Complementarity of light and water use in tropical agroforests - I. Theoretical model outline, performance and sensitivity

Agroforestry models are needed to help define the biophysical boundary conditions where agroforestry may be beneficial. A generic, process-based model is outlined which couples light and water use in a tree-crop-soil system. The model predicts the net primary production (NPP) of a uniform overstorey of tropical broadleaved trees, using parts of the model Hybrid [Friend, A.D., et al., 1997. A process-based, biogeochemical, terrestrial biosphere model of ecosystem dynamics (Hybrid v3.0). Ecol. Model. 95; 249-287.] and the growth and grain yield of an understorey sorghum crop, using PARCH [Bradley, R.G., Grout, N.M.J., 1994. PARCH-User Guide. Tropical Crops Research Unit, University of Nottingham, Sutton Bonington, Leicestershire, UK, 122 pp.]. Belowground competition for water is simulated. The model is driven by daily climate, which can be generated for any half-degree pixel in the world. The performance of the model is illustrated for a site in Ghana. The model gives validated output of the NPP of natural forest/woodland vegetation and of potential sorghum grain yield. Tests of the sensitivity of the model to simplifying assumptions are presented and discussed. It is concluded that the model provides a useful tool to explore opportunities for complementarity of light and water use by trees and sorghum in a range of climates. (C) 1998 Elsevier Science B.V.