Floristic heterogeneity of flooding Pampa grasslands: a multi-scale analysis

Abstract

The present paper describes vegetation heterogeneity at contrasting spatial scales in order to analyse patterns of floristic diversity and vegetation – environment relations in the Flooding Pampa grasslands of Argentina. The overall grassland flora contains a large proportion of exotic species, which are mostly annual forbs. At a regional scale, there is a latitudinal turnover in relative abundance of major grass tribes, and C3 grasses become increasingly common in grasslands at higher latitudes. The principal gradients in overall vegetation composition were associated with landscape-level changes in topography and soil chemistry. Both native and exotic species richness increased with topographic elevation, and markedly decreased in flood-prone habitats with acidic or saline soils. In remnants of tussock grassland, the predominant physiognomy of the Pampas in pre-settlement times, the presence of the tall native grass Paspalum quadrifarium is a strong determinant of local community structure, reducing the percentage of exotic species. Heterogeneity analysis at the community scale revealed a fine-grained species packing that was in sharp contrast with spatial mosaics observed in other grasslands under livestock grazing. At this scale, spatial variation in floristic composition correlated with changes in various soil parameters, but not with subtle differences in topographic relief. The studied grasslands possess some unique features that contribute towards a better understanding of how current vegetation heterogeneity is controlled by historical, environmental and management factors.