Selecting temperate urban trees for resistance to drought: Relevance of leaf turgor loss point

Source abstract

Urban trees increasingly experience drought conditions amplified by climate change and the urban heat island effect. Qualitative descriptions of drought tolerance are common, yet quantitative metrics such as leaf water potential at turgor loss point (Ѱ TLP ) offer greater precision for characterizing and comparing species drought responses. We measured leaf osmotic potential and calculated leaf Ѱ TLP in 22 urban tree species. We then integrated these measurements into an additional dataset encompassing 150 woody species to facilitate interspecific comparisons of potential drought performance. Finally, we compared the leaf Ѱ TLP with drought tolerance categorizations from widely available tree selection guides. Large variation in leaf Ѱ TLP was observed among species and genera. Some genera such as Quercus clustered primarily towards moderate drought tolerance classifications while others like Magnolia were clustered around more sensitive classifications. In contrast, the genus Acer demonstrated high interspecific variability spanning nearly two-thirds of the observed Ѱ TLP range. When comparing qualitative drought tolerance designations, we observed many similarities across data sources; however, we also observed inconsistencies across these sources or between these designations and our Ѱ TLP observations. Notable deviations (≥ 2 drought designations) occurred for roughly half of the 23 most commonly planted trees in the Chicago region and nearly 20% of the larger set of temperate trees. This highlights the persistent uncertainty and need for further study in tree drought responses, particularly future research that can integrate leaf Ѱ TLP measurements into a whole-plant approach that also accounts for other tree adaptations to mitigate drought stress including stomatal behavior, xylem vulnerability to embolism, and belowground rooting strategies.