Maize and cowpea were grown as sole stands or in agroforestry systems containing grevillea trees (Grevillea robusta A. Cunn.). Crop and system performance were examined over a 4.5-year-period (nine growing seasons) commencing in October 1991; failure of the rains caused the loss of one cropping season. A rotation of cowpea (Vigna unguiculata L. Walp.) and maize (Zea mays L.) was grown during the first five seasons after planting the trees, while maize was grown continuously during the final four seasons. Sole maize was also grown under spectrally neutral shade netting which reduced incident radiation by 25, 50 or 75% to establish the relative importance of shade and below-ground competition for water and nutrients in determining the performance of understorey crops. The above-ground biomass and grain yield of understorey crops were not significantly affected by the presence of grevillea during the first four seasons, but were greatly reduced in subsequent seasons as the trees became increasingly dominant; maize yields reached 50% of the sole crop values only once during the final four seasons, when rainfall was unusually high. The hypothesis that competition for water was the primary limiting factor for understorey crops was supported by the observation that above-ground biomass and grain yield were greater in the shade net treatments than in agroforestry maize, demonstrating that shade was not solely responsible for the substantial yield losses in the latter treatment. Performance ratios (ratio of values for the agroforestry system relative to sole stands) for total above-ground and trunk biomass in grevillea were initially low, reflecting the impact of competition with associated crops during tree establishment, but increased to unity within 2.5 years. Performance ratios for the understorey crops exhibited the reverse trend, initially being close to unity but approaching zero for three of the final four seasons. Performance ratios were never close to unity for both trees and crops during the same season, indicating that there was always competition for available resources irrespective of crop species or tree size. The implications for agroforestry system design and future research are discussed.
Forest Ecology and Management (2000) 139 (1-3) 187-201 [DOI: 10.1016/S0378-1127(00)00267-X]