LIGHT INTERCEPTION AND PARTITIONING IN A SHRUB GRASS MIXTURE/

When crops are grown in mixture systems, microclimate is modified, par ticularly the light regime. Experiments were conducted in Guadeloupe t o examine the variability of the light field and test a light intercep tion model in a traditional forage mixture made of angleton grass sava nna planted with rows of gliricidia shrubs. Simulated figures of refle cted, transmitted and absorbed PAR were in close agreement with experi mental data measurements during the four periods of the study. The mod el 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 abs orbance of the grass, which absorbed between 42% and 50% and 14% and 1 9%, respectively, in pure and mixed stands. Light capture by gliricidi a 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 gras s, respectively). The influence of the non-photosynthetic organs of th e shrubs was limited (about 2%). A simple version of the model is pres ented, based on a modification of the extinction coefficient instead o f the complete calculation of scattering. Differences between the two models are small. Results suggest that the effect of row orientation a nd season in this kind of perennial forage mixtures is limited.