Variation of floristic diversity, community composition, endemism, and conservation status of tree species in tropical rainforests of Sri Lanka across a wide altitudinal gradient

Tropical rainforests in Sri Lanka are considered as biodiversity hotspots. However, their existence is threatened by multiple climatic and anthropogenic drivers. Our objective was to assess the diversity, endemism and conservation status of these rainforests across a wide altitudinal range. We conducted a complete census of all trees having ≥ 10 cm diameter at breast height in ten one-hectare permanent sampling plots of tropical rainforests from 100 to 2200 m above mean sea level and determined altitudinal trends in tree diversity, floristic composition, endemism and conservation status of tree taxa. While community-scale tree diversity decreased with increasing altitude, substantial altitudinal differentiation of tree taxa was observed. Increasing day-night temperature difference (DTR), decreasing rainfall and increasing cumulative maximum soil water deficit (CSWD max ) with increasing altitude decreased tree diversity and the percentage of endemic species while increasing the percentage of endangered species. These trends show that the ability to colonize the higher altitudes, where lower temperatures and higher soil water deficits act as climate constraints, is a major determinant of tree diversity and endemism in Sri Lankan rainforests across altitude gradients. This hypothesis is supported by the observed increases of the percentages of endangered and vulnerable species from mid- to high altitudes. Most of the dominant species at different altitudes were endemic and are endangered or vulnerable. We conclude that the decrease in diversity and endemism is most likely driven by cold- and drought-sensitivity of these predominantly tropical-adapted tree species so that conservation efforts are most needed at altitudes above 1000 m.