Normalized tree water deficit: an automated dendrometer signal to quantify drought stress in trees.

Trees often encounter periods of low soil water availability and high vapor pressure deficit, which induce drought stress and significantly impair their physiological functioning, growth, and survival. Automated dendrometers are valuable tools for quantifying signals related to tree water status, such as tree water deficit (TWD). Despite previous attempts, a robust method to clearly identify drought stress timing and intensity from TWD is still lacking. We established a novel normalized tree water deficit (TWD) index to quantify drought stress of trees that relies on TWD and normalized maximum daily shrinkage (MDS). We validate our approach with measurements of leaf water potential, stomatal conductance (g), and critical hydraulic thresholds from 118 trees from nine European tree species at the Swiss Canopy Crane II site. TWD proved to be effective in identifying periods of decreased g and the onset of leaf turgor loss. We have found robust species-specific thresholds for TWD at which leaf turgor is lost, and the combination of TWD and MDS could be interpreted as a general index for drought stress. Deriving TWD and MDS solely from automated dendrometer data provides a unique biological drought stress signal that complements meteorological, hydrological, and satellite-borne drought indicators.