Tree mortality in Pinus sylvestris stands in Sweden after pre-commercial thinning at different densities and thinning heights

Abstract

Individual tree mortality was analysed for 9924 Scots pine trees from 26 pre-commercial thinning (PCT) experiments established in Sweden (57°–66° N) between 1953 and 1972. Almost all of the stands examined were naturally regenerated and homogenous pine stands. The PCT treatment factors included stand density and height at the time of PCT. The stand densities varied between 600 stems ha −1 and >9000 stems ha −1 before PCT. The height at the time of PCT varied between 1 and 8 m and mortality was examined 8.6–23.5 years after PCT. The results showed low overall mortality (<5%) irrespective of treatment. Mortality was higher in stands with high stem number before PCT than in stands with low pre-PCT stem number. In addition, higher mortality was associated with early PCT (heights below 4 m) than with late PCT (heights above 4 m). Mortality after PCT was not clearly correlated to stand densities; the highest mortality was found for stand densities of 0–700 stems ha −1 and the lowest for 700–1500 stems ha −1. Furthermore, mortality was higher for small trees than for average height or tall trees. The main conclusion from this study, therefore, was that timing of PCT and thinning grade has only marginal effects on the risk of mortality.