Modeling shoot water contents in high-elevation Picea rubens during winter

During the winter of 1990-1991, a meteorological tower was established at an 880-m elevation site within the spruce-fir zone on Mt. Moosilauke, New Hampshire, USA. Hourly means of air, needle and trunk temperatures, wind velocity, relative humidity and solar radiation were recorded. On a weekly basis, shoots that had elongated during the preceding growing season were collected from four red spruce (Picea rubens Sarg.) trees and their relative water contents (RWC) determined. Cuticular resistances of needles from these shoots were measured four times during the winter.Measured meteorological parameters were used in a previously developed model to simulate changes in red spruce shoot RWC during the winter. The modeled results were compared to measured shoot RWCs. The predictive power of the model was improved when it was modified to include measured values of cuticular resistance and needle and trunk temperatures. The new version of the model accurately predicted RWC from late December 1990 to the beginning of April 1991, after which spring recharge appeared to occur. We conclude that water lost from foliage was easily replaced by stored reserves and that uptake of water by the roots was not required to maintain an adequate foliar water content during the winter.