Central Andean vegetation response to rapid paleoclimate changes

A key question for biologists, conservationists and resource planners is how Andean forests will respond to predicted climate extremes within the current century. Unfortunately, little is understood regarding past extreme climate events in the Central Andes and the response of Andean plant communities to them. Do vegetational changes arise from sudden changes in climate, or can they occur after a period of steady, incremental change, when a climatic threshold is exceeded? These predictions were tested in the tropical Andes, one of the richest regions of biodiversity on the planet. The study spanned the Late Pleistocene (c. 94 thousand years ago, or ka) to modern time. The potential causes of rapid vegetational change obtained from the fossil pollen of lake sediments compared with the indices of climate change measured in the isotopic content of stalagmites. As humans have become an important factor in the Andean forest landscape in the last several millennia, their impact on the response of Andean forest communities to climate and the likelihood of abrupt ecological change was also studied.;A new δ18O record from the western Amazon (-3.02°, -78.14°, 980 meters above sea level or masl) revealed significant climatic oscillations within the aseasonal region of the Amazon Basin from c. 94-6 ka. A second cave stalagmite record from the eastern Andes (-5.70° -77.90°, 1920 masl) revealed relative climatic stability buffered by long-term cloud presence from c. 17 ka to the present. Both isotopic records revealed variations in precipitation and South American Summer Monsoon conditions driven by precessional insolation. The records also revealed finer-scale variability and abrupt events primarily driven by the North Atlantic, which were manifested within the tropics via latitudinal shifts in the Intertropical Convergence Zone. An abrupt, wet-climate event was identified within the southern Neotropics at c. 13 ka, which may have been related to the Antarctic climatic oscillations just prior to the Younger Dryas. A rapid shift toward sustained Andean aridity at c. 11 ka was also observed in other isotopic records.;The response of vegetation to climate and human actions was investigated via fossil pollen and charcoal recovered from three ancient lakes. Ecological variability within cloud forests of northern Peru responded to the fluctuations in El Niño Southern Oscillation activity. The oldest fossil pollen evidence of maize agriculture within Andean cloud forests was identified c. 3.2 ka. Changing precipitation conditions appeared to change the human use at all lake sites, prompting switches in staple crops and contributing to the establishment of agroforestry c. 1.2 ka.;Comparisons of high-resolution, independently-dated paleoclimate and paleoecological records indicated that, for at least the last 15 ka, Andean forests responded quickly to abrupt, large climate events. In each case, the ecological response was non-linear and longer lasting than the underlying climate event, requiring nearly twice as long as the climate event to return to an approximate equilibrium with environmental conditions in the absence of human disturbance. No abrupt shifts in Andean forest communities were observed in response to gradual climatic changes.;Regional coherence of paleoecological shifts within southern Ecuador and both northern and southern Peru clearly indicated human control over Andean midelevational landscapes in the late Holocene by c. 4 ka. Humans impacted the local forest communities by disrupting the natural cycle of climatic change and the ecological responses. As a consequence, Andean forests have displayed a clear compositional shift, which are presently dominated by taxa well-adapted to disturbance and fire. Increased external stressors, including greater fire frequencies, have reduced the resilience of montane forests, making abrupt change to smaller-magnitude climate events more probable than in the past.