Subcanopy light availability, crop yields, and managerial implications: a systematic review of the shaded cropping systems in the tropics

Agroforestry systems (AFS) represent combinations of trees, arable crops, and/or pastures. Being assemblages of diverse life-forms, they exhibit complex biophysical interactions. For instance, the multistrata canopies shade the understory crops by intercepting a significant amount of the incoming solar radiation. Optimizing understory productivity, thus, requires understanding the elements that affect the canopy transmittance of photosynthetically active radiation (PAR) and its spatiotemporal dynamics. We systematically reviewed the peer-reviewed literature involving 145 tropical and subtropical tree + crop combinations. The theoretical underpinnings of interspecific interactions in developing agroforestry stands were elucidated using a conceptual model. Additionally, the linkage between subcanopy PAR levels and yield was established for 11 arable crops. PAR reaching the understory and the subcanopy yield levels were tremendously variable across AFS. Relative yields ranged from 6 to 188% of the sole crops. Stage of stand development, canopy architecture, and management factors are cardinal determinants of canopy light extinction, understory PAR availability, and yield. The yield of shade-tolerant crops either increased (“over-yielding”) or remained the same as PAR levels decreased within certain limits, albeit with intraspecific variations. The tree-crop interaction effects on yield were positive, negative, or neutral. In total, 19 cases showed positive responses, 29 were neutral, and 113 were negative, with a few overlapping responses depending on the tree, crop, and management. This implies that the key to ecological intensification is component selection and management. Agroforestry, while containing the loss of, maintaining, or even increasing understory yields, thus maximizes overall (tree + crop) outputs and land equivalent ratio.