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 light interception, hedgerows are modeled as long prisms, solar angles and hedgerow shadow lengths are calculated, and light intercepted by the hedgerows and points in the alley is summed. Biomass allocation to leaves and wood and the new hedgerow dimensions are calculated from empirical allometric equations. Hedgerow shoots are modeled as a population undergoing density-dependant mortality. Parameters for the model were developed from an alley cropping 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 in 1995 and 1996. In 1995, light interception simulated by the model for a point on the floor of the alley tracked measured light interception well over the course of the crop both when hedgerow sizes were input from field measurements and when they were simulated. Hedgerow biomass predicted by the model closely followed that measured in periodic harvests. In 1996, light interception 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 growth was simulated. Radiation use efficiencies (ε) for the hedgerows over the entire cropping season were calculated by regressing harvest biomass against 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 hedgerow biomass was affected by the presence of the crop, the fact that ε was the same in both treatments indicated that competition was overwhelmingly for light rather than for water or nutrients.