Forests, savannas, and grasslands are prevalent across the landscapes of South America. Land uses associated with these ecosystems have influenced economies from household to country scales, shaping social-ecological organization across the region since pre-Hispanic societies. Recent studies suggest that tropical and subtropical grasslands, savannas, and forests represent alternative ecosystem states. Transitions between these ecosystem states can be promoted by changes in disturbance regimes and by land uses determined by the organization of societies and their activities. We analyzed how changes in agriculture, fire, and livestock management influenced forest cover over a 45-year span (1966-2011) in the Campos region, an extensive subtropical ecotone between rain forests and grasslands of South America. We found that forests contracted in areas with high crop agriculture, whereas forests increased in those grasslands where livestock densities had been reduced. These patterns were strongly associated with soil and topographic conditions because they broadly determine the potential land productivity and use. Our results show that current land use and disturbance regimes explain the large extent of grasslands across the South American Campos and suggest that changes in land use and disturbance regimes could facilitate or prevent transitions between subtropical forests, savannas, and grasslands altering the provision of ecosystem services linked to them.