Light interception and partitioning in a shrub/grass mixture

Abstract

When crops are grown in mixture systems, microclimate is modified, particularly the light regime. Experiments were conducted in Guadeloupe to examine the variability of the light field and test a light interception model in a traditional forage mixture made of angleton grass savanna planted with rows of gliricidia shrubs. Simulated figures of reflected, transmitted and absorbed PAR were in close agreement with experimental data measurements during the four periods of the study. The model was also used to simulate the influence of row orientation (N-S or E-W), location (North Tropic or Equator), and dates (winter solstice, equinox or summer solstice) on the absorbed photosynthetically active radiation (PAR). The presence of the shrubs strongly decreased the absorbance of the grass, which absorbed between 42% and 50% and 14% and 19%, respectively, in pure and mixed stands. Light capture by gliricidia ranged between 51% and 66%, and was not affected by the presence of the grass. In all cases the interception of the PAR was better for the mixed crop (+15% and +30% more than the pure shrubs and the pure grass, respectively). The influence of the non-photosynthetic organs of the shrubs was limited (about 2%). A simple version of the model is presented, based on a modification of the extinction coefficient instead of the complete calculation of scattering. Differences between the two models are small. Results suggest that the effect of row orientation and season in this kind of perennial forage mixtures is limited.