DYNAMICS OF CENTRAL ANDEAN HIGHLAND VEGETATION SINCE THE LAST GLACIAL MAXIMUM

Paleoclimatic evidence suggests that most area of the Neotropics experienced a 4-8¢ªC cooler period during the last glacial maximum (LGM). Based on pollen records from Ecuadorian and Peruvian Andes, vegetation developed throughout the following stages since the LGM: 1) The high mountains of central Andes were barren or covered with sparse super-puna vegetation during the LGM. 2) Rapid deglaciation occurred between 16,000 and 17,000 cal yr BP, and since then, vegetation coverage increased. 3) Rapid warming occurred at ca. 12,000 yr BP. At elevation of ca. 4000 m, high montane Polylepis woodlands declined in abundance and Poaceae-dominated puna was established. The warming was continuous, and the Younger Dryas event was not recorded. 4) Early Holocene between 10,000 and 5,500 cal. yr BP was warm and dry as recorded in ice cores. During this period, Alnus expanded in downslope forests. Drought or fire disturbances may have induced decline of other species and made spaces for the expansion of Alnus. 5) During the Mid-Holocene, Alnus collapsed at ~5500 cal yr BP when wetter and cooler conditions returned. The decline time was not exactly simultaneous at different sites (ranging from 6500 to 4500 cal yr BP). Weedy components Ambrosia, Chenopodiaceae/Amaranthaceae expanded following the decline of Alnus. Climatic changes or intensified human activities may be responsible for the change. 6) At the Late Holocene, Maize (Zea mays) pollen occurred widely in Ecuador and Peru after ~2600 ca yr BP, indicating a minimum age for local agriculture. An increase in Alnus pollen abundance at ~1000 cal yr BP was recorded regionally, which could be due to human activity or perhaps due to climatic change associated with cultural turnover in the Andes.