High phylogenetic but low functional turnover of melastomes along an elevational gradient
There is a pressing need to study the potential response of plants to changing climates, especially in the tropics, where impacts are predicted to be most severe. Plant functional traits, which integrate across physiological processes, offer a promising way to understand plant responses to climate change. The Melastomataceae are an ideal clade in which to investigate the evolutionary patterns of plant responses to climate change. In the neotropics, more than 3500 species occur in diverse habitats, from lowland rainforests to high-elevation paramos. The melastomes also display remarkable morphological and functional diversity: woody species can be early succession pioneers, forest interior trees, or epiphytic.
We measured functional traits of more than one hundred melastome species along an elevational gradient in Costa Rica, extending from the La Selva Biological Reserve at 30m ASL, to a site near the summit at ~2500m on Volcan Barva. At each of five localities, we sampled herbaceous, epiphytic, and woody melastome species for stem density and a suite of leaf traits. Information on fruit type, seed size and other reproductive traits was gathered for each of these species from herbarium material. We asked whether these traits turned over across the elevation gradient, and whether these patterns differed by growth form. We also asked whether the patterns of species turnover matched the pattern of trait turnover across sites.
We found strong species turnover across sites: out of 102 species, 66 were sampled in only one site, and 27 of the remaining species were only measured in two sites. Despite considerable phylogenetic turnover, however, we found no pronounced turnover of functional traits across elevation. This suggests that melastome species tend to play the same functional role in communities across the gradient. We can test this hypothesis by quantifying the turnover of traits of other co-occurring plants and asking whether melastomes fill the same trait-space in communities even as other groups show functional shifts. Our results suggest that melastomes may respond similarly to climate changes across environments despite their considerable morphological differences.