How does functional diversity change along environmental gradients?

Marko Spasojevic, Michel Au, Jared D. Huxley, Tesa Madsen-McQueen, Erin McCann, Kristen Prattipati, Advyth Ramachandran, Sena Sadri and Melisa Salguero, Department of Evolution, Ecology, and Organismal Biology, University of California Riverside, Riverside, CA

Background/Question/Methods
Trait-based community assembly rose to prominence as a way to infer the multiple processes that structure communities in observational datasets. Despite over a decade of research examining how functional diversity patterns change along ecological gradients, few generalizations have emerged. To address this gap, we searched web of science using the search terms “functional diversity”, “gradient”, and “plant” for all papers published before 2021. We found a total of 547 papers, which we then narrowed down by removing all papers that were focused solely on methods or did not conduct on the ground measurements (i.e., remote sensing, modeling) of alpha diversity. We then created a database noting which traits were measured (e.g., SLA, height, etc.), which metrics of functional diversity were quantified (e.g., community weighted means, functional richness, functional dispersion, etc.), and what type of ecological gradients patterns were measured along (e.g., nitrogen, soil moisture, etc.). We then used vote counting to assess how functional diversity changed along gradients examining the directionality of those changes.
Results/Conclusions
Generally, we found that functional diversity in leaf traits and plant height increased with increasing resource availability (water, nitrogen) or biomass. However, in studies dominated by exotic species functional diversity generally decreased along the same gradients. In both cases community weighted means (CWM) shifted toward a more resource acquisitive functional strategy (i.e., higher CWM SLA). Changes in functional diversity along elevational gradients were largely dependent on climate. In drier climates functional diversity increased with increasing elevation, while in wetter climates it decreased. Despite these general patterns, a wide variety of approaches were used with no clear consensus among the traits measured or functional diversity metrics calculated. These idiosyncrasies represent a large barrier to synthesis as many metrics and traits are not comparable. We recommend that future research pairs community weighted means that describe shifts in functional strategies along with functional dispersion which quantifies shifts in functional dissimilarity for examining how functional diversity changes along ecological gradients.