<p>Vulnerability curves to cavitation (VC) and their derived parameters, such as <em>P₅₀</em>, are increasingly used and reported to assess forest vulnerability to drought and predict forest responses to climate change. Forest practitioners and policy-makers are encouraged to rely on these parameters to support species selection based on their sensitivity to drought. However, in the majority of studies, we consider that the variability of VC parameters is not clearly reported nor considered, which can lead to counterproductive decisions. In this opinion paper, we demonstrate the importance of precisely reporting the variability around VC parameters and the sources of this variability (plant materials, methods, etc.). We also identify the information that should be provided when reporting mean values of VC parameters. To support our argument, we built VCs for three <em>Picea</em> species and <em>Pinus strobus</em>, using different methods, and compared the value of <em>P₅₀</em> determined in our experiments with values from a literature review.</p>

The future climate of northern temperate forests is projected to be drier and warmer by the end of this century. As a result, more drought-induced forest dieback events are anticipated in northeastern North America, and assessing the vulnerability of dominant tree species to drought is critical for understanding the future composition of these forests. In a greenhouse experiment, we exposed two-year-old seedlings of Picea glauca (Moench) Voss, Picea mariana (Mill.) B.S.P. and Pinus strobus L. to three future climate treatments for southern Quebec, Canada, and evaluated their mortality, growth, and foliage water status responses to soil water availability and atmospheric drought. Using a unique approach, climate treatments emulated droughts of different frequencies, durations, and intensities. Treatments closely simulated one growing season, with changes in air temperature and relative humidity every six hours and daily adjustment in the amount of water delivered to the seedlings. The three species experienced high mortality (75%) in all water-limited treatments compared to a control treatment that provided non-limiting soil moisture (0% mortality). The biomass of the seedlings that survived was 40% lower than that of control seedlings. Our results confirmed that the hydraulic safety margins, defined as the difference between seasonal minimum water potential and xylem water potential leading to 12, 50 and 88% of hydraulic conductivity loss, were good predictors of probability of tree mortality. Therefore, hydraulic safety margins are useful functional traits that can be used to compare the vulnerability of various species to drought and then provide crucial information to practitioners and policymakers to adjust forest management to climate change. We showed that three dominant conifer species of northern temperate forests were highly vulnerable to drought in future climates. Because drought is projected to be a significant threat to forests, understanding potentially adaptive physiological responses to drought, such as hydraulic safety margins of tree seedlings, is important for predicting the response of forest regeneration and composition in warmer and drier climates.