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 (Ψ50) and the seasonal dynamics of water and osmotic potentials. The leaf water potentials (Ψ1) of all species declined significantly along the summer, reaching significantly lower Ψl at the end of summer in the drier sites. Surprisingly, we did not find plasticity along the drought gradient in Ψ50 or osmotic potentials. This resulted in much narrower hydraulic safety margins (HSM) 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 geographic dry edge of their distribution.

The PLC curves and resistance to embolism were measured using the Cavitron. The pre-dawn and midday water potentials were measured using a pressure bomb. The C13 was measured with a 13C cavity ring-down analyzer. The osmotic potential was measured using an osmometer. All methods are described in Alon et al., Acclimation limits for embolism resistance and osmotic adjustment accompany the geographic dry edge of Mediterranean species. 2023. Functional Ecology

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