UC Berkeley

As plants take in carbon dioxide through their stomata, they move large amounts of water from the soil, through their vascular system, and into the atmosphere through the process of transpiration. Transpiration makes plants a major mediator in the movement and storage of water as part of the global water cycle. The coupling of water and carbon fluxes at plant stomata also has major implications for ecosystem function, climate change, and other environmental challenges. The fluxes of water and carbon, and their variation, are a function of both environmental conditions and the physiology of the plant. Despite the complexity of the interactions between plants and their environment and the important role of plant-environment interactions in the Earth system, existing paradigms in hydrology and ecology tend to characterize plant water use through either a purely physical or a purely biological lens, even though such approaches have notable limitations and failure modes. Advancing our understanding of the ways in which plant physiology interacts with the environment can help us to better predict plant fluxes, how they might change in the future, and how to leverage these dynamics to better manage environmental challenges.

This dissertation explores the interdependence of biological and physical systems in the context of plant health and plant water use outcomes. The explorations are done through a series of ecohydrological modeling studies and experiments that examine the interplay of plant physiology and the physical (hydrological) environment for different applications. The first two chapters explore how incorporating consideration of plant-water interactions into management practices for different environmental challenges can improve design outcomes. In the first, I consider the modeling of plant pathogen spread, a problem that is typically considered through the lens of spatial ecology. By introducing both an ecohydrological model and a surface runoff transport model to the issue of pathogen spread, I demonstrate the importance of a novel transport mechanism for pathogen spread and make recommendations for improving management. In the next study, I explore how plant-water fluxes can be utilized in landscape restoration designs to limit the potential for contaminant mobilization from former mining and landfill sites. I develop and use a model that describes species-specific physiological responses to water stress through a plant hydraulics framework in order to assess how system designs can limit both plant hydraulic risk and the risk of contaminant mobilization. The final two studies address more fundamental questions of how plant hydraulic properties influence plant function and a plants' hydrologic role. In the third study, I model within-plant hydraulic transport and test hypotheses about how variation in traits between plant tissues impacts plant function during water stress. With the model, I demonstrate how observed traits that appear to contradict a longstanding hypothesis in plant hydraulics, could indicate an alternative type of drought response with implications for water fluxes and plant survival. In the final study, I conduct an experiment that intensively monitors a number of physiological, hydrological, and meteorological metrics along with changes in plant fluxes over the course of a drydown period. By addressing the need for concurrent and high-resolution measurements of physiological and environmental variables, the experimental data provide insight into the coordination of environmental and plant dynamics, as well as an important resource for model development.

Overall, this dissertation provides new insights and data into how plant physiology interacts with the environment to determine water and carbon fluxes. The roles of these dynamics within management scenarios are elucidated, and recommendations made for how to incorporate these dynamics to create more effective plans and practices. Understanding of these ecohydrological dynamics will be critical for predicting the effects of a changing environment and responding to the impacts.