Fine scale measurement and simulation of penumbral radiation formed by a pine shoot

A new instrument for measuring photosynthetically active radiation (PAR) was used to investigate changes in the distribution of direct sunlight on a plane shaded by a Scots pine shoot situated at different distances, and to test a previously developed model of shoot geometry. The experiment was designed to imitate a simple case of between shoot shading within a canopy. Measured distributions were compared to simulated ones, which were produced using the Scots pine shoot model and measured characteristics of the experimental shoot. Comparison showed that the distributions agreed reasonably well with one another, although, the standard deviation was somewhat larger in the simulated distributions. The shape of the distributions changed from clearly bimodal (full sun-full shade) to distributions concentrated around the mean (penumbral irradiance), as the shoot was moved further away from the sensors. Changes in the distribution of direct sunlight (proportions of full sun, full shade and penumbra) with shading distance are important to consider for a reliable estimation of the rate of photosynthesis at different levels in the canopy. (C) 1999 Elsevier Science B.V, All rights reserved.