Hydraulic failure is one of the main causes of tree mortality in conditions of severe drought. Resistance to cavitation is known to be strongly related to drought tolerance and species survival in conifers, but the threshold of water-stress-induced embolism leading to catastrophic xylem dysfunction in angiosperms has been little studied. We investigated the link between drought tolerance, survival and xylem cavitation resistance in five angiosperm tree species known to have contrasting desiccation resistance thresholds. We exposed seedlings in a greenhouse to severe drought to generate extreme water stress. We monitored leaf water potential, total plant water loss rate, leaf transpiration, stomatal conductance and CO2 assimilation rate during drought exposure and after rewatering (recovery phase). The time required for the recovery of 50% of the maximum value of a given ecophysiological variable after rewatering was used to determine the critical water potential corresponding to the threshold beyond which the plant failed to recover. We also investigated the relationship between this potential and stem xylem cavitation resistance, as assessed from vulnerability curves. This minimum recoverable water potential was consistent between ecophysiological variables and varied considerably between species, from -3.4 to -6.0 MPa. This minimum recoverable water potential was strongly correlated with P50 and P88, the pressures inducing 50 and 88% losses of stem hydraulic conductance, respectively. Moreover, the embolism threshold leading to irreversible drought damage was found to be close to 88%, rather than the 50% previously reported for conifers. Hydraulic failure leading to irreversible drought-induced global dysfunction in angiosperm tree species occurred at a very high level of xylem embolism, possibly reflecting the physiological characteristics of their stem water-transport system.

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.

Questions The objectives of this study were to examine altitudinal shifts in tree species distributions over one decade to quantify the potential for tree migration. Location Spain. Methods We analysed presence–absence data using two successive surveys of the Spanish Forest Inventory in five Fagaceae tree species (two temperate: Fagus sylvatica and Quercus petraea, one sub‐Mediterranean: Q. faginea and two Mediterranean: Q. suber and Q. ilex) in two mountain ranges (the Pyrenees and the Iberian system). Half of the fitted altitudinal distributions were skewed and required use of an asymmetric model for unbiased estimates of optimum altitude and changes in the probability of presence along the altitudinal gradient. For each species and mountain range, shifts were considered to have occurred when the difference in optimum altitude was significant or when differences in probability of occurrence between the two surveys demonstrated the occurrence of colonization or extirpation events. Results Overall, depending on species and mountain range, shifts in optimum altitude ranged between −34 m and +181 m. The altitudinal distribution of the Mediterranean species at the core of their latitudinal distribution range presented no sign of change. For the temperate and sub‐Mediterranean Fagaceae species, the patterns demonstrated the existence of distribution changes over a 10‐yr period. The largest, although not statistically significant, upward shift in optimum altitude was observed for Q. petraea in the Iberian system. More interestingly, its distribution indicated colonization events at higher altitudes. For Q. faginea in the Pyrenees, the shift in optimum altitude was the second largest and statistically significant, and was associated with large extirpation events at the lower altitudes. No evidence of shifts was observed for F. sylvatica. Conclusion This work demonstrates that changes in altitudinal distribution could occur over a 10‐yr time period for tree species located at the southern limit of their distribution, such as some temperate and sub‐Mediterranean oaks, whereas no movement was detected for Mediterranean oaks in the core of their distribution area.