Relating plant capacitance to the water relations of Pinus contorta

Internal water storage of Pinus contorta Dougl. ex. Laud. (lodgepole pine) was estimated by both calculation and measurement. Readily available water from internal storage was calculated to be capable of providing 0.6 h of maximum transpiration. This estimate was substantiated by direct measurement. Needle water content contributed only 4% of total storage, but had an important role in stomatal response through the control of leaf conductance by leaf water potential. Leaf water potential was highly correlated with needle water content when measurements were taken under low transpiration conditions which minimized the flow-related component of water potential. When trees were cut and allowed to dry upright, the needles retained water and a stable leaf water potential for up to 33 days as the stem sapwood was depleted. Sapwood water content of intact trees remained fairly constant at 60-70% of saturation throughout the summer.