Comparing clades: Using a trait-based approach to measure functional diversity of forest trees and soil microbes across the Latitude Biodiversity Gradient

Background/Question/Methods The Latitudinal Biodiversity Gradient remains one of the most recognized patterns in ecology; however understanding the mechanisms driving these patterns remains unclear. Ecological theories used to explain the origin of biodiversity gradients such as competitive exclusion, neutral dynamics, and environmental filtering make predictions for how functional diversity should vary at different spatial scales and across broad diversity gradients. We use empirical data to examine soil microbial and plant taxonomic and functional diversity along a latitudinal gradient. We test predictions by quantifying differing measures of diversity via functional traits. Measures of functional diversity were based on plant and microbial functional traits that representing major axes of plant strategy variation as well as microbial resource use and functioning. We utilized high throughput sequencing and functional gene array, to analyze microbial functional diversity, composition, and structure of microbial communities. We assessed functional gene families related to microbial carbon (C), nitrogen (N), and phosphorus (P) cycling. We ask, are shifts in taxonomic diversity best explained by trait shifts and patterns of functional trait diversity. Do plants and microbes have similar richness gradients across latitude (temperature)?

Results/Conclusions Our results enable one of the first comparison of functional diversity within and across microbial and plant species communities across latitude. In contrast to recent studies that have shown that climate only has a relative weak signal in patterns of single trait shifts across climate gradients, we show that when differences in species abundances are taken into account that multivariate shifts in trait composition is largely explained by climate (~ 30 – 93%). We find that soil temperature and mean annual temperate have the strongest effect in shifts in community trait diversity. Overall, our results suggest that observed shifts in taxonomic diversity across latitude are associated with shifts in the functional composition in plant and soil bacterial communities but less so with fungal composition. This is consistent with hypotheses that suggest richness gradients are significantly structured due to traits/function. While we highlight several challenges that remain in fully comparing patterns of diversity across differing taxonomic groups, nevertheless, our findings have important implications for assessing biodiversity theory and for the main drivers that structure diversity gradients.