Penumbra in within-shoot and between-shoot shading in conifers and its significance for photosynthesis

Simulations of the statistical distribution of direct solar irradiance in a Scots pine (Pinus sylvestris L.) crown were used to evaluate the effect of a penumbra (a partial shading of the solar disc) on the rate of photosynthesis. Three different situations were considered: the rate of photosynthesis was estimated at points on needles subjected to (a) shading from the same shoot, (b) shading from another shoot, and (c) both within- and between-shoot shading. Results showed that the penumbral effect of within-shoot shading was small, i.e., the assumption of parallel solar beam geometry within a shoot did not lead to serious underestimates of the rate of photosynthesis. In contrast, shading from another shoot (needle layer) situated further away than ca. 25 cm from the target point could be better characterised as diffuse shading. It is proposed that, in modelling the penetration of direct radiation in a coniferous stand, shading from a short distance (“within-shoot shading”) and that from a longer distance (“between-shoot shading”) be treated separately. A method is presented to estimate mean rates of photosynthesis based on this approach. This method was found to be more accurate than two commonly used methods to estimate photosynthesis, one of which is based on the assumption of parallel solar beam geometry (no penumbra) while the other assumes no stochastic variation in the direct solar component.