Modelling the distribution of Amazonian tree species in response to long-term climate change during the Mid-Late Holocene

Aim: To (a) assess the environmental suitability for rainforest tree species of Moraceae and Urticaceae across Amazonia during the Mid-Late Holocene and (b) determine the extent to which their distributions increased in response to long-term climate change over this period. Location: Amazonia. Taxon: Tree species of Moraceae and Urticaceae. Methods: We used MaxEnt and inverse distance weighting interpolation to produce environmental suitability and relative abundance models at 0.5-degree resolution for tree species of Moraceae and Urticaceae, based on natural history collections and a large plot dataset. To test the response of the Amazon rainforest to long-term climate change, we quantified the increase in environmental suitability and modelled species richness for both families since the Mid-Holocene (past 6,000 years). To test the correlation between the relative abundance of these species in modern vegetation versus modern pollen assemblages, we analysed the surface pollen spectra from 46 previously published paleoecological sites. Results: We found that the mean environmental suitability in Amazonia for species of Moraceae and Urticaceae showed a slight increase (6.5%) over the past 6,000 years, although southern ecotonal Amazonia and the Guiana Shield showed much higher increases (up to 68%). The accompanied modelled mean species richness increased by as much as 120% throughout Amazonia. The mean relative abundance of Moraceae and Urticaceae correlated significantly with the modern pollen assemblages for these families. Main Conclusions: Increasing precipitation between the Mid- and Late Holocene expanded suitable environmental conditions for Amazonian humid rainforest tree species of Moraceae and Urticaceae, leading to rainforest expansion in ecotonal areas of Amazonia, consistent with previously published fossil pollen data.