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Tests of Solar Radiation Models in Three Forest Canopies

Miller, Philip C.
Ecology 1969 v.50 no.5 pp. 878-885
branches, equations, forest canopy, leaves, models, prediction, primary productivity, solar radiation, vegetation
Several models expressing the decrease of solar radiation in vegetation canopies have been proposed. The models are similar in (1) including a method of projecting the shadow of one more leaves onto a horizontal plane and (2) including a method of relating the projected shadow to the decrease of solar radiation. The models differ in the equations used and in the assumptions made. The models used the binomial, the Poisson, or some modification of the binomial to relate the projected shadow to the radiation at different levels. When the models were tested against the percentage of sunlit area at different levels in the canopy, the best fitting models was one proposed in this paper in which the shadow of the leaf is projected as a shadow of a cone and the binomial is used to express the decrease of radiation. Negative exponential models usually overestimated the percentage of sun. Including the shadow cast by twigs and branches in the prediction improved the agreement between the observed and the expected percentage of sun. Primary production models in which the negative exponential is used to calculate solar radiation may overestimate the actual production.