Carbon Sequestration- Current Estimates of Forest Carbon

(Mickler-SFS Ch. 33)

Since the early 1950s, forests in the United States have been surveyed periodically on a Statewide basis. The inventories have been conducted more intensively on the more productive forest land known as timberland. Timberland is defined as land capable of producing in excess of 20 cubic feet per acre per year of industrial wood products and not designated as reserved. Approximately 94 percent of southern forests are timberlands. Significant areas of less productive or reserved forest lands that have not been inventoried in the past exist in Texas, Oklahoma, and Florida. FIA surveys were designed for estimating forest area and merchantable timber volume. Tree volume can be converted to carbon (C) mass by means of basic models or conversion factors. C in other forest components can be estimated similarly on the basis of forest attributes. Forest C is often assessed separately for a number of pools: (1) aboveground live trees, (2) aboveground standing dead trees, (3) down dead wood (including stumps), (4) understory vegetation, and (5) the forest floor. Aboveground pools are often focused on because of the uncertainty in belowground C estimates, and because there are accounting difficulties relating to the transfer of large blocks of soil C as a consequence of land use change (Heath and others 2003).

Southern forests presently contain 5810 Mt of aboveground C on 87 million ha. Forests of the conterminous United States contain 20 340 Mt of aboveground C on 250 million ha. Thus the South accounts for approximately 29 percent of aboveground forest C stock in the conterminous United States. Allocation of this stock among forest ecosystem pools is shown in table 33.1. The sum of standing C in live and dead trees is provided in table 33.2 by forest type and ownership. The majority of C is in privately owned forests and in hardwood forest types.  In the Southern States, aboveground forest C stocks generally accumulated steadily between 1953 and 1997 (fig. 33.3). 

One can only speculate about the effects of global climate change and socioeconomic responses to the change on the future composition of forest ecosystems. The analysis reported here provides an estimate of potential forest production for 2000, 2050, and 2100 for the Southeastern United States based on a warmer and wetter climate scenario. The trend in total forest C stocks between 2000 and 2050 was generally steady C accumulation throughout the period for States in the northern portion of the region and decreased C accumulation in the Southern States (figs. 33.6A and 33.6B). There is a projected decreasing trend in total forest C stocks between 2050 and 2100 for all States in the region (figs. 33.6B and 33.6C). Future trends in climate suggest increasing air temperature that will affect woody tissue respiration, evapotranspiration, and the development of diurnal and seasonal water stress. Air temperature will likely be a contributing factor in lowering future forest productivity, especially along the Gulf Coastal Plains.