2020 ESA Annual Meeting (August 3 - 6)

COS 182 Abstract - Can grasslands help resolve the latitudinal patterns of nitrogen-fixing plants?

Benton Taylor1, Kimberly Komatsu1, Meghan Avolio2, Sally E. Koerner3, Corre Data Consortium4 and The Grazing Exclosure Consortium5, (1)Smithsonian Environmental Research Center, Edgewater, MD, (2)Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD, (3)Department of Biology, University of North Carolina Greensboro, Greensboro, NC, (4)Multiple Institutions, (5)Various Affiliations
Background/Question/Methods

Global patterns of symbiotic nitrogen-fixing plants (N fixers) represent a long-standing ecological and biogeochemical puzzle. Ecosystem theory suggests that N fixers should have a competitive advantage in nitrogen-limited regions of the world such as the temperate and boreal zones but be relatively rare in the nitrogen-rich tropics. Substantial analysis of latitudinal patterns of N-fixing trees has shown the exact opposite pattern—N-fixing trees are relatively common in the tropics and rare at high latitudes—a seemingly paradoxical pattern that has yet to be fully explained ecologically. However, anecdotal evidence of high abundances of herbaceous N fixers at high latitudes suggests that the latitudinal pattern for herbaceous N fixers may not reflect that of N-fixing trees. To better understand these patterns, we analyzed the relative abundance and richness of N fixers in two global networks of grassland experiments: the Community Responses to Resource Enrichment (CoRRE) and Grazing Exclosure (GEx) plot networks. We use a maximum-likelihood framework to assess the latitudinal pattern of grassland N fixers across more than 200 sites, 1,500 individual plots, and 4,500 species globally. We also assess the influence of biotic and abiotic drivers of these global patterns of grassland N fixers.

Results/Conclusions

Overall, we found contrasting latitudinal patterns between grassland N fixers and previously established patterns of N-fixing trees. N fixers in grasslands exhibit similar relative abundances and relative diversities at high latitudes as they do near the equator, patterns that contrast with those of N-fixing trees. Abiotic conditions such as temperature, precipitation, N limitation and phosphorus limitation were not strong predictors of grassland N-fixer abundances. Rather, biotic drivers such as N-fixer diversity and the degree to which grazers preferentially feed on N fixers were the strongest drivers of grassland N-fixer abundances. These analyses provide strong evidence that the influence of ecological conditions on N-fixer abundances likely differs substantially between N-fixing trees and the herbaceous and small woody N fixers of grassland ecosystems. Our results suggest that general ecological “rules” for what makes N fixation competitively advantageous in an ecosystem may not exist, given that the effects of many factors (e.g. temperature, nutrient limitation, diversity, herbivory) seem to differ substantially between tree and grassland N fixers. Further comparisons of tree and grassland N-fixers may help refine the nuanced ecological drivers of N fixers, N inputs, and N limitation at regional and global scales.