Non-linear vegetation responses to climate change: A paleoecological perspective from Lake Titicaca

Background/Question/Methods A 370,000-yr palaeoecological record from Lake Titicaca provides a detailed record of past climate change that includes marked changes in vegetation. Fossil pollen and charcoal data reveal the marked contrast between glacial and interglacial conditions. Landscape productivity ranged between glacial foreland to a productive (relatively) interglacial Andean grassland/shrubland. Throughout the record, but most evident in the interglacials, changes in paleolake area on the Altiplano vary by an estimated 400% of the modern Titicaca to 15%. At the height of two out of the last three previous interglacials (marine isotope stages 5e and 9), profound drying desertified the Altiplano. To what extent would the changing size of lakes in the Altiplano influence microclimate and vegetation on the Altiplano? Can previous interglacial vegetation responses provide a guide for future changes? Methods Fossil pollen and charcoal, and sedimentary grayscale analysis of a 370,000-year sedimentary sequence from lake Titicaca. Results/Conclusions These changes follow the general pattern of orbital precession and were most likely to have highstands when December-February insolation was maximal. Although paced by insolation, the direct drivers of lake level were probably the balance between the arrival of Amazonian moisture and evaporation. We advance a conceptual feedback model to account for the observed changes that includes previously ignored lake effects. Lake Titicaca serves to warm the local environment by about 4-5 oC and also to increase rainfall. We observe that as water levels in the lake are drawn down due to warm, dry, interglacial conditions, there is a possible regional cooling as the lake effect on local microclimates diminishes. Positive feedback mechanisms promote drying until much of the lake basin is reduced to saltmarsh. The usual conception of a steady upslope migration of species with warming would not be applicable in the Altiplano. If, as projected, the next century brings warmer and drier conditions than those of today, a tipping point appears to exist within c. 1-2 oC of current temperatures, where the relatively benign agricultural conditions of the northern Altiplano would be replaced by inhospitable arid climates. Such a change would have profound implications for the citizens of the Bolivian capital, La Paz.