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Carbon Sequestration Results

Average aboveground carbon in southern forests is approximately 25 tons per acre (fig. 18.10). Higher averages are found in the Appalachian Mountains and the Mississippi Alluvial Valley. Over the last 40 years, increases in biomass and organic matter on U.S. forest lands have added only enough stored carbon to offset 25 percent of national emissions for the same period (Birdsey and Heath 1997). This result has important implications because the overall carbon inventory in southern forests is predicted to remain relatively stable through 2040 (fig. 18.11).

Nonindustrial private forests store more total aboveground carbon than all public and industrial lands combined due to a much higher percentage of forest land being privately owned (table 18.3). Approximately 42 percent of the aboveground carbon in southern forests is in the oak-hickory forest-type group (table 18.4), which dominates nonindustrial land (table 18.5). Whereas the percentage of the oak-hickory forest-type group is expected to decrease slightly by 2020, it will continue to dominate nonindustrial private forests (see chapter 14 for more information). Volume and stocking density measurements on these tracts indicate that they are typically understocked and managed with low intensity (National Research Council Board on Agriculture 1998). Private landowners could make a significant contribution to carbon sequestration efforts by increasing stocking levels.

Southern pines dominate southern industrial forests due to their fast growth and high product value and therefore make up more than 60 percent of all forest industry and Timber Investment Management Organization forest land (see chapter 14 for more information). This proportion is predicted to increase by 10 to 20 percent by 2020 (table 18.5). Because intensive management strategies have been shown to increase planted pine yields 70 percent more than traditional management (see chapter 14 for more information), manipulating commercial sites will be an important carbon sequestration tool.

In the South, harvesting forests initially results in a net carbon loss, but sites begin to show a net carbon gain 10 to 15 years after harvest. Most of the carbon in harvested wood is either lost through emissions, stored in finished products, or burned for energy as a substitute for fossil fuels. Residual wood left on site decays and returns to the soil or goes off to the atmosphere as CO₂. Waste and discarded products are buried in landfills where the carbon continues to be stored. Figure 18.12 shows an example of the estimated disposition of carbon on a highly productive southeastern pine site after 80 years with a rotation age of 40 years. Whereas 53 percent of the carbon sequestered in trees is lost in emissions and energy (wood burned as a substitute for fossil fuels), 39 percent of the carbon remains stored in products and landfills. Because the total amount of carbon in wood removed from southern forests is expected to increase between now and 2035 (fig. 18.13), high levels of emissions could continue to counteract carbon sequestration efforts. However, the emissions should be estimated carefully because burning wood for energy mitigates fossil fuel emissions.