Water and energy availability mediate biodiversity patterns along an elevational gradient in the tropical Andes

Aim: How tree taxonomic and phylogenetic diversity vary with elevation at multiple spatial scales may provide new insights into the ecological and evolutionary processes influencing biogeographical patterns. The effect of water- and energy-related climatic variables on forests diversity across elevations, as well as how clades have evolved on and established across mountain regions lack consensus. Here, we tested whether changes in biodiversity with elevation are consistent with one of, or multiple, competing hypotheses: the water–energy dynamics (WED), species–energy relationship (SER), Tropical Niche Conservatism (TNC) and Out of The Tropics hypothesis (OTT). Location: Patia watershed, Colombia. Taxon: Seed plants (trees). Methods: We used a large dataset of 490 0.1 ha forest plots in nine elevational belts (545–3410 m a.s.l) that correspond to three different life zones, and quantified alpha and gamma scales using taxonomic and phylogenetic diversity indices. We fitted linear mixed-effects models to evaluate how taxonomic and phylogenetic alpha diversity changed with elevation, precipitation and aboveground biomass. We assessed taxonomic and phylogenetic beta diversity using the Sørensen index and its spatial turnover and nestedness components. Results: Taxonomic and phylogenetic alpha diversity decrease with elevation. Yet, at the gamma scale, taxonomic and phylogenetic diversity predominantly increased with elevation. Taxonomic and phylogenetic beta diversity were strongly influenced by species turnover, and followed a hump-shaped pattern with elevation. Main conclusions: Overall, diversity shows a decreasing trend at the local scale, while coarse-scale gamma diversity followed a pattern of nonlinear increases for both taxonomic and phylogenetic diversity. Evidence supports the influence of SER and WED on diversity patterns across elevations, yet neither evolutionary hypotheses had sufficient empirical support to be conclusive.