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6.1 Carbon Sequestration Methodology: Current Conditions

Forest carbon is generally reported in terms of carbon in above- and below-ground tree components, understory vegetation, forest floor litter, and soil; with over 90 percent stored in the tree and soil components (Plantinga and others 1999). The carbon cycle involves carbon fluxes between the atmosphere, oceans, and terrestrial biosphere, with active reserves transferred through biological, physical, and chemical mechanisms (Sarmiento and Wofsy 1999). Processes that naturally increase the emission of CO₂ have historically been balanced by processes that accelerate carbon sequestration, thus resulting in little change to atmospheric CO₂ levels (U.S. DOE 1999). The current large increase in atmospheric CO₂, however, implies that CO₂ emissions exceed carbon sequestration (U.S. DOE 1999).

6.1.1 Forest Structure and Land Use

Forests contain approximately 85 percent of global aboveground carbon (Huntington 1995); however, the relationship between carbon sequestration and forest structural characteristics is complex. On average, regenerating southern forests initially act as net carbon sources, but generally become carbon sinks within 10 to 15 years due to rapid carbon accumulation (Figure 9). Carbon accumulation continues to increase until stands reach maturity. After this time, net carbon uptake begins to decrease and may approach zero (Plantinga and others 1999). Site differences (including climate, topography, and soil) greatly influence the forest productivity and carbon sequestration potential of an area. These differences are further enhanced when considering previous land use practices and their effect on soil fertility. Land use change, not climate change or atmospheric chemistry, has been and probably will continue to be the most important determinant of carbon storage, uptake, and release in terrestrial ecosystems (Sampson and others 1993).

6.1.2 Forest Soils and Long-Term Carbon Sequestration

Forest soils appear to be the best available long-term option for storing carbon in terrestrial ecosystems because the residence time of carbon in soils is much longer than in aboveground biomass (U.S. DOE 1999). Approximately 50 to 60 percent of the carbon in temperate forest ecosystems is found in the soil organic matter (SOM)(U.S. DOE 1999, Huntington 1995). Soils with high concentrations of carbon in SOM have improved nutrient absorption, retention, and resistance to erosion (U.S. DOE 1999), factors especially important for forest productivity and carbon sequestration (U.S. DOE 1999, Johnson 1992). However, understanding and quantifying soil carbon pools has been complicated by a lack of available data (Huntington 1995, Sanchez 1998). For example, temperature is an important controller of soil organic carbon dynamics, but the effects of different temperature scenarios on soil carbon are not fully understood (Garten and others 1999).

Land management practices and land use changes can directly affect the ability of soils to sequester carbon. Practices that protect soil and reduce erosion greatly improve the potential of those soils to sequester carbon (U.S. DOE 1999). Comparing disturbed (previously harvested) and relatively undisturbed (no known cultivation or harvesting since European settlement) watersheds in Georgia, Huntington (1995) found that disturbed sites have potential for large increases in soil carbon storage. Harvesting followed by cultivation also results in substantial losses of SOM; intensive cultivation after forest harvesting can cause SOM to decrease by 50 percent in the upper 7.87 inches of soil (Huntington 1995, U.S. DOE 1999). This practice can also result in overall soil carbon losses of 30 to 60 percent (Huntington 1995). Converting cultivated land to forests, on the other hand, provides an important carbon sink. There are clearly opportunities to increase carbon storage in soil through reforestation of former agricultural land and adoption of forest management practices like fertilization and genotype improvement that increase net rates of biomass production (Johnson 1992). Timber harvesting followed by forest regrowth does not necessarily reduce soil carbon storage (Huntington 1995) and may increase soil carbon storage (Johnson 2001). When followed by erosion and subsequent loss of SOM, however, harvesting does result in substantial losses of soil carbon and fertility. Harvesting practices may increase soil carbon when specifically designed to do so, by burying forest floor material and downed dead wood in the soil. At the broad scale, while soil fertility losses may be partially mitigated by increases in CO₂ and nitrogen deposition, air and water pollution may lead to soil degradation and further carbon loss (Huntington 1995, Sarmiento and Wofsy 1999).

6.1.3 Long-Term Carbon Storage in Harvested Wood

Harvested wood provides options for long-term carbon storage and, when burned, a substitute for non-renewable fossil fuel-derived emissions (Heath and others 1996, Skog and Nicholson 1998). Carbon can be stored for centuries in furniture or housing. When discarded in anaerobic landfills like those currently used in the United States, wood stores carbon for long periods.