Drought survival in elm saplings: xylem structure and growth history

Summary In a greenhouse experiment, two-year-old Ulmus laevis Pall. saplings were exposed to severe drought stress till the point when half of the saplings had died. Since nearly no morphological and physiological traits were found separating the dead saplings from the surviving ones in the same experiment, the anatomical differences defining drought embolism susceptibility were expected to play a decisive role. The cross-sectional xylem structures in fine roots, stems, and petioles were analysed and tested to find anatomical traits linked to mortality, with a special focus on vessel dimensions and tree-ring growth characteristics. However, the studied petiole, stem, and fine root xylem anatomical traits only differentiated the more drought-sensitive elm saplings in a limited way. Specifically, the dead saplings had a higher petiole xylem area and a smaller median vessel lumen diameter in the stem. Nevertheless, the growth conditions during the saplings’ life histories, as reflected by the growth ring width, appear to have played an important role in drought survival. The more susceptible saplings were either those that had grown poorly or those that had grown quickly during the two years preceding the drought stress. Overall, this study reveals that the vulnerability of elm saplings to severe drought stress is largely influenced by past growth conditions, although the role of xylem structure cannot be omitted.