Drought stress vulnerability for mature versus young trees in a mixed temperate forest stand

Drought can affect mature and young trees differently, and many studies suggested that large trees might be more prone to drought-related damage and mortality than conspecific smaller individuals. A higher drought vulnerability for tall trees is generally assigned to increasing constraints to water transport with tree height, but a multitude of other biotic and abiotic factors can additionally contribute and modulate their drought resistance. The interrelation of these factors and its consequences for the different drought vulnerabilities between small and dominant trees is so far not well understood and awanting exploration. Within our study, we took advantage of the Swiss Canopy Crane II (SCC II) site to simultaneously measure critical physiological parameters related to drought stress, namely, pre-dawn and mid-day leaf water potentials (LWP) and stomatal conductance, of mature, ca. 30 m tall individuals and small saplings of 9 common temperate European tree species. These measurements took place throughout the growing season 2023 that was characterized by high temperatures and extended dry periods in late summer and early autumn. Independent of tree size, the increasingly drier conditions caused pre-dawn and mid-day LWP to decrease significantly along the season, with mature trees of some species showing values close or at the species-specific P50 threshold (i.e. the LWP where 50 % loss of xylem conductivity is expected). Our results show that mature trees experienced overall more negative LWP than saplings. Most of these differences could be explained by increasing hydrostatic constraints with increasing tree height. However, even after accounting for this hydrostatic effects (-0.01 MPa per meter tree height), there remained significant differences in pre-dawn and mid-day LWP between mature trees and sapling in some of the investigated species, that were likely caused by either 1) microclimatic effects (especially VPD differences between the upper canopy and the understory), 2) different abilities to refill tree-internal water stores during the night, or 3) differences in soil water access due to rooting differences. Following this first assessment, we are planning to expand our investigations in the next years to better disentangle these different factors that significantly contribute to differences in drought vulnerability with tree size in different tree species, with important implications for the future state and population dynamics of temperate forests.