Structural and anatomical responses of Pinus sylvestris and Tilia platyphyllos seedlings exposed to water shortage

Phenomena of tree decline and mortality are increasing worldwide as a consequence of the higher temperatures accompanying drought events. Studying changes in biomass allocation and xylem anatomy may shed light on the relative importance that C and water impairment have during drought, and can help to better understand how plants will respond to future droughts. We measured the dry weight of the leaf, aboveground and belowground xylem biomass in tree seedlings of the drought-avoidant Pinus sylvestris and drought-tolerant Tilia platyphyllos exposed to different intensities of water shortage. Moreover, the area of vessels was measured at three positions along the stem. In P. sylvestris, we found no differences in total biomass across treatments, but a preferential allocation to needle mass under drought, while there were no differences in xylem anatomy. Tilia platyphyllos under ambient and mild drought increased leaf and total xylem biomass according to an isometric pattern, whereas the largest vessels near the stem apex were found in seedlings under severe drought. Our results suggest a categorisation of the two species regarding the coordination of carbon and hydraulic economies. Pinus sylvestris invests relatively more into leaf biomass to increase photosynthesis and thus decreases the risk of carbon starvation, while T. platyphyllos invests more into hydraulic efficiency to decrease the risks of embolisation.