Dry corridors opened by fire and low CO2 in Amazonian rainforest during the Last Glacial Maximum

Abstract

The dynamics of Amazonian rainforest over long timescales connect closely to its rich biodiversity. While palaeoecological studies have suggested its stability through the Pleistocene, palaeontological evidence indicates the past existence of major expansions of savannah and grassland. Here we present integrated modelling evidence for a grassier Neotropics during the Last Glacial Maximum, congruent with palaeoecological and biological studies. Vegetation reconstructions were generated using the land processes and exchanges model, driven by model reconstructions of Last Glacial Maximum climate, and compared with palynological data. A factorial experiment was performed to quantify the impacts of fire and low CO2 on vegetation and model–data agreement. Fire and low CO2 both individually and interactively induced widespread expansion of savannah and grassland biomes while improving model–data agreement. The interactive effects of fire and low CO2 induced the greatest ‘savannafication’ of the Neotropics, providing integrated evidence for a number of biogeographically relevant open vegetation formations, including two dry corridors (paths of savannah and grassland through and around Amazonia that facilitated major dispersal and evolutionary diversification events). Our results show a bimodality in tree cover that was driven by fire and further enhanced by ‘CO2 deprivation’, which suggests biome instability in this region of climate space. Lower CO2 and more-frequent fires may have supported grassland expansion in the Amazon during the Last Glacial Maximum, according to vegetation modelling using a range of boundary conditions tested against existing pollen records.