Accumulated Photosynthetically Active Radiation and Its Heterogeneity Collectively Decrease Soybean Yield in Apple-Based Intercropping Systems

The under-canopy light environment in agroforestry systems is a key limiting factor for the growth of intercropped crops. However, the impact of under-canopy light heterogeneity on crop yield remains unclear. This study focused on 4 (Y4)-, 6 (Y6)-, and 8 (Y8)-year-old apple tree/soybean intercropping systems with root barriers, measuring under-canopy photosynthetically active radiation, yield, and yield components at different phenological stages of soybean, and establishing a quantitative relationship between light heterogeneity and soybean yield. In the apple/soybean intercropping system, the spatial heterogeneity of accumulated photosynthetically active radiation (APAR) is greatest in Y6, with the APAR divided into five categories parallelized along the tree rows. Y4, which had the least spatial APAR heterogeneity, was divided into three categories. The APAR was split into two classes in Y8. The seed number per plant and 100-seed weight of soybean decreased with the increase in tree age. Compared to Y4, yields of Y6 and Y8 treatments decreased by 22.6% and 46.2%, respectively. The reduction in APAR showed a negative effect on yield and its components of soybean, especially in Y4. The shading intensity and under-canopy light heterogeneity jointly constrained the intercropped soybean yield; this effect was gradually strengthened with increasing tree age. Different measures should be taken according to different tree ages and soybean growth stages in intercropping systems to reduce the adverse effects of under-canopy light on soybean yield.