Separating the effects of trees on crops: the case of Faidherbia albida and millet in Niger

Faidherbia albida (Del.) A. Chev. is an important tree species of the scatt ered tree or parkland systems in the Sahel. The improved crop growth under its canopy is well known, and has been attributed to various components not ably: higher soil fertility, improved microclimate and better soil physical properties. The relative contributions of each of these components are not known, but knowledge about this is essential for making proper decisions c oncerning management options. The overall tree effect on crop production (e xpressed as fraction of the sole crop production) is analysed here as a wei ghted sum of (positive and negative) relative net tree effects on the resou rces for crop growth. In this sum, the weights reflect the degree of limita tion of the resources in the environment at the tree-crop interface. The pa per shows how the relative net tree effects and the accompanying weights ca n be estimated from field experiments. In an on-farm field experiment in Ni ger, millet production under the F. albida canopy was about 36% higher than in the open field. The nitrogen availability under trees was estimated to be more than 200% higher than in the open causing a 26% production increase . The phosphorus availability was estimated to be almost 30% higher and bec ause of its high limitation causing a production increase of 13%. The net e ffect via other resources (notably light and water) was negligible (3% prod uction reduction) and not significant.