THREE-DIMENSIONAL SIMULATION OF CANOPY PHOTOSYNTHESIS IN AN APPLE ORCHARD

A coupled model was developed to simulate the three-dimensional (3-D) distribution and the diurnal variations in canopy photosynthesis in an apple (Malus domestica Borkh. cv. 'Fuji') orchard (40 degrees 13' N, 116 degrees 13' E, altitude 79 m). The coupled model was developed on the basis of a biochemical model of C-3 photosynthesis and 3-D distribution of canopy radiation as determined by direct measurement. Foliage photosynthetic capacity at different canopy positions was described by an experiential equation. A flat step in the top canopy of the 3-D Pn distribution was observed, which decreased rapidly with the decreasing PAR caused by leaf interception. However, the 3-D distribution of the total photosynthetic rate in a unit cell was mainly depended on the pattern of the leaf area density. The diurnal variations of the Pn mostly depended on net radiation, represented by double-peak curves. The values of the canopy photosynthesis increased in the morning and reached a maximum level at about 11: 00, then decreased to lower values by midday. These values recovered to the second maximum level at about 15: 00 and decreased again in the late afternoon, creating an asymmetrical diurnal pattern on a sunny day. The midday depressions of Pn in the top canopy were expressed more strongly than those in the middle and bottom canopy regions. During a whole day (24 hrs), an apple tree (leaf area = 37.95 m(2)) was fixed at 9-11 mol of CO2 on a clear day and 2-4 mol of CO2 on a cloudy day.