Plant Species Biogeographic Origin Shapes Their Current and Future Distribution on the World's Highest Island Mountain

In New Guinea, low elevations are thought to be dominated by plant species with biogeographic origins in Sunda (Borneo, Sumatra, Java, Mainland Asia) and high elevations by species from Sahul (Australia and New Guinea). In regions where frequent surveys of the flora are unfeasible, herbarium specimens collected over time offer an opportunity to study the co-occurrence of species. Over 7000 collections have been made on New Guinea's Mount Jaya allowing us to examine species presence under current and future temperatures. Applying a Bayesian phylogenetic mixed effects model to the Mount Jaya elevation gradient, we show that species of Sunda origin are more likely to occupy warmer lowlands and Sahul species cooler higher elevations. Using the model to predict species distributions under a 2 degrees C increase, we find that Sahul species are less likely to tolerate future temperatures, especially below 1000 m. Predictions also show a potential increase in species richness across the gradient. It is unclear if these increases are possible due to limits imposed by the species area relationship. Synthesis. These results emphasise how biogeographic origin influences current and future species distributions on tropical mountains. It remains to be seen how our results link to traits that dictate species' ability to tolerate increasing temperatures.