Aridity drives phylogenetic diversity and species richness patterns of nitrogen-fixing plants in North America


Aim: Nitrogen (N)-fixing plants are an important component of global plant communi- ties, but the drivers of N-fixing plant diversity, especially in temperate regions, remain underexplored. Here, we examined broad-scale patterns of N-fixing and non-fixing plant phylogenetic diversity (PD) and species richness (SR) across a wide portion of temperate North America, focusing on relationships with soil N and aridity. We also tested whether exotic species, with and without N-fixing symbiosis, have fewer abi- otic limitations compared with native species. Location: USA and Puerto Rico. Time period: Current. Major taxa studied: Vascular plants, focusing on N-fixing groups (orders Fabales, Fagales, Rosales and Cucurbitales). Methods: We subset National Ecological Observatory Network (NEON) plant plot data from all sites along two axes (N fixing–non-N fixing and native–exotic), calculat- ing plot-level SR, PD and mean pairwise phylogenetic distance (MPD). We then used linear mixed models to investigate relationships between diversity values and key soil measurements, along with aridity, temperature and fire frequency. Results: Aridity was the sole predictor of proportional phylogenetic diversity of N fixers. The SR of N fixers still decreased marginally in arid regions, whereas native N-fixer MPD increased with aridity, indicative of unique lineages of N fixers in the dri- est conditions, in contrast to native non-N fixers. The SR of both native N fixers and non-N fixers increased in low-N soils. Aridity did not affect SR of exotic non-N fixers, unlike other groups, whereas exotic N fixers showed lower MPD in increasingly high- N soils, suggesting filtering, contrary what was found for native N fixers. Main conclusions: Our results suggest that it is not nitrogen, or any soil nutrient, that has the strongest effect on the relative success of N fixers in plant communities. Rather, aridity is the key driver, at least for native species, in line with empirical results from other biomes and increased understanding of N fixation as a key mechanism to avoid water loss.

Global Ecology and Biogeography