Climate controls plant life form patterns on a high-elevation oceanic island

Aim: Plant life forms characterize key morphological strategies that enable large-scale comparisons of plant communities. This study applies Raunkiær's plant life form concept that was developed for temperate climate to a subtropical island flora, in parts, dominated by summer aridity. We quantify how plant life form patterns as well as patterns of important plant functional traits (PFTs) relate to important climate and topographic characteristics. Location: La Palma, Canary Islands Taxon: Flora of La Palma.

Methods: We assigned each native plant species a plant life form, i.e., phanerophyte, chamaephyte, hemicryptophyte, geophyte and therophyte, as well as PFTs (succulence and N-fixer). We used stacked species distribution models to assess occurrence probability for each species using the Atlantis database (500x500 m grid). We related richness and percentage values for each plant life form and PFT to climate and topography.

Results: Plant life forms and PFTs showed a clear pattern within geographic but also climate space, while topography had a minor effect. Phanerophytes mainly contributed to the flora in humid areas. Chamaephytes and hemicryptophytes most strongly contributed to the summit scrub flora and, to some degree, also to the arid coastal regions. Geophytes and therophytes were mainly found in dry coastal regions. N-fixers contributed mainly to warm-arid and cool-arid regions, while succulent species were mainly found in arid coastal regions.

Main conclusions: Raunkiær's plant life form concept can be comprehensively transferred to a subtropical island flora by adapting to local unfavorable growing conditions, i.e., aridity. Using the strong environmental gradients offered by our study island, we identify substantial climate-driven variation in patterns of plant life forms and PFTs that might be used for large scale comparisons in macroecological studies. The growth strategies reflected in Raunkiær's plant life forms suggest differences in species establishment and coexistence dynamics within different parts of the island's climate space.