Dynamic simulation of canopy transpiration in apple tree

GAO Zhao-quan; LI Tian-hong; ZHANG Xian-chuan

Abstract：A coupled model of canopy stomatal conductance and transpiration rates (Tr) in apple canopy was presented.This model could simulate the response of Tr to microclimatic factors and the diurnal Tr evolution.The model used the Penman-Monteith equation to compute effects of microclimafic factors; stonlatal conductance model was developed by experiential model; the feedback of transpiration to stomatal conductance was described by leaf water potential through steady flow model.Sap flow measured by the compensation heat-pulse technique was taken as the tree transpiration on a daily time scale ,and it proved to be a very good way to connect simulated transpiration rates with the individual apple tree sap flow.The mathemati-cal simulation showed strong interactions among various microclimatic factors, and indicated that Tr was mainly driven by va-por pressure deficit (VPD)and stomatal conductance.The diurnal variations of the Tr increased as net radiation and VPD in-creased, and it decreased as leaf water potential increased, represented by one-peak curves.Maximum transpiration rates of experimental trees (leaf area index was 2.53) were about 8 retool, m~(-2)·s~(-1)on clear days and about 3 mmol· m~(-2)· s~(-1)on cloudy days.Over 24 h, an apple tree (leaf area = 37.95 m~2) lost 50 to 70 L of water on clear days and about 15 L on cloudy days.