In the weeds: Nutrient enrichment and herbivore exclusion influence plant-pollinator interactions in an old field community

Global environmental changes, such as land use change and nutrient enrichments, may influence the abundance, diversity, and dominance patterns of plant communities, with consequences for plant-pollinator interactions. Increases in the abundance of large herbivores and widespread nutrient enrichments may influence plant-pollinator interactions via changes to floral abundance, phenology, or plant community composition. In this study, we use a long-term field experiment to examine the simple and interactive effects of nutrient enrichment and herbivore exclusion, as a part of a collaboration with the Nutrient Network. We established 25 m² plots in an old field in central Maryland and constructed deer-exclusion fences in 2019. Fertilizers were then applied in a factorial design, and floral abundance, floral species richness, and pollinator visitation were recorded in repeated surveys. Over the short term (2 months post-fencing), floral abundance, pollinator visitation, and fruit set did not differ between fenced and open plots, but over the longer-term (2 years), we observed reduced floral species richness in fenced plots. In plots receiving multiple nutrient enrichments (NPK) for 2 years, both total floral abundance and floral species richness were depressed in early-season floral surveys. Our current and future research will investigate the long-term responses of pollinators to the experimental treatments, as well as potential mechanisms underlying these responses. Notably, goldenrods (Solidago altissima) are dominant members of the old field plant community and, though they flower in the late-season, goldenrods may interact competitively with early-flowering plants, with consequences for the floral resources available for pollinators.