University of California, Santa Barbara, United States
Abstract: Climate change is altering both the supply of and demand for water in ecosystems across the globe. How ecosystems respond to climate-induced water stress will depend on how individual plants regulate water fluxes under changing moisture conditions. Plants employ different water use strategies across species and functional types, and even closely related and coexisting species exhibit a range of responses to water limitation. Knowledge of the differential rates of water uptake at the individual plant level under varying moisture conditions can facilitate prediction of how ecosystem structure and function might shift in response to climate-induced water stress, yet critical measurements of vegetation water use and indicators of tree water stress are lacking.
Here, we characterize individual tree responses to fluctuating water availability and atmospheric demand in a dryland riparian woodland using a novel, drone-based approach for estimating transpiration. Integrating thermal imagery, structural data, and a suite of environmental sensors mounted on an unmanned aerial vehicle (UAV) platform, this approach was designed to capture fine-scale functional data and variation in individual-level plant water use and allows for efficient calculation of evapotranspiration (ET) for a site solely using data collected from the UAV. Using UAV-based measurements of transpiration across seasonal, diurnal, and spatial gradients of water stress, we quantified individual-scale hydraulic sensitivity to water availability and vapor pressure deficit (VPD). Water-stressed tress showed peak ET at midday and early summer, decreasing into afternoon and later in the dry season, reflecting down-regulation of photosynthesis to preserve hydraulic function and response declining plant-available water, respectively. Drought-sensitive species showed greater sensitivity to increasing VPD than drought-tolerant species, particularly during the dry season.
These results reveal plant functional responses well before any structural changes have occurred, providing early indicators of when, where, and which species are most vulnerable to water stress. Such insights provide a functional understanding of the vulnerability and resilience of ecosystems to water stress in the face of global environmental change.
