Dynamic Simulation of Transpiration and Water Use Efficiency in Apple Tree Canopies

We developed a model that couples canopy stomatal conductance, transpiration and canopy Water Use Efficiency (WUE) in an apple orchard (Malus pumila Mill. 'Fuji'). The model used the Penman-Monteith equation to compute effects of the interaction between canopy transpiration rate (Tr) and microclimatic factors. For convenience, WUE was expressed as the ratio of photosynthesis to transpiration rate; we simulated the relationship between WUE and microclimatic factors. Models developed were tested against observational data from an apple orchard (latitude 40° 13' N, longitude 116° 13′ E, altitude 79 m). The models and parameters well simulated Tr and WUE in apple trees. The mathematical simulation showed strong interactions among various microclimatic factors and indicated that transpiration was driven mainly by Vapor Pressure Deficit (VPD) and stomatal conductance. During the course of 1 day, transpiration increased (decreased) as net radiation and stomatal conductance increased (decreased). Maximum transpiration rates of experimental trees (leaf area index, LAI = 2.53) were about 8 mmol/m2/s on clear days and about 3 mmol/m2/s on cloudy days. Over 24 h, an apple tree (leaf area = 37.95 m-2) lost 50 to 70 L of water in clear weather and about 15 L in cloudy weather. On clear days, the peak of WUE was reached early after sunrise; it then decreased to a stable level maintained through most of the day until falling to zero at sunset.