LAUSR

We took advantage of the European 2018 drought and assessed the mechanisms causing differences in drought vulnerability among mature individuals of nine co-occurring tree species at the Swiss Canopy Crane II site in Switzerland. We monitored leaf water status throughout the growing season and determined hydraulic vulnerability thresholds (Ψ12 -, Ψ50 - and Ψ88 -values), corresponding hydraulic safety margins (HSMs) and carbohydrate reserves for all species. In addition, we determined total average projected leaf area per tree and used stable isotopes to assess differences in root water uptake depth among the nine species. Marked differences in drought vulnerability were observed among the nine species. Six species maintained their water potentials above hydraulic thresholds associated with critical levels of embolism formation, while three species F. sylvatica, C. betulus, and P. abies were pushed beyond their hydraulic thresholds and showed loss of hydraulic conductivity. Embolism resistance thresholds and associated HSMs did not explain why the co-existing species differed in their drought vulnerability, neither did their degree of isohydry nor their regulation of carbohydrate reserves. Instead, differences in structural-morphological traits, in particular root water uptake depth, were associated with the risk of embolism formation among the nine species. Our study shows that lab-based hydraulic thresholds are valid tools for predicting the water potential thresholds beyond which the hydraulic system is at risk. However, structural-morphological traits such as root water uptake depth determine how quickly different species approach these thresholds during a drought event and can thus explain species differences in drought vulnerability among mature field-grown trees.