The thermal niche and phylogenetic assembly of evergreen tree metacommunities in a mid-to-upper tropical montane zone

Frost and freezing temperatures have posed an obstacle to tropical woody evergreen plants over evolutionary timescales. Thus, along tropical elevation gradients, frost may influence woody plant community structure by filtering out lowland tropical clades, and allowing extra-tropical lineages to establish at higher elevations.

Here we assess the extent to which frost influences the taxonomic and phylogenetic structure of naturally-patchy evergreen forests (locally known as shola) along a mid-upper montane elevation gradient in the Western Ghats, India. Specifically, we examine the role of large-scale macroclimate and factors affecting local microclimates, including shola patch size and distance from shola edge, in driving shola metacommunity structure. We find that the shola metacommunity shows phylogenetic overdispersion with elevation, with greater representation of extra-tropical lineages above 2000m, and marked turnover in taxonomic composition of shola woody communities near the frost-affected forest edge above 2000m, from those below 2000m. Both minimum winter temperature and patch size were equally important in determining metacommunity structure, with plots inside very large sholas dominated by older tropical lineages, with many endemics. Phylogenetic overdispersion in the upper montane shola metacommunity thus resulted from tropical lineages persisting in the interiors of large closed frost-free sholas, where their regeneration niche has been preserved over time.