Growing Density and Thinning Models for Betula Pubescens Stands on Peatlands in Northern Finland

1991

36 S.

27 Lit. Ang.

Bandaufführung

2628

37278

Introduction - A lot of Betula pubescens-dominated stands in Finland are to be found in Ostrobothnia and the southern parts of Lapland. There are about 500 000ha of Betula pubescens stands growing on peatlands in this region. It has been estimated that the annual need for thinnings in hardwood-dominated stands in the whole of Finland is around 30 000ha (Kuusela et al. 1983 and 1986). The growth of Betula pubescens has earlier been studied in Finland using temporary sample plots. Keltikangas & Seppaelae (1977), Saramaeki (1977 and 1982) and Ferm (1990) carried out studies on peatlands. Koivisto (1959) and Gustavsen & Mielikaeinen (1984) carried out studies on the growth of both Betula pubescens and Betula pendula on mineral soils in southern Finland. The study in hand differs from those mentioned above in that it consists of material from controlled, permanent experiments. The aim of this study was to determine the effects of stand density on the growth, commercial timber production and crown development of Betula pubescens stands. The results were used to compare different thinning programmes. The thinning models for Betula pubescens stands on peatlands are presented. Material - The eight thinning experiments (115 plots) used as the material were established on fertile peatland sites in northern Finland during the 1970s (Table 1). The tree stands were thinned for the first time when the experiments were established, and they were measured at five-year intervals (Table 3). The experimental design consists in most cases of 5 growing densities with a number of replications. Thinning intensity was determined on the basis of stem number (Table 2). Breast height diameter (d) was measured on all the trees, and height (h) and upper diameter (d6) on the sample trees (about 40/plot). Height development of Betula pubescenc - The development of dominant height in stands was in good agreement with the site index curves published by Saramaeki (1977) for Betula pubescens (Fig. 1). Dominant height at an age of 50 years varied between 12m and 18m. The first thinning was done in the youngest stand at a dominant height of 7m, and in the oldest stand at 14.5m. Heavy thinning decreased dominant height growth (Fig. 2). Thinning had no immediate effect, but during the second 5-year period the dense birch stands grew 8-15cm/a more than those with a low growing density (below 1000 stems/ha). Thinning had no marked effect on the increase in mean height. Volume growth and commercial wood production - The volume growth in the young stands was highest in the unthinned or very lightly thinned ones (N groesser gleich 2500 stems/ha, Fig. 3a). Heavy thinnings caused growth losses of as much as 2-3 Kubikmeter/ha/a. The transition to pulpwood size class (length 1=3m, min. diameter dt=6,5cm with bark) was greater than the volume growth. In the older birch stands over 13m in ..