UNDERSTANDING FOREST-GRASSLAND MOSAICS: THREE CASE STUDIES ON THE BASALTIC PLATEAUS IN HUMID SUBTROPICAL BRAZIL ANALIZANDO MOSAICOS DE BOSQUES Y PASTIZALES: TRES CASOS DE ESTUDIO EN EL SUBTROPICO HÚMEDO DE BRASIL

Landscapes exhibiting complex mosaics of contrasting natural ecosystems, like forests and grasslands, continue to evade a straightforward interpretation concerning the key drivers behind the landscape mosaic. Our research shows that the highlevel drivers of the land cover - land use mosaics, characteristic of the basaltic tablelands of northern Rio Grande do Sul, Brazil, can be identified based on analysis of the vegetation pattern in relation to geomorphic and pedogenetic phenomena controlling water and nutrient supply, together with human action. The analysis is implemented on two spatial scales: one regional, embracing a large portion of tablelands, and the other at the landscape scale by selecting three contrasting sites. Natural grasslands (campos) still extend over much of the highest plateaus, on young but nutrient-depleted inceptisols. While on the lower altitudes campos over deep oxisols have been extensively replaced by cash crops. The Araucaria angustifolia montane forest characterizes the higher tablelands, where it occurs in habitats with a permanent water supply and out of the reach of the frequent grasslands fires. Semideciduous forests occur on the lower plateaus, restricted to the wet conditions of the bottomlands. In any case, the increasingly widespread human disturbance renders the natural pattern fuzzy, but the close correspondence between more fertile habitats and forest occurrence suggests that nutrient availability plays a key role. Nutrient availability, in turn, results from enhanced rates of erosion wearing out the ancient peniplain and further developing the drainage system. A conceptual model relating geomorphic, pedogenetic and human factors is proposed to explain the original and actual vegetation mosaics.