A simulation model for hedgerow light interception and growth

In order to investigate competition for light between trees and crops in an agroforestry system, we wrote a simulation model of light interception and growth by a system of hedgerows and an interplanted crop. To calculate lig ht interception, hedgerows are modeled as long prisms, solar angles and hed gerow shadow lengths are calculated, and light intercepted by the hedgerows and points in the alley is summed. Biomass allocation to leaves and wood a nd the new hedgerow dimensions are calculated from empirical allometric equ ations. Hedgerow shoots are modeled as a population undergoing density-depe ndant mortality. Parameters for the model were developed from an alley crop ping experiment of hedgerows of Flemingia macrophylla, a legume shrub with broad leaves, grown alone or intercropped with maize on the island of Kauai , Hawaii. The model was tested with data from field experiments conducted i n 1995 and 1996. In 1995, light interception simulated by the model for a p oint on the floor of the alley tracked measured light interception well ove r the course of the crop both when hedgerow sizes were input from field mea surements and when they were simulated. Hedgerow biomass predicted by the m odel closely followed that measured in periodic harvests. In 1996, light in terception simulated by the model tracked measured Light interception well when the model was run with measured hedge dimensions, but the model under predicted hedgerow growth and, thus, over predicted transmission when growt h was simulated. Radiation use efficiencies (E) for the hedgerows over the entire cropping season were calculated by regressing harvest biomass agains t Light interception as calculated by the model. These averaged 0.18 (S.E. 0.005) g above-ground biomass mol(-1) photosynthetically active photons for both treatments in 1995 and 0.21 (S.E. 0.008) g mol(-1) in 1996. While hed gerow biomass was affected by the presence of the crop, the fact that E was the same in both treatments indicated that competition was overwhelmingly for light rather than for water or nutrients. (C) 2001 Elsevier Science B.V . All rights reserved.