Transpiration of apple trees in a humid climate using heat pulse sap flow gauges calibrated with whole-canopy gas exchange chambers

For scientific study and practical irrigation management, the estimate of plant transpiration and water use rates is needed. Several methods are available to measure water use by plants, and among them, sap flow estimation techniques are currently popular, although the interpretation of data may be complex due to several assumptions. In this study, we present a combined approach to measure transpiration rates in mature, field-grown dwarf apple trees ('Empire'/M.9) in a humid climate. Sap flow gauges (heat pulse method) were used to estimate sap flow rates on apples, over a period of 10 weeks. The gauges were first calibrated with simultaneous measurements of whole-canopy gas exchange using chambers mounted on the same plants for short periods (5 days). Mid-summer transpiration rates were peaked at about 2.5 l day−1 m−2 of leaf area but generally averaged about 2 l day−1 m−2 of leaf area. Calculated basal crop coefficients were generally lower than those reported in literature, variable and were a function of the vapor pressure deficit, thus confirming the concerns of using crop coefficients with reference grass in humid climates especially when applied to tall, discontinuous canopies that are well coupled with the bulk air.