Effect of Thinning Method on the Yield of Middle-aged Stands in Southern Finland

1991

21 S.

6 Abb., 9 Tab.

Bandaufführung

2628

37286

In Finnland, forest stands have been thinned almost exclusively according to the principle of thinning from below, i.e. removing trees mainly from the lower crown classes. However, previous studies suggest that volume increment would be at least as great or even slightly greater if thinnings from above were applied. This would allow for more flexible thinning patterns and an increase in returns from thinnings. The purpose of this study is to compare the effect of these methods on increment and yield of stands in terms of volume and returns. The research material consisted of 10 stands in Southern Finland. The dominant tree species was in seven cases Scots pine (Pinus sylvestris). Norway spruce (Picea abies), Silver birch (Betula pendula) and Pubescent birch (Betula pubescens) were dominant in one case each. Stand ages at establishment of the experiments ranged from 40 to 90 years (Table 1) . The stands were treatet with one to four thinnings of relatively low intensity during 20-24 years, preceded by thinnings from below in the first half of the rotation (Table 2). In thinnings from below, trees were removed mainly from the lower crown classes. In thinnings from above, some dominants and codominants in addition to suppressed individuals were removed. Trees that did not yet include sawtimber but were expected to reach the required minimum diameter soon, if released, were favoured in the treatment. In each stand, both methods were applied on an equal number of plots. In both treatments, the remaining basal area was equal. When thinning from above, the remaining trees (mostly former codominants) of the conifer stands responded to release with a greater relative increase in diameter increment, which resulted in a greater basal area increment of the stand (Table 3). No difference between treatments was observed in the increment of dominant height. However, each thinning from above reduced the dominant heigh of stand by 0.1-0.7 m due to the removal of a number of dominant trees. Stem form developed more or less similarly in both treatments (Figure 3). Consequently, a greater volume increment was obtained in the thinning from above in the pine stands (Figure 4, Table 3). The difference had increased since previous assessments, which suggests a more sustained response in the thinning from above. In the spruce stand, volume increment was about the same in both treatments, whereas in the birch stands, thinning from below was substantially more advantageous. Total yield during the experiment of the pine stands thinned from above was greater than the yield of the stands thinned from below. In the spruce and birch stands, thinning from below resulted in a greater total yield (Table 5). However, the production of sawtimber volume was not enhanced by the application of thinning fromabove, as had been initially anticipated. Furthermore, the yield ..........