Holocene climate-vegetation-fire linkages along the Patagonian forest/steppe ecotone (41 - 43°S)

Abstract

Patagonian vegetation has dramatically changed in composition and distribution since the Last Glacial Maximum. Although patterns of vegetation change are relatively clear, our understanding of the processes that produce them is limited. In this study, I reconstructed the vegetation and fire history of the North Patagonian forest-steppe ecotone (lat. 41 - 43°S) and linked vegetation changes to variations in the fire regime, large-scale synoptic controls of climate, and human activity. Postglacial vegetation and fire dynamics were inferred from seven high-resolution pollen and charcoal records from lakes located along the forest-steppe ecotone. Regional trends in vegetation composition and biomass burning were compared to independent records of climate to assess longterm climate-vegetation-fire linkages. Pollen data indicate that late-glacial steppe was replaced by parkland in the early Holocene and by shrubland and forest in the middle and late Holocene. Fire activity was lowest during the late-glacial/early-Holocene transition and gradually increased throughout the Holocene. Based on current knowledge of human settlement in the area, there is no evidence that indicates that increased aboriginal population densities resulted in higher biomass burning at regional scales. Instead, results show that climate was the main driver of Holocene ecological change, either by its direct effects on vegetation or its indirect effects on fire. Watershed vegetation flammability explains much of the fine-scale variability in the fire regime, which, in turn can amplify or override the direct influence of climate on ecotone composition. During the late Holocene, in particular, oscillations in forest dominance were largely driven by changes in humidity, possibly associated with the onset or strengthening of ENSO. At intermediate-moisture levels fire became an important control of community composition. These findings emphasize the importance of biophysical feedbacks in ecosystem dynamics and suggest that these relations must be understood in the context of millennial-scale climate variations that shape broad patterns of vegetation and fire in the region.