Abstract

The transition from the temperate rain forest to the Patagonian steppe of the southern Andes in Argentina is one of the most remarkable vegetation gradients. The vegetation changes correspond to a west-to-east precipitation gradient, documenting that climate is the main control on the regional-scale variability in vegetation composition. We collected sediment cores from two lakes: Lake Bruja (40°14’S, 71°30´W; 1069m) and Lake Avutarda (40°23’S, 71°25´W; 1610m). Core Bruja was dated to 3600 years ago and core Avutarda dated to 2800 years ago. Preliminary results show the presence of several tephra deposits in both cores suggest high volcanic activity. Both cores show fluctuations in the percentages of Nothogafus dombeyi-type, with the increase of pollen from herbaceous taxa at the decline of N. dombeyi–type. Both lakes are within and or close to the distribution of Nothogafus obliqua and Nothofagus alpine. At around 1000 years ago the pollen diagram from Bruja shows an increase in the abundance of the N. obliqua pollen type, which includes N. alpina. Avutarda record shows the pollen type for the last 2000 years with an increase around 1000 years ago. Thus, both diagrams documents that N. obliqua and potentially also N. alpina have become more abundant in the region during the last millennia. Human impact is possible to recognize in both records in the presences and increased abundance of Rumex acetosella, Plantago lanceolata related to severe overgrazing by sheep and cattle during the first three or four decades of this century, documented for this zone. Pollen of Pinus spp is another indicator of human land use documenting the initiation of plantations for timer production. The appearance of these indicator taxa coincides with the decrease in the percentage of Nothofagus dombeyi-type.