Prediction of PPFD variance at forest floor in a thinned Japanese cypress plantation

A model for predicting the spatial variance of photon Aux density (PPFD) was developed in a thinned Japanese cypress (Chamaecyparis obtusa) plantation, to quantify the effect of thinning on light availability. Hemispherical diagrams were simulated from the foliage densities of cubes 0.2 x 0.2 x 0.2 m which were estimated from the coordinates of tree apexes. Predicted PPFD values and gap distributions were compared with the actual values, which were measured by quantum sensors and hemispherical photographs through a fish eye lens. After validation, the model predicted gap distribution and daily total PPFD well. The model also produced a frequency pattern of PPFD with sufficient accuracy for evaluating the thinning effects on Light availability. This model has the following four merits. (1) The frequency pattern of PPFD can be simulated with this model. The frequency pattern seems to be a more adequate index for eco-physiological discussion than the integral of PPFD, since the photosynthetic rate does not change linearly with PPFD. (2) The compound effect of multiple gaps can be evaluated in this model. (3) The process of decreasing PPFD by canopy closure can be predicted, since the canopy closure process was predicted by the canopy dynamic model, upon which this model is based. (4) The influence of azimuth and inclination of slope on PPFD can be predicted. (C) 2000 Elsevier Science B.V. All rights reserved.