Kristina Ahnlund Ulvcrona

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.

Stand density and fertilization effects on aboveground allocation patterns and stem form of Pinus sylvestris in young stands

The increasing demand for forest biomass for energy generation could be partially met by growing denser stands and use of fertilizer. Before this is done at large scale, more knowledge of the effects of stand density and fertilization on aboveground allocation patterns and stem form is needed. Therefore, effects of pre-commercial thinning (PCT) to 3000 stems ha−1, an unthinned dense control (C), and PCT combined with two levels of fertilization (100 kg ha−1 of nitrogen applied either during the establishment of the field experiment (F1) or annually (F2)) were examined in 23- to 26-year-old Scots pine (Pinus sylvestris L.) stands six years after the establishment of the field experiment. In total, 114 sample trees were harvested using destructive biomass sampling. The growth allocation and stem form of trees with diameter at breast height (DBH; 1.3 m height) >5.0 cm were not affected by either the PCT or fertilization. Small trees (DBH < 5 cm) in denser, unthinned control plots had more slender stems (lower DBH/height ratios) and allocated less growth to branches and foliage than trees in PCT plots. Fertilization had little effect on the stem form and growth allocation of the smallest trees. Therefore, effects of stem density and fertilization on stem form and growth allocation to foliage were only found for small suppressed trees, and the treatments had very little influence on dominant and codominant trees.

Identifying the biological effects of pre-commercial thinning on diameter growth in young Scots pine stands

Data from seven Scots pine (Pinus sylvestris L.) stands in central Sweden that had undergone pre-commercial thinning (PCT) were used to investigate the biological effect of PCT on the diameter at breast height (DBH) growth of the remaining trees. Two treatments were considered: a PCT treatment and a control (C) with no PCT. The DBH of the trees in each stand was measured on up to four occasions over 15 years. We examined mean DBH and DBH growth of the largest 300, 600, 900 and 1200 trees ha−1. Two methods of selecting the trees used to calculate the mean DBH values for each measurement occasion were considered: the actual mean DBH (Dma), which is based on the DBH distribution of the trees on the measurement occasion in question, and the genuine mean DBH (Dmg), which is based on the DBH distribution of the trees on the final measurement occasion of the study and therefore focuses on the same set of trees for all measurement occasions. There was no clear difference between the Dma- and Dmg-based DBH increments, but the Dmg values tended to be somewhat larger both for the C and PCT treatments. Over a 15-year period, the relative mean yearly increments (Dma/Dmg) for different tree size classes ranged from 0.91 to 0.98, with lower values for the C treatment and larger DBH size classes. We found that PCT promotes DBH growth: over a 15-year period, the increased growth amounted to about 2.0 mm year−1 compared to the trees in unthinned plots.