Importance of the first thinning in young mixed Norway spruce and European beech stands

Experimental results from the first thinning in mixed stands are not broadly experienced by forestry practice. To extend the experience with the thinning of a mixed stand, we studied thinned and unthinned mixtures of Norway spruce with European beech on two study sites in the Czech Republic, which represented different conditions: Všeteč (age of 19-35 years) - originally beech dominated site at 440 m a.s.l. and Deštné (age of 17-33 years) - originally spruce with beech site at 990 m a.s.l. Spruce and beech were mixed individually or in small groups. As the for number of trees, mixtures were 35-54% beech and 46-65% spruce at a lower altitude and 7-30% beech and 70-93% spruce at a higher altitude. In the period 1997-2013, we observed annually: mortality, diameter at breast height of all trees and height of trees (minimum 30 individuals) that represented diameter distribution. Results showed that the growth and development of young mixed spruce/beech stands were positively influenced by the first pre-commercial thinning on both locations. The most pronounced effect of thinning consisted in a decreased amount of basal area of dead trees. On control plots, salvage cut accounted for 34 and 46%, while on thinned plots it reached only 7-8% (thinned from above) and 18% (thinned from below) of basal area periodic increment during the 16-year study period. In contrast, diameter distribution was still relatively wide (i.e. an important amount of thin trees was left) at the end of observations on all plots of both study sites. Thinned stands also showed the better static stability (expressed as an h/d ratio) of dominant spruces compared to unthinned stands on both locations. Additionally, thinning supported the spruce share at a lower altitude and the beech share at a higher altitude.