Holocene vegetation dynamics and disturbance regimes in north Patagonia Argentina (40°S)

Abstract

Northern Patagonia in Argentina (40°S) offers a great opportunity to analyse the influence of volcanic ash deposition and fire on the vegetation composition. Here, three lakes located strategically along the forest-steppe ecotone within the Lanín National Park (LNP) were analysed with the aim to reconstruct past vegetation and fire history. Additionally, given the presence of tephra layers, it was possible to assess the impact of ash deposition on the vegetation composition. The results indicate that ash deposition did not trigger significant changes on the local vegetation nearby Lake Bruja (3600 cal. yr BP), Avutarda (2700 cal. yr BP), and Vizcacha (11700 cal. yr BP). However, it was observed a slight decrease in the percentage of Poaceae pollen. Overall, the results obtained from Lake Vizcacha suggest that during the Early Holocene, the vegetation nearby the Lácar basin was characterized by an open Nothofagus forest with a diverse shrub and herbaceous stratum associated to warm and dry climatic conditions. The Mid-Holocene featured a closed Nothofagus forest concomitant with more humid climatic conditions, and the presence of a small population of Austrocedrus chilensis. Finally, the Late Holocene on the three records indicates stable forest conditions, associated with the establishment of the modern rain winter/summer drought climatic regime in the region. Past fire regimes were reconstructed applying the analysis of macro charcoal particles (≥ 125μm) in the cores Bruja and Vizcacha. For the Early Holocene, the results suggest moderate local fires, likely due to the dry and warm climatic conditions inferred for this period. The Mid-Holocene was characterized by a shift from low-moderate fires to fires of high magnitude. Long fire return intervals (FRI) resulted in the accumulation of biomass that given the proper climatic and ignition conditions, resulted in the severe fires detected for the Mid-Holocene. The Late Holocene featured low severity fires in both records, however, the last 2000 years show differences in the fire activity documented on Bruja and Vizcacha, probably attributed to vegetational dissimilarities. The redundancy analysis indicated a nonsignificant effect of fire on the local vegetation composition documented in Bruja. Nevertheless, this analysis showed a positive correlation between FRI and Austrocedrus chilensis, suggesting the persistence of this conifer during long periods without fires. The anthropogenic signal was detected in the last 200 years but their impact on the vegetation was almost negligible, except for the decline in Nothofagus obliqua-type, associated with the timber activities developed during the 1800s. Moreover, the establishment of the modern climate during the last 3000 years may be responsible for the expansion of the Nothofagus obliqua-type forest within the Lácar basin, being one of the main findings of this work.