Silvicultural measures to increase the mechanical stability of pure secondary Norway spruce stands before conversion

This study evaluates the effect of two different thinning treatments (2 and 3), and a control without thinning (1), on stand stability of secondary even-aged Norway spruce stands, in relation to the main risk factors of snow and wind, which should be considered in the period of stand conversion. Treatment 2 is a heavy thinning at top heights of 10, 12.5 and 15 m; treatment 3 starts with the first heavy thinning at the top height of 10 m, but the second and third treatments are delayed till a top height of 20 and 22.5 m are reached. The experimental stands are in secondary Norway spruce forest growing on a site considered unsuitable for that species and especially at risk from snow and storm damage. The investigated thinning variants significantly influenced the stability of the experimental stands. Both thinning treatments encouraged diameter increment and therefore their h/d ratio reached lower levels than the control. In treatment 2, the h/d ratio stabilized in the period of intensive treatment at around 80; i.e., it is the most suitable treatment from the viewpoint of stem-break resistance. Treatment 3 did not stop h/d ratio increase, but slowed it compared to the control variant without thinning. Subsequently the later interventions at the top heights of 20 m and, especially, 22.5 m stopped the increase of the h/d ratio and kept it under the critical value of 90.