Sean Michaletz

Trait-based ecology seeks to understand how environmental variation influences organismal performance and fitness via functional traits. The morphology-performance-fitness (MPF) framework hypothesizes directional causal relationships linking functional traits, performance traits, and fitness. However, the mediating role of functional traits in linking environmental variation to performance has not been fully articulated or tested within this framework. To address this gap, I extended the MPF framework by explicitly incorporating causal links between environmental variation and functional traits. I tested these hypotheses using data for three leaf economics traits — leaf mass per area (LMA), leaf dry matter content (LDMC), and chlorophyll content (CHL) — and a performance trait (aboveground growth rate) in three plant species grown across a broad nitrogen addition gradient. Leaf traits responded strongly to nitrogen addition, with variation structured primarily along a leaf economics axis and secondarily along a photosynthetic pigment axis. Increasing nitrogen drove phenotypic convergence, shifting trait expression toward a common region of multivariate trait space at high nitrogen levels. Structural equation modeling strongly supported MPF hypotheses, demonstrating that nitrogen addition influenced growth rates primarily through its effects on chlorophyll content and LDMC, as well as unmeasured functional traits. By clarifying the pathways linking environmental variation to plant functional and performance traits, these findings enhance our understanding of plant responses to nutrient gradients and their broader implications for global change ecology.

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Understanding drivers of variation in plant mortality rates is a central challenge for global change biology. Metabolic scaling theory (MST) hypothesizes that mortality rates will scale as the -2/3rd power of stem diameter but doesn’t account for other potential drivers of variation in mortality rates, such as climate and functional traits. For example, it is hypothesized that the temperature-dependence of mortality rates will reflect the activation energy of photosynthesis (E = 0.58 eV), and that shifts in functional traits will maximize growth rates and buffer mortality rates across climate gradients. However, these hypotheses have been neither formalized nor assessed within a common mathematical framework. In this thesis, my objectives are to: 1) build on MST and ecological stoichiometry theory to develop new theory that formalizes prominent hypotheses for global variation in plant mortality rates, and 2) test my theory using long-term data from nine forest sites that are arrayed across global climate gradients. As hypothesized by theory, mortality rates generally exhibited diameter-scaling exponents that were statistically indistinguishable from -2/3. The temperature-dependence of mortality rates followed an activation energy of E = 0.61 eV, with 95% confidence intervals (0.39 to 0.84) that included the value of 0.58 eV hypothesized for photosynthesis. Plant functional traits all varied significantly with climate and did influence mortality as hypothesized in the theory, though this signal was lower than that of size and temperature. Thus, for the hypotheses tested here, global variation in plant mortality rates appears to be driven primarily by size and temperature, as well as plant functional traits to a lesser degree.

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