The theory of the interrelations between plant canopy structure and light penetration is reviewed using principles developed for point quadrat analysis. Estimates of leaf area index and leaf orientation can in principle be obtained from measurements of the average transmission of direct sunlight through vegetation using an indirect measurement procedure. The architecture of a kiwifruit (Actinidia deliciosa (A. Chev)
C.F. Liang et A.R. Ferguson c.v. Hayward) canopy was measured using direct methods and the indirect measurement procedure. The methods gave good agreement for estimates of the mean leaf angle and the G-function. However the indirect procedure produced values for the leaf area index that were about 1.7 times greater than those obtained from direct methods. This was consistent over a range of leaf area indices (measured directly) from 1.7 to 3.5. The evidence provided suggests that this discrepancy is most likely due to the leaves in the canopy having a regular distribution rather than being randomly arranged.
The Poisson law and the positive and negative binomial models provided the basis for further analysis of light penetration into plant stands. The results were compared with observed values of light transmission through the canopy. It was suggested that the measured values of light transmission could be used in conjunction with expected values of light transmission calculated from the direct measurements of canopy architecture to estimate leaf distribution in the canopy. The distribution of leaves in a stand could be determined at two levels, one is a local level within each plant and the other level is associated with growth of the plants in the stand. At the level of the plant, light in the stand was attenuated about 1.7 times faster than expected for a random leaf distribution. At a higher level, which corresponded to the entire stand, light was attenuated about 1.4 times faster than expected for a random stand. The difference is attributed to the non-uniform (discontinuous) structure of the stand which results from localized variations in leaf area index.
The measurements of the distribution of leaf area in the stand are used in conjunction with the direct measurements of canopy architecture to construct a computer model that can be used to simulate the stand. The model is used to simulate the light environment in the canopy so that the daily integral of photosynthetically active
radiation penetrating into the canopy can be determined as a function of the leaf area index. The available information on the effects of light intensity on kiwifruit growth was used to determine the leaf area index at which low light levels could begin to affect fruit owth and yields. For an orchard with a pergola trellis, the maximum leaf area index that could be allowed without affecting fruit growth was calculated to be about 1.6.