3) Mr. Yannik Müllers

Severe droughts, and other processes like freezing or insect feeding can lead to the blockage of xylem vessels and thereby compromise water flow and leaf gas exchange of trees. Bypassing blocked vessels through lateral pathways is crucial to minimizing those impairments. Here, we investigate the hydraulic pathways used for bypassing blocked vessels in two Mediterranean angiosperm tree species, carob (Ceratonia siliqua) and oak (Quercus calliprinos). Using stem notching, we artificially induced xylem blockage in one half of the stem cross section and measured the resulting drop in stomatal conductance (g_s) of leaves above the notches. While g_s in carobs dropped to close to zero, it was kept at more than 50% in oaks, indicating more efficient bypassing. Visualizing hydraulic pathways using microscopy and µCT imaging after dye or tracer injection respectively, revealed that water flow in the notched half of the stem had ceased in carobs, but not in oaks. The remaining water flow in oaks was outside vessels, probably through tracheids with bordered pits which we found in the xylem of oaks but not carobs. Our results suggest that vessel-bearing species with additional tracheids can more efficiently bypass partially blocked xylem and thereby secure leaf water supply. This may be an advantage for trees growing under conditions triggering vessel blockage, such as severe droughts.