Abstract for presentation at XVII INQUA Congress 2007

The temperate forest of the Patagonian Andes in Argentina shows compositional changes along several environmental gradients. The two strongest gradients are the sharp eastward decline in precipitation and altitudinal changes in temperature. Forest composition also changes from north to south. Along the mountain chain numerous volcanoes show signs of recent eruptions, which had devastating impacts on the forest. In contrast intensive human action altered the ecosystems only during the last 100 years. The Lanín national park, between 38°5’ to 40°5’S, is home to the highest diversity of tree species on the eastern side of the Patagonian Andes and offers therefore a good opportunity to study the diverse ecotones. This study aims: i) to explore how the diverse ecotones reflect Holocene climate variability; ii) to document timing and patterns of Holocene changes in vegetation composition along major environmental gradients; and iii) to study the impact of vegetation disturbance by volcanic eruptions, fire and human activities. In order to achieve this, surface sediment samples, short cores and long sediment sequences were collected from small basins and analysed in detail for magnetic susceptibility, pollen, plant macrofossil remains and charcoal. Analysis of a four meter long sediment core recovered at Lake Torta (39°06’S, 71°21’W; 1090 m asl; 4 m deep; 1 ha surface; located in a mixed forest of Nothofagus antartica and Araucaria araucana) indicate that frequent volcanic eruptions often followed by forest fires impacted heavily on the forest cover. Araucaria araucana is more fire resistant than Notofagus species and was thus favoured by fire. While volcanic tephras are more frequent in the lower part of the core Araucaria araucana pollen increases towards the top of the core. This may indicate that the climate may have become dryer or forest fires more frequent towards the present.