Remotely sensed photoprotection reveals drought stress across contrasting mature temperate tree species and functional types subjected to precipitation manipulation

Monitoring forest drought stress requires indicators able to explain tree water relations across different species, scales, and functional types. While changes detected by remote sensing indicators of vegetation greenness often represent drought legacies, i.e., lagged responses of trees occurring in the years following a severe drought, measurements of tree function can capture drought signals in real-time. Among these is the photochemical reflectance index (PRI) which detects changes in xanthophyll cycle pigment dynamics. This process reflects increases in photoprotective non-photochemical quenching activity resulting from drought-induced photosynthesis downregulation. However, the complexity of species-specific light and water resource use strategies over short and long timescales, challenges interpretation of this index across species and raises the need for a more mechanistic understanding of remote sensing signals under water limitations.

We combined drone-based multispectral imagery with measurements of tree water relations, pigments, and environmental parameters over one summer in an old-growth mixed forest subject to precipitation manipulation. Our goal was to assess the potential of remote sensing indicators of greenness (Normalized Difference Vegetation Index; NDVI) and photoprotection dynamics (PRI) to track variations in tree hydraulic traits (predawn leaf water potential, Ψ_{leaf_pd}; minimum tree water deficit, TWD_min) for seven common European tree species (Abies alba, Picea abies, Pinus sylvestris, Acer pseudoplatanus, Fagus sylvatica, Carpinus betulus, Quercus sp.).

We found NDVI-greenness captured irreversible crown defoliation and browning for the two most drought-stressed deciduous species. In contrast, PRI revealed a strengthening of xanthophyll-cycle induced thermal dissipation during drought, followed by the downregulation of photoprotection activity upon soil water replenishment, for all species and functional types and in agreement with dynamics of measured tree hydraulic traits. The combination of both remote sensing indices and tree height performed best in explaining tree water relations at our site, evidencing the importance of legacy effects captured by greenness in addition to short-term light utilization requirements revealed by PRI. Species-specific diurnal PRI-light response curves, obtained from drone flights conducted at various times of the day, were further analysed to derive metrics aimed at isolating the xanthophyll cycle response (facultative PRI response) from pigment pool size effects (constitutive response), enhancing the comparability across different species and functional types. We found species experiencing greater drought stress mostly exhibited higher photoprotection rates early in the day and a broader operational range of photoprotective systems throughout the day.

This study demonstrates the potential of PRI for drought stress monitoring in mixed forest sites, yet it also underscored the need for further investigations of the combined impact of water and light stress to develop robust cross-species drought monitoring approaches based on remote sensing signals.