Holocene changes in forest composition in northern Patagonia responded to climate with little impact of disturbance

Abstract

Postglacial climate change and changing disturbance regimes have shaped the vegetation composition in the forest-steppe ecotone in northern Patagonia (Argentina and Chile; lat. 40°‒lat. 43°S). Several investigations between 41° and 43°S document shifts in the position of the forest-steppe ecotone and the population expansion of the cypress Austrocedrus chilensis, while little is known about the vegetation dynamic of Nothofagus alpina and Nothofagus obliqua within the Lácar basin. With the aim to contribute to this respect, the sediments of a small lake within a dense Nothofagus forest, in the Lanín National Park were collected and analysed in high resolution for pollen charcoal and sediment composition. Additionally, this work assessed the role of natural disturbance on vegetation composition. Results document the environmental history for the last 11,600 years. The record indicates high fire activity during the early Holocene, associated with dry conditions and the presence of a diverse Nothofagus shrubland. The middle Holocene starts with increased percentage of Cupressaceae pollen (up to 15%) which drops following the dominance of Nothofagus associated with wet conditions and low fire frequency. The late Holocene is marked by the rise in the abundance of N. obliqua and N. alpina, documenting the spread and increased importance of these trees within the Lácar basin during the last two millennia. A statistically significant effect of ash deposition on overall vegetation composition could not be detected, while Hydrangea and Lomatia hirsuta seem to benefit from ash fall. Fire affected stands of A. chilensis and N. obliqua/N. alpina, but climate was likely the dominant factor controlling average vegetation composition. Recent anthropogenic disturbance is noticeable by the presence of introduced taxa Rumex, Plantago and Pinus, and by the decrease in the percentage of Nothofagus obliqua-type, associated to extensive timber activities around the Lácar basin.