SAP FLOW IN TREMBLING ASPEN - IMPLICATIONS FOR STOMATAL RESPONSES TO VAPOR-PRESSURE DEFICIT

We monitored sap flow and estimated diurnal changes in transpiration r ates of two trembling aspen (Populus tremuloides Michx.) stands, locat ed in the southern boreal forest and aspen parkland of Saskatchewan, C anada. In both stands during the peak growing season (June and July), sap flow during the day (0700-1700 h local time) increased linearly wi th vapor pressure deficit (VPD) from 0 to about 1 kPa, but then remain ed remarkably constant at VPD > 1 kPa (up to 4.8 kPa in the aspen park land stand). The results imply an inverse relationship between stomata l conductance and VPD under well mixed conditions, for VPD > 1 kPa. We postulate that the stomata are operating to maintain leaf water poten tials above a critical minimum value, which in turn places a maximum v alue on the rate of sap flow that can be sustained.