The effects of climate, natural disturbances, and human occupation on the rainforest boundary at the eastern foothills of Northern Patagonian Andes since the Late Glacial period

BIANCHI, MARÍA M.; GIACHÉ, YAMILA; IRURZÚN, ALICIA; GOGORZA, CLAUDIA; FONTANA, SONIA; GIESEKE, THOMAS

QUATERNARY SCIENCE REVIEWS

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2023 vol. 306

0277-3791

Available studies on the vegetationdevelopment in Patagonia cluster along the forest-steppe ecotone, whilst littleinformation is available from within the mountains. This study investigatesshifts in the eastern boundary of rainforest taxa during the Late Glacial andHolocene periods. Continuous sections of highly organic sediments with severalinterbedded tephra layers were obtained at El Laguito del Morro Lake (41 320 24.400S, 71 47’ 50.0” W), which were analyzed for pollen and macro charcoal.Additional macrofossil analysis was conducted to support the interpretations ofpollen data and gain reliable material for radiocarbon dating. Fluctuations inregional precipitation were inferred from changes in the adjacent peatbog. Thepollen record documents five distinct periods of forest diversity andstructure. Late Glacial and early Holocene vegetation were characterized by acombination of Podocarpus and Cupressaceae pollen which has no modern analogue.Taxa in question became rare, and or gradually disappeared around 10,300 cal yrBP when Nothofagus dombeyi type pollen increases but fluctuates. With the overalldecline of charcoal concentrations around 7500 cal. BP Nothofagus dombeyi-typepollen stabilizes and dominates the diagram. After the rise in charcoalconcentrations around 4000 cal yr BP also the overall vegetation compositionchanges around 3600 cal yr BP, with the rise of Cupressaceae pollen attributedto Austrocedrus chilensis marking the establishment of the modern vegetationstructure. The last stronger change in vegetation composition is characterizedby the expansion of the shrub Discaria sp. 770 cal yr BP. Charcoal and pollendata show a clear link between changes in fire regime and vegetationcomposition, however, changes in climate were most likely the main driver forboth. During the Late Glacial period, a rainforest developed under cool temperaturesand high moisture. Severity of fires decreased during a cold reversal periodfrom ca. 12,800 to 11,300 cal yr BP. During Early Holocene, increasedseasonality, volcanism-derived disturbances, and low-frequency surficial fires,enhanced Nothofagus dombeyi forest establishment. The expansion of the mixed Nothofagusdombeyi – Austrocedrus chilensis forest occurred under increased precipitationvariability and the increase of low severity fires related to the establishmentof El Nino Southern Oscillation and the Southern Annular Mode during the ~ LateHolocene. Scrub expansion occurred during historical times because of induced fires.