Abstract

Nothofagus alpina and Nothofagus obliqua have been largely investigated in genetic studies while palaeoecological studies have hitherto not addressed their history along the eastern distribution limits, especially at 40°S, where both species (same pollen type) have their largest abundance. We present a pollen record for the last 11.500 years from a small lake located within the main population of both species: the Lake Lácar basin in the Lanín National Park, Argentina. Our results indicate that the Nothofagus obliqua-type was present around the site since the beginning of the Holocene at low abundances and increased gradually in abundance towards the present. This gradual expansion may be due to changes in precipitation seasonality, as it is suggested for other taxa in the region. The pollen diagram was subdivided into four zones using a depth-constrained cluster analysis, and these zones were used to group the samples in a PCA. A visible separation between groups suggest a transition from (1) a dry stage, characterized by the dominance of steppe taxa (Chenopodiaceae, Asteraceae subfam. Asteroideae, and Poaceae); (2) a wet and warm phase characterized by the presence of the Nothofagus dombeyi-type and Austrocedrus chilensis; 3) a cold and rainy phase characterized by coldresistant taxa such as Podocarpus nubigenus and Saxegothaea conspicua and 4) a warm stage characterized by the Nothofagus obliqua-type and the appearance of the human indicator taxa Rumex acetosella and Plantago lanceolata. Our research provides the first account of the Holocene history of Nothofagus obliqua/alpina, suggesting that one or both species were already present close to our site since 10,000 years ago. In addition, Austrocedrus chilensis, representing just 0.6% in modern samples, exhibit short-term percentage fluctuation along the Holocene with a maximum of 16% 8,000 years ago. Microcharcoal particles show their maximum during the first zone, which coincides with the dry period suggested by the vegetation composition. Despite the evident fire activity, and the frequent deposition of volcanic ash the vegetation composition remained stable with only gradual compositional changes during the Holocene.