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Abstract
This dissertation focuses on the evolution and ecophysiology of leaf economics and leaf defenses in the diverse herbaceous genus Helianthus (the sunflowers). Members of the genus have radiated into a wide variety of habitats across North America and possess a concomitant broad diversity in growth form, life history, and leaf physiology. This work addresses questions regarding variation in leaf economics and leaf defenses at multiple scales: ontogenetic variation within individuals, microevolutionary differentiation among populations, macroevolution among closely-related species, and macroevolution across plants globally. Two chapters examine ontogenetic variation in leaf economics and leaf defenses, respectively, finding that leaf economic strategy shifts strongly with whole-plant development and that leaf defense investment and leaf economic strategy are intrinsically linked at the leaf level. Two more chapters use a phylogenetic comparative approach to explore the evolution of leaf economics and leaf defenses, respectively, across the genus in relation to environmental gradients in temperature, precipitation, and soil fertility. Most of the leaf-level trade-offs at the core of the leaf economics spectrum are found to exist at the macroevolutionary scale across Helianthus species, as well as at the microevolutionary scale among populations within species. However, several major deviations from expectations exist in both trait-trait relationships and associations with environment, suggesting that evolutionary dynamics in herbaceous plants are not well-represented by the global leaf economics spectrum paradigm. The evolution of leaf defenses across Helianthus suggests high evolutionary lability and few trade-offs among defenses. Investment in both physical and chemical defenses is associated with more resource-conservative leaf economic strategy, mirroring findings at the ontogenetic scale. A final chapter uses phylogenetic structural equation modeling to test the generality of the mechanistic functional relationships among leaf economic traits across evolutionary and ecophysiological scales. These analyses find no need for an unmeasured latent trait to explain global leaf economic trait covariation, as suggested by previous work, and that physiologically intuitive models best explain trait relationships across scales in Helianthus. Together this dissertation investigates variation in leaf physiology in response to both abiotic and biotic forces, and contributes to our understanding of key features of plant evolutionary ecophysiology.