Variable conductivity and embolism in roots and branches of four contrasting tree species and their impacts on whole-plant hydraulic performance under future atmospheric CO₂ concentration.

Anatomical and physiological acclimation to water stress of the tree hydraulic system involves trade-offs between maintenance of stomatal conductance and loss of hydraulic conductivity, with short-term impacts on photosynthesis and long-term consequences to survival and growth. Here, we study the role of variations in root and branch maximum hydraulic specific conductivity (k(s-max)) under high and low soil moisture in determining whole-tree hydraulic conductance (K(tree)) and in mediating stomatal control of gas exchange in four contrasting tree species growing under ambient and elevated CO₂ (CO₂(a) and CO₂(e)). We hypothesized that K(tree) would adjust to CO₂(e) through an increase in root and branch k(s-max) in response to anatomical adjustments. However, physiological changes observed under CO₂(e) were not clearly related to structural change in the xylem of any of the species. The only large effect of CO₂(e) occurred in branches of Liquidambar styraciflua L. and Cornus florida L. where an increase in k(s-max) and a decrease in xylem resistance to embolism (-P₅₀) were measured. Across species, embolism in roots explained the loss of K(tree) and therefore indirectly constituted a hydraulic signal involved in stomatal regulation and in the reduction of G(s-ref), the sap-flux-scaled mean canopy stomatal conductance at a reference vapour pressure deficit of 1 kPa. Across roots and branches, the increase in k(s-max) was associated with a decrease in -P₅₀, a consequence of structural acclimation such as larger conduits, lower pit resistance and lower wood density. Across species, treatment-induced changes in K(tree) translated to similar variation in G(s-ref). However, the relationship between G(s-ref) and K(tree) under CO₂(a) was steeper than under CO₂(e), indicating that CO₂(e) trees have lower G(s-ref) at a given K(tree) than CO₂(a) trees. Under high soil moisture, CO₂(e) greatly reduced G(s-ref). Under low soil moisture, CO₂(e) reduced G(s-ref) of only L. styraciflua and Ulmus alata. In some species, higher xylem dysfunction under CO₂(e) might impact tree performance in a future climate when increased evaporative demand could cause a greater loss of hydraulic function. The results contributed to our knowledge of the physiological and anatomical mechanisms underpinning the responses of tree species to drought and more generally to global change.