Woody Plant Invasions in Pampa Grasslands: A Biogeographical and Community Assembly Perspective

Abstract

Modern primary grasslands are changing as a result of woody plant invasions on a global scale. Grassland invasion by either native or exotic trees represents the assembly into a local community of a novel life form that passed through various ecological filters. We argue that both the historical existence of treeless grasslands and the present shifts in tree cover can be integrated within a common framework, which emphasises the hierarchical influence of multiple forces acting at different spatio-temporal scales. In this chapter, we examine the ongoing process of tree invasion in the Pampa grasslands of Argentina. Two broad phenomena are considred, namely, the dynamics of native taxa across natural grassland/forest boundaries, and the spread of exotic tree species across habitat mosaics. The latter occurs through ?diffuse? ecotones created by anthropogenic landscape fragmentation, and is facilitated by the introduction of pre-adapted tree genotypes, the addition of dispersal corridors, and the disturbance of resident grasses. Examples from different districts of the Pampas are used to illustrate these processes. We conclude that the Pampa grasslands will be inevitably converted into savannas or parklands unless energy subsidies are applied to control encroachment by exotic tree species.