THE TREE-CROP INTERFACE - REPRESENTATION BY COUPLING OF FOREST AND CROP PROCESS-MODELS

Three process-based approaches to agroforestry modelling are described . These are (a) coupling a continuous-canopy forest model (Hybrid) and tropical crop model (PARCH); (b) coupling an individual-tree model (M AESTRO) with a crop model (PARCH); and (c) incorporating a combined mo del of evaporation and radiation interception by neighbouring species (ERIN). The coupled Hybrid/PARCH was parameterised for maize and eucal yptus, and run in five contrasting weather-types, As expected, shade i s the most important factor limiting yield in wet sites; water in dry sites. Year-to-year variability in crop yield is increased by light an d water competition, MAESTRO/PARCH was run with similar assumptions, a nd gave comparable yield predictions, except at the driest site where it allows small areas distant from the tree sufficient water to produc e a modest yield. Hybrid/PARCH predicted total crop failure in the sam e climate. Yields on drier sites were higher in the shade, but water c ompetition was severe close to the tree. ERIN is simpler than the abov e models, but is unique in including the transfer of heat and water va pour between the two canopies. Transpiration from a moist understorey can humidify air in the overstorey, and reduce its transpiration; whil st a dry understorey will give off sensible heat, which increases the vapour pressure deficit in the overstorey and causes its transpiration to increase. Changes in overstorey transpiration due to fluxes from t he understorey may approach 15-20%.