Climatic and geographic correlates of global forest tree species-abundance distributions and community evenness

Questions: Species-abundance distributions (SADs) have long been at the centre of ecological interest because they summarize various patterns of community assembly in condensed form. Here we link observed patterns of SAD shape and evenness to latitude and climatic conditions to infer global trends in abundance distributions. Location: Global. Methods: We compiled data on tree species abundances and climate covariates from 605 sites worldwide. We applied linear regression modelling and nonlinear least squares fitting of log-series and lognormal abundance distributions to species rank-order log-abundance plots to assess latitudinal and climatic gradients in abundance distributions and Pielou evenness. Results: We observed significant latitudinal trends in SAD shape and evenness, even after accounting for richness and spatial effects. Evenness tended to increase towards lower latitudes and was positively correlated with actual evapotranspiration and negatively with climatic variability. We observed an excess of log-series SADs at lower latitudes and an increase in lognormal distributions towards northern latitudes. Accordingly, the proportion of species less abundant than expected from a log-series distribution decreased towards the tropics. Conclusions: We speculate that the observed latitudinal trends are caused by respective gradients in the importance of dispersal for local tree community assembly. This interpretation implies the hypothesis that tropical communities tend to be more open and input-driven in comparison to temperate communities.