Evolutionary effects of nitrogen are not easily predicted from ecological responses

Anthropogenic nitrogen (N) addition might alter the evolutionary trajectories of plant populations, in part because it alters the abiotic and biotic environment by increasing aboveground primary productivity, light asymmetry, and herbivory intensity, and reducing plant species diversity. Such evolutionary impacts could be caused by N altering patterns of natural selection (i.e., trait-fitness relationships) and the opportunity for selection (i.e., variance in relative fitness). Because at the community level N addition favors species with light acquisition strategies (e.g., tall species), we predict that N would also increase selection favoring those same traits. We also hypothesize that N could alter the opportunity for selection via its effects on mean fitness and/or competitive asymmetries.To investigate these evolutionary consequences of N, we quantified the strength of selection and the opportunity for selection in replicated populations of the annual grass Setaria faberi Herrm. (giant foxtail) growing in a long-term N addition experiment. We also correlated our measures of selection and opportunity for selection with light asymmetry, diversity, and herbivory intensity to identify the proximate causes of any N effects on evolutionary processes. N addition increased aboveground productivity, light asymmetry, and reduced species diversity. Contrary to expectations, N addition did not strengthen selection for trait values associated with higher light acquisition such as greater height and specific leaf area (SLA); rather, it strengthened selection favoring lower SLA. Increased light asymmetry was associated with stronger selection for lower SLA and lower species diversity was associated with stronger selection for greater height and lower SLA, suggesting a role for these factors in driving N-mediated selection. The opportunity for selection was not influenced by N addition (despite increased mean fitness) but was negatively associated with species diversity. Our results indicate that anthropogenic N enrichment can affect evolutionary processes, but that evolutionary changes in plant traits within populations are unlikely to parallel the shifts in plant traits observed at the community level.R statistical software is required to run the R script.Funding provided by: NSF Long-term Ecological Research Program*Crossref Funder Registry ID: Award Number: DEB 1832042Funding provided by: Michigan State University AgBioResearch*Crossref Funder Registry ID: Award Number: Funding provided by: Indiana University's Prepared for Environmental Change Grand Challenge Initiative*Crossref Funder Registry ID: Award Number:Data was collected in 2020 from a field experiment in a long-term ecological research site (Kellogg Biological Station LTER site in Michigan, USA). The Data folder contains 3 separate datasets as CSV files, each with accompanying .txt metadata files: 1) a dataset of individual-level data (Waterton2022_NitrogenEvolution_Individual_Data.csv); 2) a dataset of annual net primary productivity (ANPP; Waterton2022_NitrogenEvolution_ANPP_Data.csv); 3) a dataset of light measurements (Waterton2022_NitrogenEvolution_Light_Data.csv). An R script for reproducing the analyses and figures is available at https://doi.org/10.5281/zenodo.7121361