Diversity, endemism, and composition of tropical mountain forest communities in Sulawesi, Indonesia, in relation to elevation and soil properties

Explaining the diversity and distribution of tree taxa in the isolated tropical mountain forests (TMF) of the Malesian archipelago remains one of the great challenges of tropical biogeographical research. We investigated tree diversity, endemism and community composition in 13 plots of 0.24 ha between 700 and 2400 m in the TMF of Central Sulawesi's (Indonesia) highlands and related the patterns to gradients in elevation, climate and soil properties. Special attention was paid to understorey trees with stem diameters as low as 2 cm, not exceeding 2/3 of stand canopy height. Based on extensive taxonomic work on the plots, we found that Sulawesi's TMF flora is with 27-78 species per 0.24 ha not species-poorer than that of other Malesian islands, and vascular plant endemism is higher (22% of the species endemic to Sulawesi) than previously thought. Alpha diversity was mainly dependent on elevation and not soil factors (7 parameters tested). The alpha diversity of tree species, genera and families declined linearly with elevation, as did the number of species and genera per family, revealing higher phylogenetic clustering at lower elevations. Indicator Species Analysis and cluster analysis identified three main forest belts (sub-montane at c. 700-1400 m, lower montane at c. 1400-2000 m, upper montane at > c. 2000 m), deviating from earlier zonation concepts by separating sub-montane (rich in taxa related to the lowlands) and lower montane communities (rich in Fagaceae and Myrtaceae). With 27-51% of all tree species in a plot never found in the upper canopy, our data suggest that the guild of true understorey tree species is species-rich (estimated at c. 130 species in our region), contradicting the hypothesis that Southeast Asian tropical forests with mass-flowering trees (here: Fagaceae) are particularly poor in understorey tree species. We conclude that the mountains of Sulawesi harbour remarkably species-rich, but still understudied, TMF with a unique tree flora rich in understorey trees, which are of high conservation priority.