Tree structure and 3-D distribution of radiation in canopy of apple trees with different canopy structures in China

Tree structure and radiation distribution are critical factors influencing photosynthetic productivity of fruit trees as well as fruit yield and quality. The aim of this study was to determine the parameters of tree structure, leaf area density (LAD) 3-D distribution of photosynthetically active radiation (PAR) within four canopy structures (small-sparse canopy, stratified-disperse canopy, spindle canopy and open-center canopy) of apple tree. The experiment was conducted in ‘Fuji’ apple (Malus domestica Borkh. cv. ‘Fuji’) orchard during 2005~2008 growth seasons. The structure parameters of trees with different canopy structures were determined via direct measurement, and PAR of each apple tree canopy measured by quantum sensors. The results showed that the lowest total shoot number (894×10³·hm^-2) and leaf area index (LAI, 2.53) were in open-center canopy. Total shoot and LAI were 2 280×10³·hm^-2 and 4.14, 2 119×10³·hm^-2 and 3.98, 2 190×10³·hm^-2 and 3.88 in small-sparse, stratified-disperse and spindle canopies, respectively. 3-D distribu-tion of LAD was different among the four canopy structures. LAD vertical distribution yielded a distribution mode with height range of 0~4.0 m, largely occurring within 1.0~3.0 m high. While the highest LAD was within 0.5~2.0 m height range in stratified-disperse and spindle canopy structures, it was 0.5~1.5 m and 1.0~2.0 m in small-sparse and open-center canopy structures, respectively. In all the canopy structures, relative PAR decreased with increasing canopy depth. Rapid relative PAR depression was concentrated in middle canopies where LAD was highest. Relative PAR 3-D distribution pattern in unit cells apparently tracked leaf area distribution. The highest average relative PAR and most uniform distribution were in open-center canopy structure. Percent average relative PAR of leaves were 24.85%, 28.84%, 27.71% and 37.28% for small-sparse, stratified-disperse, spindle and open-center canopy structures, respectively. While only 35% leaf area was in the low-light region of open-center canopy structure, over 50% was in the low-light region in the other canopy structures. The main aim of tree pruning was to remove useless shoots and leaves, which were easily identified in 3-D plots. Percent leaf areas in different relative PAR ranges reasonably predicted canopy radiation, which was critical in fruit studies. Leaf and PAR distributions were more uniform in open-center than in other canopy structures. This was a distinct advantage of open-centre canopy structure over the other canopy structures.