Williams, Laura2018-11-282018-11-282018-08https://hdl.handle.net/11299/201174University of Minnesota Ph.D. dissertation. August 2018. Major: Ecology, Evolution and Behavior. Advisors: Peter Reich, Jeannine Cavender-Bares. 1 computer file (PDF); iii, 231 pages.Forests are complex systems made of many parts – trees, leaves, birds, insects, and more – which interact, change and adapt. My dissertation research was inspired by this fascinating complexity as I sought insights into the consequences of interactions among trees to improve our understanding of how diverse forest ecosystems function. Leveraging an international network of tree diversity experiments, my collaborators and I investigated the form and function of trees, their environments, and how trees and their environments function together as forests. Inching toward a fuller picture, I first show that community diversity mediates light and helps to explain leaf trait plasticity. Second, I illustrate how neighbors affect the growth of trees and how these neighborhood effects may be predicted by both intrinsic and plastic trait differences among trees. Third, I demonstrate how the shapes of tree crowns fit together and plastically respond to neighbors, resulting in a measure of spatial complementarity that explains why species mixtures grow more than monocultures (i.e., overyield). And, finally, I combine empirical data with a model of canopy-level photosynthesis to quantify how structural, physiological and phenological differences among and within species each affect stand-level productivity through partitioning light in space, time and intensity. Together, this dissertation illustrates how resource-mediated interactions among plants can scale through ecosystems from trait expression within organs to the growth of individuals and the productivity of forests.enThesis or Dissertation