Long-term vegetation and climate change in western Amazonia

Major issues in the paleoecological literature relate to observed interhemispheric differences in the timing and duration of past climatic events. Polar records have suggested the two hemispheres were out of phase for the onset of deglaciation and associated climatic oscillations, such as the Younger Dryas event. These differences increase in importance in the context of understanding whether natural systems were forced to respond to past rapid climate change. In general, northern hemisphere systems experienced large thermal shifts at the onset of the Pleistocene, whereas such changes were not evident in Antarctica.; Amazonia, is a known center of biodiversity and explanations of its evolutionary history have hinged on climate change. Recently, attention has shifted to the role of future climate change on that biodiversity, as climate models predict as much as a 5°C warming in the next century. The history of the region, and whether Amazonia has experienced climate change on that scale, is largely unknown. Current knowledge of paleoclimates is based on a thinly scattered network of paleoecological records. The research presented here seeks a better understanding of the vegetation history of western Amazonia in the context of past climate changes and adds to the small number of available records from the region.; A modern-pollen rain assessment showed that changes in the relative pollen abundance of individual taxa were closely associated with their distributional probability. Furthermore, altitude and the associated temperature decline was the main source of variation in the pollen rain from five locales. A mid-elevation (c. 1500-m elevation) divide was also recognized in the pollen spectra and was hypothesized to correspond to the montane-lowland boundary of past species migrations.; Reconstructions of past ecological and climatic changes were derived from three paleoecological records from the Bolivian and Peruvian Amazon. Lakes Chalaldn and Santa Rosa, in the Bolivian Amazon, documented a continuous mesic forest cover throughout the Holocene, despite a period of reduced moisture between c. 6 and 2 kcal. yr BP and the incidence of human activities. Fires were a natural component of these systems until the onset of wet conditions in the late Holocene when human activities intensified. A collapse of human populations after European arrival was also inferred from the charcoal record. The Lake Consuelo record from the Peruvian Amazon constituted one of the few available archives spanning the full last glacial period in western Amazonia. The glacial forest from Lake Consuelo resembled modern montane forests from c. 1000 m higher upslope, consistent with a glacial temperature depression of about 5.5°C. Range expansions of montane taxa during the full glacial period and their retreat after the onset of warming at c. 22 kcal. yr BP were documented in this record. The forest composition experienced almost a complete turnover from the Pleistocene to the Holocene periods. The data revealed that although moist conditions prevailed during the LGM around Lake Consuelo, the same mid-Holocene drought documented in the lowland records induced the greatest forest-composition change at this site.; Altogether, the data presented here showed that, instead of the Younger Dryas, Holocene climatic variability, coupled with the influence of human activities, caused the highest rates of change in western Amazonia during the past 40,000 years. In conclusion, great environmental changes have taken place in this region and have caused significant vegetation shifts. The modern rates of climatic changes are however greater than any experienced by these forests over the Late-Pleistocene and Holocene, and foresee unprecedented changes.