Early spread of rainforest trees during the Last Glacial Termination along the eastern slopes of the Central Patagonian Andes (45ºS)

ISABEL VILANOVA; RODRIGO VILLA-MARTINEZ; PATRICIO MORENO

Dublin

Congreso; 20th Congress of the International Union for Quaternary Research (INQUA); 2019

INQUA

Few studies have examined in detail the structure of vegetation and climate changes during the Last Glacial Termination (T1, ~11-18 ka, ka= 1000 cal yr BP) in central Patagonia (45°- 48°S). This region is adjacent to the zone of maximum southern westerly wind (SWW) influence in South America, and features strong climatic gradients along and across the Andes. These gradients influence vegetation composition and structure, resulting in zonation of evergreen and deciduous rainforests from the pacific coast upslope into the high-Andean meadows. Deciduous forests dominate the eastern slopes of the Andes andtransition into the Patagonian Steppe toward the east. Here we present palynological results from Lago Unco (45°34ʼ29.4ʼʼS; 71°43ʼ7.6ʼʼW, 756 masl), a small closed-basin lake located in the modern forest-steppe ecotone, in direct association with moraines deposited during the Last Glacial Maximum (LGM). Our aim is to document the composition, tempo and mode of vegetation changes during T1 to infer the climate evolution through this interval and the spatial/temporal patterns of afforestation.The pollen record features predominance of herbs and shrubs characteristic of cold high Andean environments and the Patagonian steppe between 17.8-11 ka, with dominance of Poaceae, Asteraceae, Fabaceae, and Apiaceae between ~17.8- 17.3 ka. The record then shows dominance of Poaceae, Ericaceae and Asteraceae, along with Acaena and Gunnera until 15 ka. A sustained rise in Nothofagus is evident starting at 16.5 ka accompanied by the hygrophilous cold-tolerant conifers Fitzroya/Pilgerodendron and Podocarpus nubigena, the rainforest tree Drimys winteri, and the hygrophilous pteridophytes Blechnum and Lycopodium magellanicum.These data suggest an increasing trend in humidity and SWW influence starting at 16.5 ka until it reaches its maximum at 15 ka. Our findings constitute the earliest occurrence of cold-resistant hygrophilous taxa at the east of the Andes during T1, suggesting that they thrived near the eastern margin of the PIS during the LGM and may have been a source for the postglacial dispersal and colonization of trees currently dominant in rainforests along the pacific coast of central Patagonia.Nothofagus forests established between 11-10 ka near Lago Unco, concomitant with a decline in hygrophilous taxa and increased fire activity. We interpret these changes as a warm pulse and decline in precipitation brought by diminished influence of the SWW at ~11 ka.