Aridity and soil characteristics drive phylogenetic diversity and species richness patterns of nitrogen fixing plants at the continental scale

Background/Question/Methods

Nodulating nitrogen-fixing plants (fixers), all contained within a single monophyletic group, are important components of global plant communities and were long thought to be particularly adapted to disturbed, arid, and fire-prone ecosystems. However, patterns and drivers of nitrogen-fixing plant diversity, especially phylogenetic diversity at large spatial scales, remain largely unknown. We assembled plant diversity data from the National Ecological Observatory Network (NEON), subsetted fixers and non-fixers and natives versus exotics, and then calculated three metrics: Species Richness (SR), Phylogenetic Diversity (PD), and Mean Pairwise Distance (MPD) for each subset. We selected a set of covariates likely to explain diversity, focusing on key soil variables including nitrogen, along with fire frequency, air temperature, and aridity. We then fit a set of linear mixed models, taking into account zero inflation and over-dispersion of count data where necessary. We hypothesize that SR and PD of nitrogen fixers is highest in arid, nitrogen-poor regions. We also test if exotic species, with and without nitrogen-fixing symbiosis, are more tolerant of extremely arid or low nitrogen environments.

Results/Conclusions

We found that the primary driver of PD and MPD of nitrogen-fixing plants at the continental-scale was aridity. Aridity decreases the richness of both fixers and non-fixers, but much less strongly for fixers. When comparing the relative phylogenetic diversity of fixers to the whole community, aridity was the only explanatory predictor, confirming that relative diversity of fixers is much higher in arid environments. MPD results provide compelling evidence that aridity acts as a filter for native non-fixers, such that only a subset of closely related non-fixers are able to survive. However, native fixers, in particular, and to a lesser extent exotics (both fixers and non) are not filtered. As expected, native fixer richness was higher in low nitrogen conditions but surprisingly, so was non-fixer richness. MPD, however, increases with nitrogen for both non-fixers and native fixers. Only exotic fixers showed higher MPD in low nitrogen conditions. These unexpected results for nitrogen coupled with the importance of aridity provide a critical framework for future work and better understanding of the current factors determining nitrogen fixing diversity in temperate regions.