Regulation of hydraulic function and consequences for drought-related mortality in mature woody plants

Precipitation and temperature patterns are expected to shift over at least the next century due to anthropogenic climate change. Effects of these shifts may be felt most acutely in arid and semi-arid regions where water is already the most limiting resource. Vegetation mortality due to drought is a likely outcome of predicted future climate, with consequences for ecosystem structure and function and feedbacks on regional and global climate. Yet, mechanisms and interactions of key plant drought responses and the end-point of mortality remain poorly understood. In this dissertation I examine the roles of xylem cavitation resistance and stomatal control of water loss in enhancing the susceptibility of species to drought-related mortality. I focused first on Pinus edulis (piñon) and Juniperus monosperma (juniper) woodlands in Southwestern United States. In Chapter 2, I used a hydraulic model to show that piñon mortality under experimental drought occurred in the absence of complete hydraulic failure, but under conditions of chronically low hydraulic conductance and transpiration. Hydraulic limitations, carbon starvation, and pathogen activity were likely all involved in this mortality. In Chapter 3, utilizing the same rainfall manipulation experiment, I showed that prolonged drought reduced both species’ ability to respond to the pulsed precipitation characteristic of semi-arid ecosystems. Transpiration was lower in the drought treatment than in the controls even after differences in pulse size and soil moisture were taken into account, suggesting physiological changes within the trees over the timescale of the experiment. In Chapter 4, I examined the relationship between leaf water potential regulation strategy and xylem cavitation vulnerability as a driver of drought-related mortality, using a global dataset drawn from the literature. Rather than simple classification as either isohydric or anisohydric, I found species to be evenly distributed along a continuum of leaf water potential regulation. Conifers maintained a lower loss of hydraulic conductance in their branches but a greater proportion had experienced drought-related mortality, relative to angiosperms. Together, this research shows that hydraulic constraints may have an important role in carbon starvation leading to drought-related mortality, while highlighting the need for further research on carbon reserves, mobility, and allocation during drought.National Science Foundation's Graduate Research Fellowship Program, Department of Energy's Office of Science, Sevilleta Long Term Ecological Research Site, and the University of New Mexico's Department of BiologyBiologyDoctoralUniversity of New Mexico. Biology Dept.Pockman, WilliamLitvak, MarcyMcDowell, NathanSperry, Joh