The high vulnerability of Quercus robur to drought at its southern margin paves the way for Quercus ilex

Populations growing at the warm margins of the species' range are more prone to experience higher water stress compared to populations inhabiting the core of their distribution. Thus, assessing tree vulnerability to drought is crucial to improve prediction of forest mortality and species range limits. We quantified the abundance of two oak species (Quercus robur and Quercus ilex) along a water stress gradient in a coastal forest located at the southern edge of the distribution of Q. robur. We assessed their ecophysiological responses to drought during a wet and a dry year and determined their vulnerability to drought under field conditions. The abundance of Q. ilex was high all along the water stress gradient, whereas the abundance of Q. robur dramatically declined with decreasing water availability. During dry years, the level of native embolism was significantly higher for Q. robur than for Q. ilex due to species differences in vulnerability to xylem cavitation. Q. robur had a narrower hydraulic safety margin than Q. ilex and operated very close to the species threshold of hydraulic failure, making it highly vulnerable to drought-induced mortality. In the current context of increasing drought frequency and severity, survival of Q. robur populations will be threatened at warm range margins.