Woodbury, Peter. B (Cornell University, Crop and Soil Sciences Department, Cornell
University, Ithaca, NY, 4853; Phone: (607) 255-1448; Email: pbw1@cornell.edu)

Effects of land use change on forest carbon budgets throughout the southern USA from 1900 to 2050.

P. B. Woodbury *, L. S. Heath, J. E. Smith

Land use change is an important driver of terrestrial carbon cycling in the United States and quantifying the effects of afforestation and deforestation is important for national and international assessments. We estimated the effects of afforestation and deforestation both historically and in future decades throughout 13 states in the Southern U.S. This
region is important because its intensively managed forests provide more timber products than any other entire nation. We developed matrices representing area transitions over time between forest, agricultural, and urban land uses and incorporated the results in a model to estimate changes in soil and forest floor carbon stocks. Land use changes such as afforestation have long lasting effects, so we developed historical data sets and ran the model beginning in 1900. Historical estimates of land use transitions were based on Governmental forest and agricultural inventories.  Future land use changes were based on projections of historical data and projections from the USDA Forest Service 2003 RPA timber assessment base run.  Transitions within forests, that is, between forest types, were assumed not to alter forest floor and soil carbon stocks. Estimates of soil carbon stocks were derived primarily from the STATSGO database. The effects of specific land use changes on soil and forest floor carbon fluxes were based on data from the literature.

Historically, in the South-Central region (Texas to Kentucky), the maximum effect of land use change occurred during the 1980s while in the Southeast region (Florida to Virginia) it occurred during the 1940s. The total maximum effect of land use change was also much greater in the South-Central region: 210 Tg C versus 70 Tg C in the Southeast region. In the future, forest area is predicted to decrease in the Southeast region by 1.4 million ha, while it is predicted to increase in the South-Central region by 0.5 million ha.  Despite this difference, the pattern of net carbon change from 2004 until 2050 is predicted to be fairly similar in the two regions, with sequestration of 41 Tg C (Southeast) and 57 Tg C (South-Central region). Afforestation is predicted to sequester 119 Tg C in the Southeast region and 117 Tg C in the South-Central region. The emission due to deforestation is predicted to be somewhat higher in the Southeast: 78 Tg C versus 59 Tg C in the South-Central. One key result of our model is that forest floor carbon changes account for nearly as much carbon changes as does the soil, even though the total mass of this pool is so much smaller. This occurs because nearly all of the forest floor mass is assumed lost with deforestation, while only a portion of soil carbon mass is lost.

The results of this model are intended to improve the forest estimates of the U.S. Greenhouse Gas Inventory, which is produced annually to meet reporting requirements under the United Nations Framework Convention on Climate Change.  From 1990 to 2004 for the entire 13-state study area, afforestation caused sequestration of 88 Tg C, of which 47 Tg C was in the soil and 41 Tg C was in the forest floor. During this same period, deforestation caused emission of 49 Tg C, of which 13 Tg C was in the soil and 36 Tg C was in the forest floor. However, the net effect of land use change on carbon mass in soil and forest floor from 1990 to 2004 was about 6-fold smaller than the net change in carbon stocks in trees on all forestland during this time period. Thus land use change effects for this period are dominated by changes in tree carbon stocks.