Abstract

The interpretation of the late Quaternary pollen records in terms of palaeoclimate requires an accurate study on the relationship between modern pollen rain, the composition of the source Vegetation and climatic factors. We present the results of present pollen rain and its relation to modern vegetation and climate on a trans-Andean transect through the steppe formations that occur between Zapala, Argentina (38 degrees 53'S, 70 degrees 02'W) and Lonquimay, Chile (38 degrees 26'S, 71 degrees 15'W). This area constitutes a climatic transition between the westerlies of mid-latitudes and the subtropical eastern circulation. It also represents the easternmost limit of the temperate rain forest and sclerophyllous subtropical forest in Chile and of the westernmost limit of the Monte Desert and Patagonian Steppe formations in Argentina. Systematic sampling of vegetation and superficial soil samples were taken every 100 m along an altitudinal gradient at both slopes of the Andes and isolated samples were taken at the summits of the Cordillera de las Raices and at the foothills of Lonquimay volcano. Multivariate analysis (cluster analysis and principal components analysis) were applied for 32 pollen samples. Five pollen units were recognized: (1) subdesert shrublands; (2) Mulinun-dwarf-shrubland steppe; (3) mid-grass steppe; (4) High Andean and Volcanic scoria steppes; and (5) Acaena-shrubland and rhamnaceous thickets. These units were correlated with the vegetation communities, precipitation and temperature. The most important discontinuity in the composition of the pollen rain corresponds to that between the subdesert shrubland and the Mulinum steppe, on the eastern end of the transect and the spectra corresponding to the High Andean formations, of the western slopes. This sharp transition in modern vegetation and recent pollen rain lies upon the 71 degrees W longitude, and is possibly determined by the strong differences in distribution and amount of precipitation and temperature.