Abstract

Large-scale environmental gradients have been invaluable for unravelling the processes shaping the evolution and maintenance of biodiversity. Gradients provide a natural setting to test theories about species diversity and distributions within a landscape with changing biotic and abiotic interactions. Elevational gradients are particularly useful because they often have an extensive climatic range within a constricted geographic region. Arunachal Pradesh is the northeastern-most province in India, located on the southern face of the eastern Himalayas. This region is considered a biodiversity “hotspot”, with an estimated 6000 flowering plant species of which 30-40% are endemic. For this thesis, I analyzed tree communities in plots distributed throughout the province using both species and phylogenetic diversity indices. I explored shifts in community structure across elevation and space as well as the biotic and abiotic forces influencing species assembly throughout the landscape. Species richness and phylogenetic diversity decreased with increasing elevation, as theory predicts. However, species relatedness did not show a clear pattern with elevation. Nonetheless, by exploring beta-diversity (both taxonomic and phylogenetic), I was able to show a strong effect of environmental filtering with elevation. Environmental filtering is generally associated with species clustering on the phylogeny, where co-occurring species in a community are more closely related than expected by chance. Here, however, I suggest that forest community structure is driven by filtering on glacial relicts, resulting in random or over-dispersed community assemblages. These patterns point to possible regions for conservation priority that may provide refugia for species threatened by current warming trends.