Utilizing Drone- and Ground-based Sensors, and Vegetation Samples to Calibrate Coyote Brush Phenological Responses to Water Deficit

As water deficit becomes an increasingly prominent aspect of hydrologic regimes in California, research on native plant responses to limited water availability will help inform our understanding of the consequences imposed by drought conditions. Coyote brush (Baccharis pilularis) is a drought tolerant perennial shrub that dominates California chaparral ecosystems, but it can quickly become desiccated and flammable when separated from its water sources. Despite its importance, little is known about the spatiotemporal responses of coyote brush to altered hydrologic regimes. To better understand ecohydrological plant-water relations of a widespread native California shrub, our study quantified physiological threshold responses to water deficiency in coyote brush plots at the Blue Oak Ranch Reserve in north-central California. Over the course of a week, several treatments were applied to plots to explore spectral, chemical, and physiological responses to desiccation and saturation. Treatments included cutting one coyote brush plot every 48 hours for a total of 4 plots, watering one plot an additional 4 gallons every 48 hours, and leaving one plot as a control. Following the treatments, this study employed ground-based moisture sensors, infrared and thermal drone imagery, and chemical analysis of vegetation samples to quantify water content, water potential, chlorophyll and carotenoid content, carbon and nitrogen content, and stable isotope composition. Results demonstrated that coyote brush had virtually undetectable water potential after 72 hours of being cut from its water supply. Drone-based imagery indicate that as water content and water potential declined, the spectral reflectance also changed, as expected. Taken together, these results signify that plant water status can be assessed and calibrated remotely. This study is the first to our knowledge to combine ground-based measurements with drone-based multispectral approaches to understand patterns of water deficiency in a common native California shrub. Findings from this study will provide land managers with new methodologies to monitor water availability via "spectral plant phenology and physiology" (SP³) and inform our understanding of the underlying mechanisms of changes in plant-water relations in drought tolerant species.