Assessing the Relative Roles of Soil Water Potential and Atmospheric Demand on Vegetation Water Stress in Semi-arid Ecosystems

Understanding vegetation's response to soil water availability and atmospheric demand is critical for assessing the impact of drought and climate change on semi-arid ecosystems. However, limited field-based research has been conducted to assess the relative importance of these drivers on vegetation productivity. Furthermore, previous research often simplifies the assessment of soil water availability by relying on soil volumetric water content (VWC) as the primary control on plant growth. Unlike soil water potential (Ψ_soil), VWC does not account for the effects of soil texture on plant water uptake. To address these gaps, we compared remotely sensed indicators of vegetation response with field-based measurements of VWC, relative humidity, and temperature (used to calculate vapor pressure deficit (VPD)) and soil temperature from 52 rangeland sites in Montana. Soil samples were collected at each site to generate a continuous time series of soil water potentials. We utilized generalized additive models to assess the relationship between our biophysical metrics and satellite-derived estimates of vegetation productivity, including the Enhanced Vegetation Index, near-infrared reflectance vegetation index (NIRv), and solar-induced fluorescence. Results suggest that Ψ_soil is a better biophysical predictor of seasonal vegetation productivity, while VPD emerges as a secondary driver in the absence of Ψ_soil limitations. Finally, anomalies in Ψ_soil were the dominant driver of anomalies in seasonal vegetation response. These findings indicate that soil water potential is a primary control on vegetation water stress across semi-arid landscapes.