UC Santa Barbara

Climate change is altering both the supply of and demand for water in ecosystems across the globe. Despite the importance of understanding how ongoing changes in water availability will impact ecosystems, critical measures of tree water use and indicators of tree water stress are lacking. This dissertation uses novel tools and theories to characterize the dynamics of plant water use at multiple scales. First, two novel approaches are presented for estimating evapotranspiration at fine scales using thermal imagery and a suite of micrometeorological sensors mounted on an unmanned aerial vehicle (UAV) platform. In a comparison of UAV retrievals and eddy covariance data across 50 UAV flights, integrating atmospheric profiles of heat and moisture into a surface energy balance algorithm is shown to reduce potential uncertainty in evapotranspiration estimates by up to 50%. Next, this approach is implemented in a desert riparian ecosystem where trees are subject to significant water and temperature stress. Retrievals of canopy temperature, conductance, and transpiration are used to characterize how individual trees co-regulate water use and temperature in response to changing water supply and demand. Desert trees have different responses to supply- and demand-driven water stress, but similar responses to thermal stress. This work reveals that under most conditions, plants must make tradeoffs between hydraulic function and avoiding thermal stress, but when both supply and demand are high, regulation of water use and canopy temperature can become decoupled. Finally, soil moisture dynamics derived from a global soil moisture dataset are used to characterize plant responses to water limitation and examine the sensitivity of these responses to environmental conditions. Using a novel model to describe plant water use as a function of soil moisture, plant responses are compared by vegetation type, productivity, and antecedent aridity. Grasslands show more aggressive water use patterns and greater sensitivity to ecological and hydrological competition than woody vegetation. As climate change scenarios project an increase in the frequency and severity of droughts, studying plant adaptations to water limitation provides insights into the responses we may expect to see across a range of ecosystems. Functional understanding of the vulnerability and resilience of plants to water stress will facilitate predictions of how ecosystems might respond to warmer, drier, more variable conditions.