Leaky taps in trees: exploring leaf minimum water conductance in temperate European trees

With ongoing climate change, summer droughts accompanied by hot temperatures (hereafter referred as hot droughts) are emerging as one of the most critical challenges to maintaining forest health. Therefore, a nuanced understanding of the underlying physiological mechanisms behind the critical responses of trees to hot droughts is fundamental for accurate prediction of tree mortality and better forest management. Under severe hot droughts, the minimal water use of a tree after full stomatal closure will directly affect the surviving time of the tree. Yet, this minimal water use has not been quantified, mainly because leaf minimum water conductance (gmin) varies considerably between and within species and exhibit great plasticity potential. In this thesis, we conducted a comprehensive survey of the variability of gmin in nine ecologically and economically important European temperate tree species using a new research tool, the Drought Box. Our findings show that gmin varied considerably among tree species but displayed a similar range of values overall. Based on our data, interspecies variation of gmin was associated with leaf cuticular and stomatal traits, but the cause of intraspecific variation is unclear. At the same time, we have revealed the remarkable acclimation of gmin to soil water availability in deciduous broadleaf tree species but not in evergreen conifers. These findings have important implications for European forest management. We further show that the seasonal variation of gmin was very limited in the studied species and, thus, unlikely to cause an ecologically significant change in tree drought tolerance. At the same time, our findings also highlight the remarkable response of gmin to increasing temperatures, providing novel insights into the mechanisms underlying rapid tree dieback under hot droughts. Finally, we confirm that even when stomates are fully closed during a hot drought, different tree species need substantial amounts of water to remain hydrated. Through in-depth studies in three chapters, we conclude that gmin display a large amount of variation among tree species and strong acclimation to the changes in soil water availability and temperature. Accordingly, tree residual transpiration, mainly dependent on gmin, may further amplify differences in drought vulnerability between tree species, and simplified modelling of residual transpiration currently appears to be challenging.