Tillväxtreaktion på kvarlämnade träd i Hagners "Naturkultur" försök
The primary goal of this study was to investigate how the radial growth of retained trees responds after thinning-from-above. The National Board of Forestry which initiated this study, was interested in whether there were differences in the growth-response between the tree species Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.). Another question of issue was if there were differences in the growth response between trees of different sizes. It is within the framework of the National Board of Forestry?s, Continuous Forests Project that these urgent questions are to be answered. Näslund (1942) saw as a result in his study that the growth response became delayed in suppressed trees after thinning. This is a theory which is tested in this study. Consequently his results are confirmed.
In total 235 drill cores were analysed. Of those, 196 were Norway spruce and 39 were Scots pine. The drill cores came from five different stands with dispersal from Härjedalen to Västerbotten. The southernmost location was in Härjedalen, (latitude 61,37º) and the northernmost location, in Västerbotten, (latitude 64,52º). The drill cores were collected from two different treatments where a different percentage of the basal area was removed. The treatments are called ?sparse? and ?dense? respectively. In the treatment called sparse, the percentage of basal area removed was 79,7% and in dense 43,6%.
Apart from answering the questions of growth response for different tree species and different tree sizes after thinning from above, the main hypotheses in this study have been that:
? The growth response is less delayed on more fertile sites than on poorer sites.
? Scots pine responds to thinning faster than Norway spruce.
? The delay in growth response is longer for small trees which have been more suppressed than for the big trees.
? A substantial removal of the basal area gives a shorter delay of the growth response.
Since the studied stands have very similar site conditions, the hypothesis about growth response delay on rich sites compared to poor sites could not be tested.
The results have not indicated any difference in delay of diameter growth response between spruce and pine. The delay is for both species two years, only in the fourth year is the increase in growth significant. This is in agreement with Näslund´s (1942) results. Pine reaches a higher growth rate quicker than spruce. Generally pine reaches a new higher and relatively stable growth rate seven years after thinning. During the eleven years compared in this study, spruce has shown a continuously increasing growth rate after the delay period. No stagnation has been seen except for the thickest trees, where the spruce, six years after liberation reaches a new higher growth rate which remains relatively constant.
In the comparison between big and small trees, no difference was seen. Before this study started there was a hypothesis that the delay would be longer in smaller trees. However this study has shown that it is not the case. The smallest diameter class has for both species shown the biggest growth response in comparison to the bigger diameter classes. This fact is independent of the treatment (thinning intensity).
In this study, a correlation was seen between a high percentage of basal area removed and a high relative basal area growth in all diameter classes independent of tree species. However, no significant indication has been found that the thinning response delay should be less when a higher percentage of basal area is removed. For spruce however there is a tendency that the abovementioned hypothesis could be true, but this tendency is not statistically significant.
In the treatment sparse, the biggest growth response was found for individual trees, despite that, growth at stand level was higher in treatment dense. Before thinning, basal area growth of the unthinned stand was 0,33 m²/ha and year. During the eleven years after treatment, the production of the treatment sparse, (80% of the basal area removed), was 36% of the growth of the unthinned stand. In treatment dense, (44% of the basal area removed) the corresponding number was 67%.