Delving into the variations in tree species composition and richness across South American subtropical Atlantic and Pampean forests

Abstract

We analyse here the variations in species composition and richness and the geographic ranges of the tree species occurring in South American subtropical Atlantic and Pampean forests. Our goals were to assess (i) the floristic consistency of usual classifications based on vegetation physiognomy, climate and elevation; (ii) the leading role of temperature-related variables on the variations in species composition and richness; (iii) the predominance of species with tropical–subtropical ranges, possibly as a result of forest expansion over grasslands after the Last Glacial Maximum (LGM); (iv) the restriction of most subtropical endemics to stressful habitats as a possible result of past forest refuges during the LGM. The region was defined by the Tropic of Capricorn to the north, the Rio de la Plata to the south, the Atlantic shoreline to the east and the catchment areas of the upper Paraná and Uruguay Rivers to the west. Multivariate analyses, multiple regression modelling and variance partition analyses were performed on a database containing 63 994 occurrence records of 1555 tree species in 491 forest sites and 48 environmental variables. All species were also classified according to their known geographic range. A main differentiation in species composition and richness was observed between the eastern windward coastlands (rain and cloud forests) and western leeward hinterlands (Araucaria and semi-deciduous forests). Pre-defined forest types on both sides were consistent with variations in tree species composition, which were significantly related to both environmental variables and spatial proximity, with extremes of low temperature playing a chief role. Tree species richness declined substantially towards the south and also from rain to seasonal forests and towards the highland summits and sandy shores. Species richness was significantly correlated with both minimum temperature and actual evapotranspiration. About 91% of the subtropical flora is shared with the much richer tropical flora, probably extracting species that can cope with frost outbreaks. The 145 subtropical endemics were not concentrated in harsher habitats.