Deciphering tree drought responses across species: linking leaf water potentials with remote sensing greenness and photoprotection dynamics

Monitoring forest drought stress requires indicators that capture tree water relations across species and scales. Remote sensing enables large-scale assessment of drought vulnerability, but species-specific water and light use strategies complicate data interpretation, underscoring the need for mechanistic insights into remotely sensed signals in mature trees. We investigated drought responses of seven common European tree species (Abies alba, Picea abies, Pinus sylvestris, Acer pseudoplatanus, Fagus sylvatica, Carpinus betulus, Quercus sp.) at a temperate forest throughfall exclusion site during the 2023 peak growing season, integrating drone-based multispectral imagery with measurements of leaf water potential, turgor loss point, and leaf pigments. Our goal was to assess whether drone-derived greenness and photoprotection indicators capture species-specific variation in tree water status and contribute to a mechanistic interpretation of remote sensing signals over seasonal and diurnal timescales. We found that the photochemical reflectance index (PRI) strongly correlated with leaf water potentials, capturing both drought-induced declines and post-rainfall recovery, while the normalized difference vegetation index (NDVI) mostly detected greening losses in A. pseudoplatanus, F. sylvatica, C. betulus, but failed to reflect recovery. A model combining NDVI-derived greenness and PRI-derived photoprotection accounted for 65–70 % of the variance in leaf water potential dynamics across the site, particularly at midday as a function of species-specific stomatal control. We further found that species experiencing higher stress on their hydraulic system (i.e., lower water potentials) and characterized by lower drought tolerance based on their climatic distributions, generally showed higher engagement of their xanthophyll cycle. This was reflected in higher photoprotection activation rates in the morning (PRI_rate) and wider daily operating ranges (PRI_range), driven by diffusional and non-diffusional limitations on photosynthesis. By integrating hydraulic and photoprotective functioning, this study highlights both the insights gained and the inherent complexity in explaining interspecific differences in drought vulnerability, underscoring the potential to refine early-warning systems and enable species-specific drought monitoring.