Were Dry Forests widespread in the Pleistocene and what is their fate under climate change? A modelling approach using a specialist plant

Seasonally Dry Tropical Forests (SDTF) are deciduous and semideciduous seasonal forests that are geographically distant from each other in South America, but share multiple species in common. How did these disjunct species distributions arise and what changes in their distribution can be expected under climate change? Gymnanthes boticario, a SDTF specialist, is a perfect model to test these questions.Occurrence data were obtained from herbarium sheets and confirmed by the authors and/or taxonomic publications. We selected bioclimatic variables for past, current and future scenarios and modeled the potential distribution of Gymnanthes boticario for three paleoclimatic conditions and two future climate change scenarios. We found an unexpectedly large geographical extent of climate suitability for G. boticario during the Last Interglacial (LIG), forming a possible connection between the Brazilian dry Caatinga forests and the dry forests of Bolivia. In the Last Glacial Maximum (LGM), our model shows a reduction in the extent of suitability compared to the LIG, making the suitable area more similar to that modelled for the current period. For all future climate change scenarios, we found a decrease in the total suitable area. The future scenario of SSP585 is most severe, with a loss of almost 60 % of G.boticario suitable areas. We show that during drier timeframes, dry forests likely became less geographically distant from each other, allowing migration between the two now isolated dry forests. This historic connectivity could explain the considerable number of species disjunctions between Caatinga and Bolivian dry forests. Our models indicate that the broadest distribution of SDTF may have occurred during the LIG, and not in the LGM as previously thought. Moreover, climate change models indicate that the future may be even more challenging than the current for SDTF habitats.