Remotely sensing drought stress in Q. douglasii is complicated by the unstable relationship between drought status and water content

Remote sensing is a powerful tool for ecosystem monitoring, yet has been limited by its ability to capture early warning metrics of tree drought stress. The foundational measurement of tree physiological water status, water potential, has not been well-linked to remotely sensed products that estimate the amount of water in tree canopies. To test remote sensing’s ability to capture changes in tree water status, we investigated the stability of relationships between water potential and area- and mass-based metrics of water content (measured in-situ and via airborne hyperspectral sensors) over diel, seasonal, and spatial variation. We assess the utility of ‘space for time’ and laboratory-based approaches for establishing relationships among in-situ and remotely sensed water status metrics. We found strong and consistent relationships within late-season temporal and spatial sources of variation, indicating that water content can be linked to water potential when leaf morphology is known (i.e. when water content is represented on a mass-basis). However, inconsistency across sources of variation did not support space-for-time approaches. Laboratory-based relationships only accurately reflected diel patterns, not those observed across the landscape or over time; as such, classic benchtop drydown methods do accurately describe leaf capacitance but are not good proxies for estimating larger-scale patterns of tree stress. Taken together, our findings highlight how covariation between water status and leaf morphology act as confounding physiological processes that can mask drought-response signals in area-based remote sensing products, and highlight the importance of fully characterizing drought stress signals across differing axes of variation.