Sean Michaletz

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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