FOREST HISTORY ALONG THE EASTERN ANDEAN FLANK (40° S) AS RECORDED BY POLLEN AND CHARCOAL RECORDS OF PEAT AND LAKE SEDIMENTS

In the last 15 years major international cooperative research programs in Northern Patagonia east of the Andes have lead to a detailed late Quaternary paleoclimatic history based on multi-proxy analyses of lake and bog sediments. Here we report on the fire-climate-vegetation linkages reconstructed from charcoal and pollen analysis of four sites in the Sub-Antarctic Nothofagus Forest. The close relationship between changes in fire frequency and vegetation composition on millennial and sub-millennial time scales reveals the importance of improving our understanding of the role of climate variations in the development of these cool temperate forests. The pollen and charcoal data of Mallín Aguado (40° S, 71° 29’W) record the vegetation history of the forest – steppe ecotone during the last ca. 17 14C kyr. A sparse herb-shrub steppe surrounding a shallow lake characterized the full-glacial environments suggesting that the climate was markedly drier and probably colder than today. After 16 14C ka, wetland taxa replaced open-water taxa, indicating development of a peat bog, and grasses and a diverse steppe-herb assemblage replaced open ground and shrub taxa suggesting warmer, but still relatively dry conditions. Nothofagus dombeyi-type pollen increased at 14 14C ka and was most abundant after 12.5 14C ka, suggesting establishment of a relatively open forest and increased precipitation. During the late-glacial interval and especially after 11 14C ka charcoal increased, indicating the onset of fires and periods of pronounced drought. Pollen records from Lake Mascardi (41° 20´S, 71°30´W), Lake El Trébol (41°S, 71° W) and Lake Escondido (41°05’ S, 72°34’W) show replacement of an open Nothofagus dombeyi-type forest by a mesic and denser forest at ca. 14 14C ka. The forest was best developed between 12 and 5 14C ka and high levels of charcoal confirm that this was a time of frequent fires. In the last 5 kyr, a mixed Nothofagus – Austrocedrus forest has replaced the mesic forest and charcoal values have decreased, indicating descreasing fire activity and wetter conditions than before. The shift may be related to increased summer precipitation and the onset of ENSO-related precipitation variability.