A general tree-environment-crop interaction equation for predictive understanding of agroforestry systems

Agroforestry is a young science in need of theories and methods providing predictive understanding. The present study is an attempt to contribute to the development of a general agroforestry theory and accessory methods. A tree-environment-crop interaction equation is derived in which the overall tree effect on crop production is explained as a balance of (positive and negative) relative net tree effects on resource availability to the crop. In this balance, each relative net tree effect receives a weight that is equal to the degree of limitation of the resource in the specific environment. The equation thus makes explicit that the balance of available resources in the environment is a major factor determining the biophysical outcome of an agroforestry technology. The equation enables the formulation of two rules that can be viewed as the agroforestry counterparts of classic crop production laws. The agroforestry counterpart of the law of limiting factors states: the more a resource becomes available in the tree-crop environment, the smaller its share becomes in the overall tree-environment-crop interaction. The agroforestry counterpart of the law of the optimum states: the more other limiting resources become available in the tree-crop environment, the greater the share of a resource becomes in the overall tree-environment-crop interaction. These rules help with extrapolation, without requiring full quantification of the tree effect balance. They allow the development of a method for the analysis and synthesis of agroforestry experiments. The method is illustrated by analysing some published alley cropping experiments. This analysis indicates that for alley cropping systems: (1) the hypothesis that trees capture nutrients below the crop rooted zone resulting in a net benefit for the crop is probably true for nitrogen and not true for phosphorus, and that (2) competition for water by trees probably exceeds their water conservation effects. Suggestions for experimentation are given and the circumstances under which yield benefits of agroforestry systems can be expected are discussed. © 2000 Elsevier Science B.V. All rights reserved.