Beyond biomass: measuring the effects of community‐level nitrogen enrichment on floral traits, pollinator visitation and plant reproduction

Abstract

Summary1. Nitrogen (N) limits primary productivity in many systems and can have dramatic effects on plant–herbivore interactions, but its effects on mutualistic interactions at the community level are not well‐understood. The reproduction of many plants depends on both soil N and pollination, and N may affect floral traits, such as flower number or size, which are important for pollinator attraction to plant individuals and communities.2. Thus, N may influence plant biomass and reproduction directly as well as indirectly via changes in pollination. The degree to which the effects of N enrichment scale from plant individuals to assemblages through emerging community‐level changes in species interactions, like pollination, is relatively unknown.3. For 4 years, we tested how N addition to subalpine plant assemblages in Colorado, USA, affected primary productivity and species diversity, floral traits and plant–pollinator interactions, and components of female and male plant reproduction.4. At the community level, we found that high‐N addition favoured the biomass and seed production of grasses, whereas low‐N addition promoted forb growth, flower production and pollinator visitation. However, using a pollen supplementation experiment, we found no evidence that N addition altered patterns of pollen limitation of seed production. Pollinators distributed themselves evenly across floral resources such that per‐flower visitation rate did not differ among N treatments. Thus, individual plants did not incur any extra benefit or cost from community‐level changes in plant–pollinator interactions that resulted from N enrichment, and the effects of N on forb reproduction were direct.5. Synthesis. Understanding how mutualistic and antagonistic species interactions influence individual and community responses to abiotic resources may provide insight to the dominant forces structuring communities and is especially important in the context of predicting the effects of environmental change. In this case, the direct effects of N addition on plants were stronger than the indirect effects mediated through plant–pollinator interactions, thus supporting the concept of bottom‐up resource limitation controlling plant response.