Broad-scale patterns and drivers of functional diversity in a key-stone plant group (palms, Arecaceae)

Background/Question/Methods . Palms constitute a diverse plant group of keystone ecological importance in many tropical regions. It is increasingly recognized that functional diversity – i.e. the mean, range and dispersion of functional trait values in an assemblage – is of key importance for defining its ecological functioning and impact on ecosystem processes. While palms exhibit high levels of variation in key functional traits (such as maximum stem height, progagule size, and clonality), our understanding of the patterns and drivers of palm functional diversity remains poor. We here report new macroecological work directly addressing this question, testing for drivers related to past and present climate as well as to interactions with herbivores. One set of analyzes was done using palm range maps for the New World (n = 541 species) and data on traits (leaf size, stem height and fruit size) to estimate median trait values for palm species assemblages in 110×110-km grid cells. Another set of analyses was done using data on the distribution of all palm species (n = 2469) in ‘botanical countries’ (small countries and regions within large countries) across the world, coupled to palm traits, present and past climate, soils, and mammal species distributions.

Results/Conclusions . Within the New World, we find that gradients in current climate are of prime importance as determinants of functional trait patterns in palms. Notably, seasonality in temperature and precipitation play major roles in explaining geographical variation of all traits, with mean annual temperature and annual precipitation also important for leaf size. There were mostly minor links to past climate. In a global analysis of assemblage mean fruit size, we find links to present and past climate, notably with  bigger fruits in areas subject to stronger Quaternary climate fluctuations. Controlling for environment we also find unexplained regional differences and a clear positive link between body size of mammal frugivores and palm fruit size. Interestingly, within the major tropical regions the latter pattern was only apparent in the New World tropics, when the many recently extinct mammals (latest Pleistocene to early Holocene) were included in the analysis. Global analysis of assemblage maximum stem height means in palms similarly show strong links to current climate and strong unexplained regional differences. However, for stem height we also find a strong link to vegetation canopy height. We conclude that broad-scale patterns in functional diversity in palms exhibit strong links to climate, but are also shaped by historical and trophic dynamics.