VEGETATION AND POLLEN DISPERSAL IN THE SUBTROPICAL-TEMPERATE CLIMATICTRANSITION OF CHILE AND ARGENTINA

The interpretation of the late Quaternary pollen records in terms of p alaeoclimate requires an accurate study on the relationship between mo dern pollen rain, the composition of the source Vegetation and climati c factors. We present the results of present pollen rain and its relat ion to modern vegetation and climate on a trans-Andean transect throug h the steppe formations that occur between Zapala, Argentina (38 degre es 53'S, 70 degrees 02'W) and Lonquimay, Chile (38 degrees 26'S, 71 de grees 15'W). This area constitutes a climatic transition between the w esterlies of mid-latitudes and the subtropical eastern circulation. It also represents the easternmost limit of the temperate rain forest an d sclerophyllous subtropical forest in Chile and of the westernmost li mit of the Monte Desert and Patagonian Steppe formations in Argentina. Systematic sampling of vegetation and superficial soil samples were t aken every 100 m along an altitudinal gradient at both slopes of the A ndes 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 app lied for 32 pollen samples. Five pollen units were recognized: (1) sub desert shrublands; (2) Mulinun-dwarf-shrubland steppe; (3) mid-grass s teppe; (4) High Andean and Volcanic scoria steppes; and (5) Acaena-shr ubland and rhamnaceous thickets. These units were correlated with the vegetation communities, precipitation and temperature. The most import ant discontinuity in the composition of the pollen rain corresponds to that between the subdesert shrubland and the Mulinum steppe, on the e astern end of the transect and the spectra corresponding to the High A ndean formations, of the western slopes. This sharp transition in mode rn vegetation and recent pollen rain lies upon the 71 degrees W longit ude, and is possibly determined by the strong differences in distribut ion and amount of precipitation and temperature.