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-b ased model is outlined which couples light and water use in a tree-cro p-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, biogeochem ical, terrestrial biosphere model of ecosystem dynamics (Hybrid v3.0). Ecol. Model. 95; 249-287.] and the growth and grain yield of an under storey sorghum crop, using PARCH [Bradley, R.G., Grout, N.M.J., 1994. PARCH-User Guide. Tropical Crops Research Unit, University of Nottingh am, Sutton Bonington, Leicestershire, UK, 122 pp.]. Belowground compet ition for water is simulated. The model is driven by daily climate, wh ich can be generated for any half-degree pixel in the world. The perfo rmance of the model is illustrated for a site in Ghana. The model give s validated output of the NPP of natural forest/woodland vegetation an d of potential sorghum grain yield. Tests of the sensitivity of the mo del to simplifying assumptions are presented and discussed. It is conc luded that the model provides a useful tool to explore opportunities f or complementarity of light and water use by trees and sorghum in a ra nge of climates. (C) 1998 Elsevier Science B.V.