Deglacial and postglacial forest history and climate changes on the eastern slopes of the central Patagonian Andes

RODRIGO VILLA-MARTINEZ; PATRICIO MORENO; ISABEL VILANOVA

Dublin

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

INQUA

We report a high-resolution pollenand charcoal records from lake sediments obtained from Lago Mellizas toreconstruct the history of vegetation, fire and climate changes on theeastern slopes of the central Patagonian Andes over the last 17ka (ka=1000 cal yr BP). These data allow examination of the timing anddirection of arboreal colonization through the increasingly ice-freePatagonian landscapes and the timing and structure of the southernwesterly winds (SWW) changes during and since the last glacialtermination. Lago Mellizas (45°32' S; 71°48' W, 760 masl) is a small closed-basinlake located along the eastern slopes of the Andes near to theclimate-modulated forest-steppe ecotone in central Patagonia.Central Patagonia (44°-49°S) is an area in which the SWW are the only source ofprecipitation, generating a strong west-east precipitation gradient across thePatagonian Andes that induces a zonation of the regional vegetation, which canbe used for inferring past changes in precipitation regimes based on fossilpollen records. Furthermore, the Chilean-European colonization process incentral west Patagonia started early in the20th century, allowing characterization of natural vegetation andclimate. The pollen record shows an openlandscape dominated by herbs (Poaceae, Gunnera, Acaena,Apiaceae) and shrubs (Empetrum) suggesting an open landscape undercold conditions between 17 and 16.7 ka. An increasing trend of Nothofagusstarted at 16.7 ka, along with hygrophilous and cold-tolerant conifers (Pilgerodendron,Podocarpus). The latter reached maximum abundance between 15-11.7 ka,indicating a rise of the precipitation, followed by the establishment of dense Nothofagusforests and declines in hygrophilous and cold-resistant trees, herbs,shrubs and increases of fire activity under warm and dry climateconditions until 9.5 ka. Nothofagus forests have persisted withlittle variation since 9.5 ka, despite of the occurrence of the increaseof fire activity at multicentennial timescales and subtle increases ofconifers. We detect a major change in the pollen stratigraphy at 0.06 kawhen started a sustained decreasing trend in Nothofagus, alongwith increases of Poaceae and the aquatic Cyperaceae and Myriophyllum.We interpret these changes as a forest opening and centripetal expansion oflittoral environments toward the lake center driven by lake-level lowering inresponse to lowered precipitation. Rumex and Plantago, whichare indicative of human perturbation, increase at 1900 AD.  Our resultssuggest an early expansion of cold-resistant hygrophilous conifers andrainforest trees sourced from the eastern flanks of the Andes in central Patagonia and the precipitation variations related tochanges in the influence of the SWW during and since the lasttermination.