TRANSPIRATION OF WELL-WATERED AND WATER-STRESSED ASIAN PEAR TREES AS DETERMINED BY LYSIMETRY, HEAT-PULSE, AND ESTIMATED BY A PENMAN-MONTEITH MODEL

Early season transpiration of 5-year-old Pyrus serotina trees receiving different amounts of irrigation was measured using lysimetry and the compensation heat-pulse technique. Withholding water resulted in a rapid decline in soil-water content within the lysimeters, and caused a 50% decrease in stomatal conductance and a 25-35% reduction in tree water use. Weekly transpiration rates, as determined by integrated heat-pulse measurements, were comparable with measurements by lysimetry for a well-watered tree, but overestimated water use by a water-stressed tree by 22%. The reason for this discrepancy was thought to be diurnal changes in sapwood water content, leading to an overestimation by the heat-pulse technique. Stomatal conductance and meteorological data obtained over 2 days were used to parameterize a Penman-Monteith model to estimate tree transpiration. On a daily basis, these estimates were within 8% of the sap flow determined by the heat-pulse technique. Substantial sap flow occurred in the stressed tree at night even when the stomata were closed. Obviously, this night-time sap flow could not be predicted by the Penman-Monteith model. Instead, it appeared to be a capacitance effect owing to the nocturnal refilling of the tree's internal store of water.