impacts on carbon storage. Foresters often cut some vegetation to enhance growth of desired trees. Enhanced growth stores more carbon, but the cut vegetation releases CO2; the net effect depends on many factors, such as prior and subsequent growth rates and the quantity and disposal of cut vegetation. Rising atmospheric CO2 may stimulate tree growth, but limited availability of other nutrients may constrain that growth.
In this context, timber harvesting is an especially controversial forestry practice. Some argue that the carbon released by cutting exceeds the carbon stored in wood products and in tree growth by new forests. Others counter that old-growth forests store little or no additional carbon, and that new forest growth and efficient wood use can increase net carbon storage. The impacts vary widely, and depend on many factors, including soil impacts, treatment of residual forest biomass, proportion of carbon removed from the site, and duration and disposal of the products. To date, the quantitative relationships between these factors and net carbon storage have not been established.
Some observers are concerned that "leakage" will undermine any U.S. efforts to sequester carbon by protecting domestic forests. By leakage, they mean that wood supply might shift to other sites, including other countries, exacerbating global climate change and causing other environmental problems, or that wood products might be replaced by other products that use more energy to manufacture (thus releasing more CO2). Others counter that the "leakage" arguments ignore the enormous disparity in ecological systems and product preferences, and discount possible technological solutions.
Several federal government programs affect forestry practices and thus carbon sequestration. Activities in federal forests affect carbon storage and release; timber harvesting is the most controversial such activity. Federal programs also provide technical and financial help for managing and protecting private forests, and tax provisions affect private forest management. Various federal programs can also affect the extent of forested area, by supporting development (which may cause deforestation) or encouraging tree planting in open areas, such as pastures.