Tree-crop interactions for below-ground resources in drylands: Root structure and function

This paper describes recent advances in below-ground research in dryland ar eas, focusing mainly on studies in dryland Kenya, in which novel sap flow a nd allometric techniques for determining root structure and function were c ompared with simple, but labor-intensive methods involving soil coring and trenching. Studies using these approaches indicated that tree (Gliricidia s epium and Guevillea robusta) and, crop (Zea mays) roots predominantly occup ied the same soil volume, implying there was little spatial complementarity , and highlighted The existence of considerable variation in tree root dens ity between seasons. The less labor-intensive approach of determining the " competitivity index", based on measurements of main roots, was not a reliab le indicator of competition unless it was adjusted for tree size. Fractal r elationships provided reasonable estimates of coarse root length, but serio usly underestimated the length of fine roots. Sap flow studies showed the m arked capacity of tree root activity to switch from one part of the root sy stem to others in response to changes in soil moisture availability, and hi ghlighted the importance of root function as opposed to structure in determ ining the severity of below-ground competition; such effects may provide bo th benefits and costs to neighboring plants. Thus, there is an urgent need to develop tree management regimes which promote effective sharing of below -ground resources.