Productivity, microclimate and water use in Grevillea robusta-based agroforestry systems on hillslopes in semi-arid Kenya

This paper describes a multi-disciplinary project to examine the changing i nteractions between trees and crops as the trees in semi-arid agroforestry systems establish and mature; the project is one of the most detailed and h ighly instrumented long-term studies of tree and crop growth, system perfor mance, resource capture, hydrology and microclimate ever carried out within an agroforestry context. Its primary objective was to compile a comprehens ive experimental database to improve the mechanistic understanding of tree/ crop interactions and support the development and validation of process-bas ed simulation models describing resource capture and tree and crop growth i n semi-arid agroforestry systems. Grevillea robusta A. Cunn, (grevillea) trees were grown as mono-cultures or in mixtures with cowpea (Vigna unguiculata L.) or maize (Zea mays L.) over a 68-month period. Allometric approaches were used to determine seasonal a nd annual growth increments for leaf area and leaf, branch and trunk biomas s in grevillea. Crop performance was examined during each growing season, w hile the spatial distribution of toe and crop roots was established during the latter stages of the experiment using coring and mini-rhizotron approac hes. Detailed hydrological studies examined effects on the soil water balan ce and its components (precipitation, interception, runoff and soil moistur e status); equivalent measurements of spatial and temporal variation in mic roclimatic conditions allowed the mechanistic basis for beneficial and detr imental effects on understorey crops and the influence of proximity to tree s on crop performance to be examined. Transpiration by grevillea and water movement through lateral and tap roots were measured using sap flow methodo logy, and light interception by the tree and crop canopies was routinely de termined. This multi-disciplinary study has provided a detailed understanding of the changing patterns of resource capture by trees and crops as agroforestry sy stems mature. This paper provides an overview of the underlying rationale, experimental design and core measurements, outlines key results and conclus ions, and draws the attention of readers to further papers providing more d etailed consideration of specific aspects of the study. (C) 2000 Elsevier S cience B.V. All rights reserved.