Less pronounced drought responses in ring-porous than in diffuse-porous temperate tree species

• Stronger VPD-driven stomatal control over sap flow in ring- vs. diffuse-porous trees. • New VPD crit index pinpoints shift from abiotic to stomatal control of transpiration. • Radial growth less drought sensitive in ring- vs. diffuse-porous trees. • Deep-rooting, ring-porous trees exhibit superior drought resistance. • Basal area of ring-porous trees likely to increase in a drier future. Tree species differ in their physiological responses to drought, but the underlying causes are often unclear. Here we explored responses of radial growth to centennial drought events and sap flow ( F s ) to seasonal drought in four mixed forests on either moist or drier sites in northwestern Switzerland. While the diffuse-porous species ( Fagus sylvatica, Prunus avium, Tilia platyphyllos ) showed marked growth reductions in 1976 and 2003, two known marker years for severe drought, growth of the two ring-porous species ( Quercus petraea and Fraxinus excelsior ) was less severely affected. During a dry early to midsummer, diffuse-porous species strongly reduced F s at the two drier sites but not (or less so) at the two moister sites. Regardless of soil moisture availability, the deep-rooting, ring-porous trees invariably down-regulated F s to 60–70% of their maxima in response to vapour pressure deficit (VPD) and maintained similar fluxes across sites, irrespective of upper soil moisture conditions. A generalised additive model of normalised F s as a function of VPD and soil matric potential yielded a drought-sensitivity ranking of F s led by the two insensitive ring-porous species followed by the diffuse-porous trees (ordered by increasing sensitivity: Fraxinus excelsior < Quercus petraea < Prunus avium < Acer pseudoplatanus < Fagus sylvatica < Tilia platyphyllos ). In conclusion, ring-porous tree species exhibited stronger VPD-driven stomatal control over F s , and tree-ring formation was less sensitive to severe drought than in their neighbouring diffuse-porous species. The F s regulation explained the greater drought tolerance of the ring-porous trees.