Mechanisms and Processes Shaping Patterns of Forest-Grassland Mosaics in Southern Brazil

Mosaics of forest and grassland are frequent in South Brazilian landscapes, both in the highlands of the Atlantic Forest and southward into the Pampa regions. The grasslands are relicts of a previous drier and colder climate and are currently subject to woody encroachment and forest expansion. These processes are dynamic and observed mosaic patterns result from a complex interplay of environmental, biogeographic, landscape, and anthropogenic drivers. Thus, the composition and abundance of woody species in the grassland portion of the mosaics vary across spatial and temporal scales. Grazing and fire disturbances over the grasslands are important determinants, filtering out many woody species without traits and strategies that enable them to establish and grow in grasslands under certain disturbance regimes. In the absence of or under low disturbance regimes, the expansion of forests over the grasslands occurs gradually from the boundaries and often by nurse plants facilitation. Nurse plants are trees or shrubs that initially establish safe sites in grasslands and then facilitate the subsequent colonization of tree species by attracting animal dispersers, improving microenvironmental conditions, and promoting the formation of forest patches within grasslands. Moreover, particularly in the highlands, forest-grassland mosaics are considered a domesticated landscape where pre-Columbian societies contributed to the expansion of Araucaria forests. In this chapter, we present the main patterns, discuss the mechanisms and drivers behind the processes, and propose a mechanistic model to synthesize patterns and processes of woody encroachment and tree expansion in forest-grassland mosaics in the South Brazilian Grassland region.