GLOBAL POLICY COMMITMENTS Carbon Sequestration and Southern Pine Forests

Carbon Sequestration and Southern Pine Forests mary controls of the major carbon pools. We then suggest management options for increasing carbon sequestration and research needed so that southern pine forest carbon sequestration can be better quantified, predicted, and managed. Land Use: A Major Determinant Two primary forces determine land use in the South. One is economic development that generates demand for urban, residential, and other humandominated land use; the other is the relative economic returns from agriculture and timber. The margin between agriculture and forest land use is uniquely elastic in the South. Over the past 20 years, forest areas have shifted into developed and urban uses and a substantial area of agricultural land has shifted back to forest; on net, the area of forests in the South has remained relatively constant over the past 40 years. However, the distribution of forest types has changed substantially. Increased pine plantations have steadily offset declines in the area of natural pine. Natural pine has declined from about 72 million acres in 1952 to roughly 34 million acres in 1998, while plantations now comprise more than 32 million acres (about 16 percent) (USDA Forest Service RPA data; see also Powell et al. 1993). Economic land-use models project that, with stable agricultural prices, shifts to forestland from agriculture will continue to offset urbanization. Urbanization will be concentrated largely in the Piedmont and coastal areas, while expanding forest cover is expected in the Coastal Plain, the most productive region for growing pine. It is estimated that the area of planted pine may double to more than 60 million acres by 2040, mainly through planting on marginal agricultural land (Murray et al., in press). How Is Sequestration Estimated? Before we delve into the controls of carbon sequestration per unit land area, we will briefly discuss how these estimates are attained. Essentially, carbon sequestered by forests is the difference between carbon gained by photosynthesis and carbon released by respiration of all the components of the ecosystem; this overall carbon gain or loss is called net ecosystem productivity (NEP). In the past, NEP could only be estimated by measuring component processes on one or a group of sites and then summing the component values (table 1, p. 16). More recently, technology and theory have developed so NEP can be estimated “directly” (some modeling is needed) in a method called eddy covariance. Besides the exact measurement protocols, sampling intensity and extent also can differ widely. Annual NEP of forests in the southeastern United States, measured with eddy-covariance instruments, tends to be higher than that in forests elsewhere in North America, reaching values above 4,460 pounds of carbon per acre per year (Clark et al. 1999). Such high NEP values are supported in the Southeast by mild climate, ample precipitation during the growing season, and the presence of fastgrowing species. Unlike most broadleaf species, most coniferous species are able to absorb CO2 nearly year-round under favorable climatic conditions. Over two years, eddy covariance measurements at a 14-yearold loblolly pine (Pinus taeda) stand at the Duke Forest in Durham, North Carolina (located in the northern third of the species range) showed positive NEP in nearly all months of the year (fig. 2, p. 17). The annual NEP at this forest was estimated between 5,620 and 6,780 pounds of carbon per acre per year (Katul et al. 1999), similar to estimates in a Florida slash pine (Pinus elliottii) plantation (Clark et al. 1999). Some of the variability in NEP was due to stress from strong periods of drought (Oren et al. 1998). Using the mean annual NEP at the two monitored southern pine forests (6,110 pounds per acre), and the area covered with southern pines in the Southeast (795.9 × 103 square miles), we estimate annual NEP in southeastern pine forests should be in the order of 0.21 Pg per year (P = 1015; 0.187 × 1015 pounds per year). This NEP estimate represents a gross approximation of the upper limit for the southeastern pine forests at their current extent and equals approximately 12 percent of the annual US fossil fuel emissions. This estimate does not include the growth of mixed hardwood-pine forests. In addition, the estimate does not account for the impacts of fires, harvesting, and other biotic and abiotic perturbations. Lastly this analysis assumes that all pine forests are at the same highly productive developmental stage represented by the two monitored Journal of Forestry 15 Above-ground biomass Major influences • Productivity • Rotation length Below-ground biomass (including forest floor) Major influences • Soil type • Management • Climate Atmospheric CO2