ROOT ARCHITECTURE IN RELATION TO TREE-SOIL-CROP INTERACTIONS AND SHOOT PRUNING IN AGROFORESTRY

Desirable root architecture for trees differs between sequential and simultaneous agroforestry systems. In sequential systems extensive tree root development may enhance nutrient capture and transfer to subsequent crops via organic pools. In simultaneous systems tree root development in the crop root zone leads to competition for resources. Fractal branching models provide relationships between proximal root diameter, close to the tree stem, and total root length or surface area. The main assumption is that a root branching proportionality factor is independent of root diameter. This was tested in a survey of 18 multi-purpose trees growing on an acid soil in Lampung (Indonesia). The assumption appeared valid for all trees tested, for stems as well as roots. The proportionality factor showed a larger variability in roots than in stems and the effects of this variability should be further investigated. A simple index of tree root shallowness is proposed as indicator of tree root competitiveness, based on superficial roots and stem diameter. Pruning trees is a major way to benefit from tree products and at the same time reduce above-ground competition between trees and crops. It may have negative effects, however, on root distribution and enhance below-ground competition. In an experiment with five tree species, a lower height of stem pruning led to a larger number of superficial roots of smaller diameter, but had no effect on shoot:root ratios or the relative importance of the tap root.