Abstract A simulation study on the combined effects of sun angle and spatial pattern of trees on the amount of intercepted photosynthetically active radiation (PAR) is presented. Different spatial patterns of trees were generated starting from a square lattice. Tree locations were first rotated by an angle α=0, 22.5 or 45°, and thereafter x-coordinates were multiplied by a constant c x and the y-coordinates were divided by c x . The values of c x were from 0.6 to 1.6 with the increment of 0.2. Simulations were made using constant stand density (400/ha), and crown size and shape. The crown envelope was described as the upper half of an ellipsoid with a height of 6 m and a radius of 1.5 m at the bottom. Only direct radiation was taken into account, and the direct solar irradiance was assumed to decrease exponentially within tree crowns. Simulations were made separately for latitudes 40°N (Thessaloniki) and 60°N (Helsinki). For the whole growing season (April to October at latitude 40°, and from May to September at latitude 60°), the intercepted PAR was largest when α=45° and c x =1.4 (wide east-west spacing) and smallest when α=0° and c x =0.6 (tight east-west spacing). At latitude 40° the optimum was 4.4% larger and the minimum 13.0% smaller than the intercepted radiation for the square pattern ( α=0° and c x =1). At latitude 60° the corresponding figures were 2.0 and 10.0%. The patterns for α=22.5° were nearly optimal for all c x . Reversed relations were found later in fall.