Estimating historical changes in land cover: North American croplands from1850 to 1992

1. We present a simple algorithm for reconstructing spatially explicit hist orical changes in croplands. We initialize our simulation with a satellite- derived characterization of present-day croplands c. 1992. This data set of croplands is then used within a simple model, along with historical cropla nd inventory data at the national and subnational level, to reconstruct his torical crop cover. We present an annual data set of cropland areas in Nort h America between 1850 and 1992, at a spatial resolution of 5 min (approxim ate to 10 km). 2. The reconstructed changes in North American crop cover are generally con sistent with qualitative descriptions of change. Crop cover is initially co ncentrated in the eastern portions of the continent, and subsequently migra tes westward into the Midwestern United States and the Prairie Provinces of Canada. We also see cropland abandonment in the eastern portions of the co ntinent during the 20th century. The simulation, however, fails to characte rize adequately the changes in crop cover in Mexico. 3. We also estimate the extent to which the different vegetation types of N orth America have been cleared for cultivation. We find that savannas/grass lands/steppes and forests/woodlands have undergone the most extensive conve rsion (1.68 and 1.40 million km(2) cleared, respectively, since 1850). We f urther discuss the wider implications of such large-scale changes in land c over.