Acclimation limits for embolism resistance and osmotic adjustment accompany the geographical dry edge of Mediterranean species

Survival and growth of woody species in the Mediterranean are mainly restricted by water availability. We tested the hypothesis that Mediterranean species acclimate their xylem vulnerability and osmotic potential along a precipitation gradient. We studied five predominant co-occurring Mediterranean species; Quercus calliprinos, Pistacia palaestina, Pistacia lentiscus, Rhamnus lycioides and Phillyrea latifolia, over two summers at three sites. The driest of the sites is the distribution edge for all the five species. We measured key hydraulic and osmotic traits related to drought resistance, including resistance to embolism (Psi(50)) and the seasonal dynamics of water and osmotic potentials. The leaf water potentials (Psi(l)) of all species declined significantly along the summer, reaching significantly lower Psi(l) at the end of summer in the drier sites. Surprisingly, we did not find plasticity along the drought gradient in Psi(50) or osmotic potentials. This resulted in much narrower hydraulic safety margins (HSMs) in the drier sites, where some species experienced significant embolism. Our analysis indicates that reduction in HSM to null values put Mediterranean species in embolism risk as they approach their hydraulic limit near the geographical dry edge of their distribution.

Automated summary

Survival and growth of woody species in the Mediterranean are mainly limited by water availability. We tested the hypothesis that Mediterranean species adjust their xylem vulnerability and osmotic potential along a precipitation gradient. Our study on five predominant Mediterranean species showed that leaf water potentials declined significantly along the summer, reaching lower levels in the driest sites. Surprisingly, we did not find plasticity in resistance to embolism or osmotic potentials along the drought gradient, leading to narrower hydraulic safety margins and embolism risk in drier sites.