Phylogeny and phenotypic adjustments drive functional traits in Rhododendron across elevations in its diversity hot-spot in W-China

Abstract

Mountains represent ‘experiments by nature’ that permit testing ecological theory. Using herbarium samples of 92 Rhododendron species collected between 800 and 4500 m a.sl. in the Himalaya-Hengduan Mountains region, SW China, we explored congeneric elevational trends in key plant traits at the among- and at the within-species level. We aimed at identifying the contributing factors to phenotypic trait expression across a climatic gradient, distinguishing effects in response to elevation from those due to phylogenetic relatedness, non-phylogenetic species effect, and within-species trait variability. We found that a substantial fraction of the trait variation (up to 88%) is deeply rooted in phylogeny, with species elevation also revealing a strong phylogenetic signal. While accounting for these phylogenetic influences, we still found consistent associations with elevation of anatomical (increasing leaf thickness and decreasing specific leaf area), morphological (decreasing plant and leaf size, annual length increment), physiological (less negative foliage δ¹³C signal), and reproductive traits (flower size largely conserved, while leaf size declines, hence relative investment in flowers increases). Our findings support a combined action of phylogenetic inertia and phenotypic adjustments of plants to life conditions at high elevation. Given that our samples come from one genus and a natural climatic gradient (rather than botanical gardens) with temperature the dominant variable, such patterns represent a robust and representative signature of the multiple causes of trait-environment associations in woody species.