LATE QUATERNARY VEGETATION, CLIMATE, AND FIRE DYNAMICS: HUMAN IMPACT AND EVIDENCE OF PASTPOLYLEPIS POPULATIONS IN THE NORTHERN ANDEAN DEPRESSION INFERRED FROM THE EL CRISTAL RECORD IN SOUTHEASTERN ECUADOR

摘要

Late Pleistocene and Hoiocene vegetation, climate, and fire dynamics, as well as human impact, were studied in the El Cristal region, which is part of the Andean Depression in southeastern Ecuador. The sediment core from a small basin at 2056 m a.s.l.was analyzed by pollen and charcoal analysis and dated by six radiocarbon dates. The record indicates that during the late Pleistocene (ca. 19 750-12 500 cal yr BP) the Upper Mountain Rainforest (UMF), with some smaller areas of paramo and subparamo were the main vegetation type in the study area. Additionally, there is evidence for Polylepis woodland in the region during this period, which does not exist in the area today. During the transition from the late Pleistocene to the early and mid-Holocene between ca. 12 500 to 3600 cal yr BP, there is a significant presence of mountain rainforest and stable proportions of subparamo and paramo taxa. The early Hoiocene from ca. 11 500 to 7800 cal yr BP is characterized by UMF vegetation at the study site. Polylepis became more frequent and reached its maximum. During the mid-Holocene between 7800 to 3600 cal yr BP, the composition of the UMF changed. Polylepis decreased after ca. 4000 cal yr BP, probably due to higher fire frequency. During the mid- to lateHoiocene, human landscape disturbance is evident, especially between ca. 2000 and 1380 cal yr BP, with Asteraceae, MuehlenbeckialRumex, Osmunda and Huperzia all increasing in abundance. The late Hoiocene, from ca. 1380 cal yr BP, was characterized by anincreased abundance of paramo taxa and a decrease in mountain rainforest taxa. The expansion of grassland during the late Hoiocene is interpreted as a result of an increase in human activity, which limited forest vegetation to isolated patches. However,between ca. 1200 and 900 cal yr BP the UMF expanded, probably due to forest recovery after the high frequency of fire. Between ca. 900 cal yr BP to present times, the UMF vegetation decreased markedly and grasslands (paramo taxa) expanded.