Scots pines colonizing the harsh environment of volcano slopes increased their hydraulic safety margin

Key message Scots pines developed for several years on volcano slopes showed a dwarf phenotype and a larger hydraulic safety margin than downslope trees, such that hydraulic safety was prioritized over growth. Abstract The ability of trees to survive and naturally regenerate under increasing drought conditions will depend on their capacity to vary key hydraulic and morphological traits that increase drought tolerance. Yet, there has been limited investigation into this variability under recurrent severe drought conditions since the establishment phase. We investigated the hydraulic and leaf trait adjustments of Scots pine trees settled in an abandoned slag quarry by comparing them across three topographic positions inducing contrasted effects on growth. We measured xylem and foliar traits to compare the water status of trees according to tree location and to evaluate the respective risk for xylem hydraulic failure using the soil–plant hydraulic model SurEau . Compared to upslope and downslope trees, slope trees exhibited lower growth, photosynthetic pigment contents, vulnerability to embolism and specific hydraulic conductivity, as well as higher water potential at turgor loss point and midday water potentials. As a consequence, slope trees showed an increase in the hydraulic safety margin and a low level of embolism, compared to downslope and to upslope trees. Additionally, these adjustments induced an increase in the time to hydraulic failure in slope trees compared to downslope and upslope trees under similar drought conditions simulated using the SurEau model. These results suggest a prioritization of hydraulic safety over growth in Scots pine developed in a harsh environment, resulting in a dwarf phenotype.