Dependence of light interception efficiency of Scots pine shoots on structural parameters

The ratio of shoot silhouette area to total needle area (STAR) is one means of quantifying the light interception efficiency of a coniferous shoot. The silhouette area (or STAR) of a shoot depends on various structural characteristics of the shoot, and also varies with the direction of the shoot relative to the direction of radiation (sun angle). The mean STAR taken over all directions in space is the mean ratio of shoot silhouette area to total needle area in an isotropic radiation field. It also represents a mean STAR with respect to a spherical shoot orientation. In this study, equations for the relationship between mean STAR and easily measurable shoot characteristics were developed. The STAR values in different directions were determined from photographically measured shoot silhouette areas. Mean STAR varied between 0.079 and 0.308, and averaged 0.146. A clear increase in mean STAR with age was found. The material consisted of 305 Scots pine (Pinus sylvestris L.) shoots from Sweden and Finland, representing shoots of different age and a wide variation in site fertility. The needle area density in the shoot cylinder, together with the diameters of the shoot cylinder and the twig explained 87% of the variation in mean STAR.