Effects of Land-Use Change on the Carbon Balance of Terrestrial Ecosystems

Most changes in land use affect the amount of carbon held in vegetation and soil, thereby, either releasing carbon dioxide (a greenhouse gas) to, or removing it from, the atmosphere. The greatest fluxes of carbon result from conversion of forests to open lands (and vice versa). Model-based estimates of the flux of carbon attributable to land-use change are highly variable, however, largely as a result of uncertainties in the areas annually affected by different types of land-use change. Uncertain rates of tropical deforestation, for example, account for more than half of the range in estimates of the global carbon flux. Three other factors account for much of the rest of the uncertainty: (1) the initial stocks of carbon in ecosystems affected by land-use change (i.e., spatial heterogeneity), (2) per hectare changes in carbon stocks in response to different types of land-use change, and (3) legacy effects; that is, the time it takes for carbon stocks to equilibrate following a change in land use. For the tropics, recent satellite-based estimates of deforestation are lower than previous estimates and yield calculated carbon emissions from land-use change that are similar to independently-derived estimates of the total net flux for the region. The similarity suggests that changes in land use account for the net flux of carbon from the tropics. For the northern mid-latitudes, the carbon sink attributed to land-use change is less than the sink obtained by other methods, suggesting either an incomplete accounting of land-use change or the importance of other factors in explaining the current carbon sink in that region.