At the margins: agriculture, subsidies and the shifting fate of North America's native grassland.

We examined patterns of shifting cropland cultivation in the US Great Plains from the dust bowl to the beginning of the twenty-first century, by comparing land-cover data from 400 sample sites across the region from the 1930s, 1950s, 1970s, 1990s and 2000s. The small area land-cover data were nested within 50 target counties across the region. To understand the use of marginal land for cultivation since the Great Depression, we argue, requires consideration of the long term dynamics of demography, technology and policy. We draw on these historical dynamics, and their interactions with programs aimed at reducing environmental impacts of agriculture, to tell the story of how and when marginal lands have been brought into use. In a multilevel panel design, macro- and micro-level covariates were used to predict levels of encroachment on marginal soils. We conclude that land retirement programs (like the Conservation Reserve Program) have had a generally stabilizing effect on the micro-level patterns of land use in recent decades, but that increased levels of encroachment on marginal soils and native grassland remain a problem in areas with higher or increasing population densities. [ABSTRACT FROM AUTHOR]