Island biogeographic gradients and local environmental conditions differentially shape phylogenetic and functional diversity in Malesian tree assemblages

The islands of Malesia support exceptional tree diversity, yet the processes shaping the phylogenetic and functional diversity of local assemblages remain poorly understood. We analysed 191 forest plots across 12 islands spanning the bioregions of Sunda, Wallacea and Sahul, with 2,001 tree species represented in the dataset. We quantified the phylogenetic and functional diversity of each plot’s tree assemblage using standardised effect sizes of phylogenetic and functional richness (SESPD and SESFD). We tested how island area, isolation, local environmental conditions and geological disturbance were associated with plot-level SESPD and SESFD. Mean plot-level SESPD was significantly below zero in Sunda, indicating phylogenetic clustering, while mean SESFD indicated functional clustering across all three bioregions. Island area was negatively associated with both SESPD and SESFD, whereas distance to the mainland was positively associated with SESPD only. Earthquake proximity had opposing effects on the two dimensions, being associated with greater SESPD but lower SESFD. SESPD and SESFD were only weakly correlated among plots, and there was no correlation when assemblages were pooled within islands. Together, these results show that biogeographic processes at the island scale can leave distinct phylogenetic and functional signatures within local forest assemblages, and that species richness alone provides only a partial representation of how island tree diversity is structured.