Biophysical consequences of large-scale land use change in the upper Midwestern United States
Land use change is recognized as an important contributor to climate change. While the consequences of forest land use change are commonly studied with respect to Carbon, recent research indicates that perturbations of surface energy balance through changes in albedo and other biophysical factors can have almost as strong an effect. Here we combine regional-scale historic surveying data with modern remote sensing data to estimate the biophysical consequences of deforestation and partial recovery in upper Midwestern forests. We hypothesize that (1) albedo will be higher in the modern landscape due to high-albedo croplands absent at the time of settlement, (2) surface temperature will be higher in the modern landscape due to losses of high-transpiration forests, and (3) net biophysical changes will indicate cooling but will not outweigh the warming effect of lost potential carbon uptake. Historic biophysical conditions were simulated by applying vegetation-based seasonal profiles of MODIS surface temperature and albedo to a vegetation map generated from 19thcentury public land survey data. Radiative forcing was then calculated from differences in albedo and surface temperature to establish biophysical forcing due to land use change.
Strong increases in land coverage were observed for open lands (cropland, mixed cropland/natural vegetation), while decreases in land coverage were observed for forests (deciduous, evergreen, mixed deciduous/evergreen). Where agriculture has replaced native forest, both albedo and surface temperature increased. Where forests had been logged for timber and allowed to regrow, albedo increased slightly but surface temperature decreased. Over the entire landscape, preliminary radiative forcing due to albedo change was negative and on the order of -1 W/m2, with the strongest forcing occurring in winter. Radiative forcing due to change in surface temperature was positive and strongest during the growing season. Our results confirm earlier findings that biophysical effects of land use change can be of comparable magnitude to those of atmospheric carbon dioxide. However, albedo and surface temperature may work counter to each other after land use change, producing a small net biophysical effect.