Effect of tree canopy structure on light condition in almond-winter wheat intercropping systems

Intercropping of fruit trees and cereals is one of the major cropping patterns in South Xinjiang. As trees age and canopy volumes increase in intercropping systems, low-light stress due to tree canopy effect becomes the main drive behind dramatic reduction in yield. However, given the fragile ecological environment of South Xinjiang and food security, this system has continued for a long time now. In this study, the relationship between canopy structure and light condition in intercropping systems was determined in order to provide theoretical basis for choosing canopy structures suitable for optimal production. Almond tree (10 a) and winter wheat intercropping system was used to study the effects of 4 canopy structures (delayed open-central shape, open-center shape, high-stem shape and semicircle small-canopy shape) on radiation intensity of various wavebands, spectral composition and diurnal variations of photosynthetically active radiation (PAR) during wheat filling stage, with monoculture winter wheat as the control. The results showed that: 1) In terms of quality of light condition in intercropping systems, the order from high to low was semicircle small-canopy shape, high-stem shape, open-center shape and delayed open-central shape. The averages of radiation intensity under the 4 canopy shapes were respectively 55.63%, 46.54%, 37.87% and 28.76% of the control. Correspondingly, PAR intensities were 55.84%, 44.57%, 35.52% and 26.40% of the control. 2) Compared with the control, radiation intensity of each waveband decreased at various degrees. Reductions in PAR, blue-violet, yellow-green and red-orange light radiations were larger than average reduction in total radiation. However, reductions in ultraviolet, near infrared and far infrared light radiation were smaller than average reduction in total radiation. For example, in the eastern region under delayed open-central shape canopy, total radiation decreased to 11.37% of the control. Then blue-violet light, yellow-green light, red-orange light and PAR decreased respectively to 7.98%, 8.42%, 8.62% and 8.30% while ultraviolet light, near infrared light, far infrared light decreased to 12.30%, 15.94% and 23.00% of the control. 3) For all the light quality parameters, the ratio of red-orange light to far infrared light was most sensitive to canopy structure (CV = 23.34%). It can be used as a major indicator for light conditions in intercropping systems. 4) Canopy structure characteristics (e.g., canopy width, stem height, tree height and whether center stem) existed or had little effect on diurnal variation in PAR in intercropping systems, especially for regions east of the tree. Based on the above analysis, light conditions in intercropping systems can be improved by tree pruning such as canopy limitation, trunk height increase and head cut. Among the four canopy structures, semicircle small-canopy shape produced the best light conditions.