Simulation of within-tree and between-tree shading of direct radiation in a forest canopy: effect of crown shape and sun elevation

Abstract The pattern of momentary within-tree and between-tree shading of foliage in direct radiation flux was simulated in fully-stocked model stands with a Poisson distribution of trees. The canopies were composed of umbrella-like (crown-height/crown-radius, or H/R = 3), average (H/R = 8) or columnar (H/R = 20) tree crowns. The momentary radiation-extinction probability was calculated for a large number of points (106−107 points per ha) within the canopy. The radiation-attenuation rate depended on the projected leaf-area density within the crown. At solar elevations of 5° and 45°, columnar crowns had the most between-tree and the least within-tree shading, while at a high sun elevation (85°) the same was true for umbrella-like crowns. At low and medium sun elevations (5° and 45°) the crown shape had a rather small effect on the total shading of foliage. At a high sun elevation, flat, horizontally extended crowns were clearly best-illuminated, because both within-tree and between-tree shadings were low.

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