Decoupling the influence of leaf and root hydraulic conductances on stomatal conductance and its sensitivity to vapour pressure deficit as soil dries in a drained loblolly pine plantation

The study examined the relationships between whole tree hydraulic conductance (K-tree) and the conductance in roots (K-root) and leaves (K-leaf) in loblolly pine trees. In addition, the role of seasonal variations in K-root and K-leaf in mediating stomatal control of transpiration and its response to vapour pressure deficit (D) as soil-dried was studied. Compared to trunk and branches, roots and leaves had the highest loss of conductivity and contributed to more than 75% of the total tree hydraulic resistance. Drought altered the partitioning of the resistance between roots and leaves. As soil moisture dropped below 50%, relative extractable water (REW), K-root declined faster than K-leaf. Although K-tree depended on soil moisture, its dynamics was tempered by the elongation of current-year needles that significantly increased K-leaf when REW was below 50%. After accounting for the effect of D on g(s), the seasonal decline in K-tree caused a 35% decrease in g(s) and in its sensitivity to D, responses that were mainly driven by K-leaf under high REW and by K-root under low REW. We conclude that not only water stress but also leaf phenology affects the coordination between K-tree and g(s) and the acclimation of trees to changing environmental conditions.