Sub-regional to local topographic features shape resources, land use and disturbances determining forest distribution in old-growth grasslands of subtropical South America

Abstract

What determines the presence (or lack) of trees in grasslands is a long-standing question in ecology. Views of trees in grasslands have shifted from generally considering grasslands as degraded forests that need to be restored, to a more recent perspective on many regions as ancient grassy biomes that have long been neglected and are, according to their climate conditions, maintained as stable biomes by internal feedbacks involving fire and/or herbivory. In addition, there are currently concerns about trends of woody expansion or “encroachment” in grasslands, i.e. potential transitions between grassy and wooded ecosystems driven by local and global changes.In this study, we use high-resolution local databases and generalized additive models (GAMLSS) to analyze the factors that determine forest cover in the Campos grasslands of Uruguay in southeastern South America. We consider climate, local conditions, land use and disturbances to understand what explains the distribution of forests. Forests were associated with areas of steeper slopes, watercourses and higher surface runoff, in deeper soils with higher water availability, along a longitudinal climatic gradient, with less livestock and land-use intensity and higher human presence. We discuss how, in ancient grasslands such as this one, sub-regional geology and local topography can be permanent features that generate spatial heterogeneity in forest cover probability by mediating resource availability and influencing land use and disturbance regimes.