Effect of half-systematic and systematic thinning on the increment of Scots pine and Norway spruce in Finland

Diameter, height and volume increment of Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) stands were investigated in long-term thinning experiments in southern Finland. The measurement period was on average 19 years, and thinnings consisted of five different treatments: (1) unthinned; (2) selective thinning from below where thinning intensity was determined according to the number of stems; (3) selective thinning from below where thinning intensity was determined according to stand basal area; (4) systematic thinning where corridors were opened at regular intervals in the stand; and (5) half-systematic thinning where corridors were opened at regular intervals and the remaining strips thinned from below, i.e. a combination of selective and systematic thinning. The selective (basal area) thinning of Scots pine resulted in a volume growth reduction of about 12 per cent, but in Norway spruce it resulted in no marked reduction compared with the unthinned plots. In both tree species, volume increment in the half-systematic and selective (basal area) thinning differed only slightly from each other. The systematic thinning resulted in the lowest volume increment. In the systematic thinning, the total yield (m(3) ha(-1)) somewhat decreased with increasing corridor width. In half-systematic thinning, on the other hand, the total yield was not clearly related to corridor width. A corridor accelerated the diameter increment of the edge trees. The edge effect progressively diminished when moving deeper into the strip and ended at a distance of about 3-4 m from the corridor edge. After the thinning, the height increment first decreased, but later on it recovered. The unthinned plots had more dead trees than the other treatments, but there were no significant differences between the other treatments. It can be concluded that the prevailing thinning method in the Nordic countries, i.e. half-systematic thinning, most probably results in no significant growth and yield losses.