Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Ecology and Evolutionary Biology

Major Professor

Jennifer A. Schweitzer

Committee Members

Joseph K. Bailey, Henri D. Grissino-Mayer, Brian C. O'Meara


Under increasing additions of reactive nitrogen (N) to the planet via anthropogenic N deposition and excess fertilization, some plant species will thrive while others will not. This may seem counterintuitive, as the growth of most plants is thought to be limited by soil N, but recent evidence shows that excess N can reduce plant community composition, alter plant-microbial interactions, and lead to fundamental alterations in plant growth and fitness. Yet, we lack the ability to predict which plant species will be winners or losers in soil N enrichment scenarios. The primary goal of my dissertation was to examine variation in plant growth responses to N enrichment and whether ecological and evolutionary factors explain such variation. These factors, according to current literature, should include aspects of past evolution such as phylogeny and evolutionary differentiation in resource use traits, nutrient co-limitation, and interactions with root-associated microbes. Because variation in plant responses to soil N enrichment challenges the paradigm in ecology that productivity of all plants is N-limited or N co-limited, a second goal of my dissertation was to determine how this and other recent work changes our understanding of the terrestrial N and carbon (C) cycles and feedbacks between soil N gradients and evolution under global change.In my first chapter, I used a global dataset of plant biomass responses to N fertilization and evolutionary models to show that species vary in the direction and magnitude with which they respond to N enrichment (with more than one in four species responding negatively or neutrally), and that two aspects of past evolution (phylogenetic relatedness and selection associated with constraints on resource use) govern responses to N enrichment. In my second and third chapters, I implemented two greenhouse fertilization experiments and subsets of the 30 functionally diverse tree species within the genus Eucalyptus that are native to Tasmania, Australia. The main result from these experiments was that phylogenetic patterns in biomass responses to N enrichment are associated with phylogenetic variation in root function (specific root length and interactions with ectomycorrhizal fungi), but not co-limitation by phosphorus (despite the fact that Tasmanian eucalypts occur across strong soil phosphorus gradients). In my fourth chapter, I reviewed how this and other current research challenges long-held and fundamental assumptions regarding the source, plant use, and microbial transformations of N and provides insights into eco-evolutionary feedbacks and C cycling under global change. Overall, my dissertation has used major theories in plant ecology and evolution to explain the variation in plant responses to global change, and synthesized research that highlights new understanding of the drivers and consequences of terrestrial N cycling.

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