The maintenance of plant community diversity and function in nutrient enriched grasslands

Understanding the mechanistic causes of change in the diversity and functioning of ecosystems is a key step in limiting ecological degradation. Eutrophication, now a pervasive global disturbance driving widespread change in plant community composition and function, might be understood in terms of changes in fundamental ecological mechanisms relating to species interactions. However, it often remains unclear how these dynamics operate in empirical settings, especially where multiple global change pressures may simultaneously alter species interactions. In this thesis, I present three articles broadly connected by a focus on the consequences of altered nutrient availability for plant communities. Specifically, in chapter one, I assessed how multiple-nutrient enrichment affected the temporal stability of aboveground biomass production in 34 globally distributed grasslands over seven years. My analysis indicated that destabilization was most severe where nutrients reduced species richness and increased species synchrony but that this effect depended on nutrient identity, with nitrogen more commonly destabilizing than phosphorus or potassium. In chapter two, working at the species level, I parameterized plant competition models with data from a greenhouse experiment designed to test how multiple-nutrient enrichment influenced species coexistence. This provided some of the first empirical evidence that both niche and fitness differences among competing plants are responsive to changes in nutrient availability but challenged general predictions by showing that both stabilization and destabilization were possible, depending on nutrient identity. Lastly, in chapter three, I conducted a field experiment to test how shifts in species’ competitive interactions explained responses to nutrient addition in a heterogenous environment, where plant fitness was impacted by several factors simultaneously. The experiment indicated that the influence of competitive interactions mediated by niche and fitness differences can become less important than the ability to tolerate pressures of herbivory. Collectively, my research contributes to an improved understanding of when and how eutrophication will impact plant community diversity and function by altering species interactions and diversity-function relationships.