Species-specific tree desiccation risk under hot drought correlates with hydraulic traits but not with thermal tolerance

Extreme drought episodes coupled with heat waves are predicted to become more common and intense in the near future, leading to ‘hot droughts’. High air temperatures rise atmospheric vapour pressure deficit, increasing the evaporative demand and worsening the effects of reduced soil water availability, and might also cause tissue temperatures to surpass critical values leading to membrane disruption and tissue death. Hot droughts are causing massive impacts on forest ecosystems, leading to extensive canopy dieback and tree mortality on a global scale. Identifying tree species that are more tolerant to hot droughts is crucial for forest management and requires an understanding of the physiological mechanisms and related functional traits underlying species-specific vulnerability to such events and to their drivers, namely water shortage and high temperatures. We leveraged the exceptional hot drought that occurred in the Classical Karst area in summer 2022 to investigate potential correlations between species-specific patterns of tree desiccation and physiological indicators of drought tolerance (turgor loss point, Ψtlp; vulnerability to xylem embolism, Ψ50) versus thermal tolerance (critical temperature inducing 50% damage to photosynthetic efficiency, T50). Desiccation rates in nine different woody species, based on visual estimates of the health status of more than 2400 individual trees, were closely correlated to both Ψtlp and Ψ50, but independent of T50. We conclude that functional traits related to drought tolerance are better predictors of tree species vulnerability to hot drought episodes than traits associated with thermal tolerance.