ABOVE AND BELOW GROUND INTERACTIONS IN AGROFORESTRY SYSTEMS

Ecological interactions between trees and crops are examined in terms of above and below ground utilizations of physical resources. Above ground interactions such as changes in light, temperature, and humidity are analyzed in terms of possible effects on understory crops. Our analysis shows that atmospheric interactions in alley cropping in the semi-arid tropics are positive but of minor importance compared with below ground interactions. Separation of below ground interactions by a shallow polythene barrier (0.5 m) indicated that competition for soil moisture is responsible for the negative interactions reported in the semi-arid tropics (SAT). Measurements of root distribution showed that roots of Leucaena leucocephala Lam. trees are abundant in the top 30 cm of the soil and the presence of a root barrier was effective in restricting lateral movement of the roots. This evidence is contrary to the assumption that trees have deep rooting systems which do not compete with crops. A secondary aim is to illustrate the contrasting problems encountered in intercropping and agroforestry systems. The importance or root studies are highlighted and the ways in which positive interactions could be achieved are investigated.