Species, growth form, and biogeographic diversity of summit vegetation along an elevation gradient in the tropical Andes: a baseline for climate change monitoring

Tropical alpine ecosystems exhibit outstanding plant diversity and endemism while being particularly sensitive to the impacts of climate change. Although understanding spatiotemporal changes in plant species composition, richness and community structure along tropical alpine altitudinal gradients is of primary importance, both the functional and historical/biogeographic dimensions of vegetation diversity remain largely unexplored. We used Generalized Linear Models and multivariate analyses to assess changes in species, growth forms, and biogeographic groups richness and abundance, in response to habitat variables along an elevation gradient in seven summits (3800 to 4600 m asl) in the Venezuelan Andes, studied using the standardized approach of the GLORIA-Andes monitoring network. The habitat variables assessed were soil temperature (-10 cm), soil organic matter, slope inclination, and substrate cover. We found 113 species, representing 72 genera, 32 families, 13 growth forms, and seven biogeographic origins, that included 25% of endemic elements. We observed richer vegetation, both in terms of species and growth forms, in summits with higher soil temperatures and higher SOM content, as well as higher biogeographic origin richness with increasing soil temperatures. The presence of holarctic elements increased toward higher elevations, while the occurrence of austral antarctic elements increased toward lower elevations. Our results indicate that biogeographic and functional approaches to vegetation diversity capture well the effect of abiotic filtering on community structuring in these tropical alpine environments. These findings constitute an important baseline for monitoring vegetation dynamics linked to climate change in the Venezuelan Andes by highlighting the functional and historical perspective on vegetation analyses, in contrast with more traditional approaches, based only on taxonomic species diversity.