Quantifying postglacial climate and vegetation dynamics of the foreststeppe ecotone in northern Patagonia, Argentina

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Vegetation responds to both average climatic conditions and extremes and changes in past vegetation thus make a good indicator for past climate changes. While sufficient pollen based quantitative climate reconstructions exist for the northern hemisphere to provide continental scale patterns, investigations are rare for the southern hemisphere, in particular South America. Ecotonal regions, e.g. the 2000 km long forest steppe ecotone in Patagonia, are particularly sensitive to climate change, although fire and other feedback mechanisms may delay responses of the vegetation to climate change. Studying the vegetation composition along climate gradients in space as well as through time is therefore important to yield a four dimensional understanding of the climate and vegetation dynamics, which will allow the influence of climate to be disentangled from vegetation internal processes and fire feedback. The proposed study will focus on the forest steppe ecotone in Argentina between -38.7 and -39.6 South. This region is rich in small to medium sized lakes at different elevations, which will provide modern pollen samples to be used a) in the construction of climate transfer functions, and b) to obtain representation factors which limit biases from differential pollen production of species. Climate variables for the transfer functions will be obtained by monitoring the relevant parameters (soil moisture and temperature) around the lakes from which modern pollen samples are collected. This novel approach is necessary as the study region contains strong climate gradients over short distances, and will provide information about the mean and variation in these parameters. Fossil pollen and charcoal will be analysed from lake sediment cores of three sites crossing the ecotone: one core from the moist side, a second from an intermediate location and a third one from the dry end of the gradient. The first two are already available and radiocarbon dated. The selection of the third site has the additional objective of obtaining Lateglacial sediments to explore the changes during deglaciation. Preliminary pollen data indicate several gradual changes as well as a strong shift in the ecotone around 6000 years ago. The extent of this shift and other changes is potentially masked due to the overrepresentation of Nothofagus pollen. The application of representation factors and models of pollen dispersal and deposition will help to quantify the extent and dynamics of this and other cases of postglacial vegetation change. The detailed reconstruction of the vegetation dynamics will be used to evaluate the quantitative climate reconstruction from the sites. In addition, this information will increase the understanding of ecotonal vegetation dynamics which will support and contribute conservational strategies for the National Parks of the region.