The effects of timing of pre-commercial thinning and stand density on stem form and branch characteristics of Pinus sylvestris

Abstract

The effects of pre-commercial thinning on tree and branch characteristics were analysed at two experimental sites in northern Sweden, where pre-thinning of Scots pine stands to 600, 1000 and 1800 stems per hectares at heights of 1.5, 3, 5 and 7 m had been combined in a factorial design. A total of 90 trees were cut and branches were measured in whorls sampled at 1-m intervals, starting with the whorl closest to 0.5 m above ground. Effects of the treatments on tree and branch characteristics were analysed by a fixed-effect analysis of variance model, which was applied separately for each experimental site and whorl height. Diameter at breast height (d.b.h.) and living crown to height ratio decreased, while the height/d.b.h. ratio (stem form) increased with increasing stand density, and generally with increased height at thinning. No treatment effect was found on the number of branches per whorl or branch angle, but branch diameter (BD) was significantly influenced by both stand density after thinning and height at thinning. BD decreased with increases in stand density and decreased with increases in height at pre-commercial thinning. Relative branch size (RBS), defined as the ratio between the sum of the diameters of branches in a whorl and stem perimeter under bark at the location of that whorl, was significantly influenced only by stand height at the time of treatment. RBS decreased by 4–6 per cent in whorls located below the current base of the living crown with each step increase in stand height at pre-commercial thinning. The results indicate that the number of stems after pre-commercial thinning has a greater impact on tree size and shape than timing of pre-commercial thinning. Stand density after thinning also has a greater impact on branch size, in the sense that it will affect branch size further up the tree than the timing of pre-commercial thinning. However, by delaying the time for pre-commercial thinning it is possible to reduce the RBS in the lower part of the stem. This might be an important measure to improve timber quality later in the rotation period, which cannot be done solely by regulating stand density.