Effects of thinning regime on the wood properties and stem quality of Picea abies

Seven thinning trials in stands of Norway spruce (Picea abies (L.) Karst.) planted on highly productive sites in southern Sweden were investigated. Cross-sectional weighted basic density was not affected by a thinning regime characterized by several thinnings from below compared with naturally thinned stands. Heavy thinnings decreased basic density. However, the decrease was moderate compared with those reported in earlier investigations, probably owing to the lower stand density prior to first thinning which affected competition between trees and crown differentiation. Thinning from above resulted in a higher basic density, mainly because trees with decreased growth ring development up until the time of the first thinning were favored. Dry matter production was not decreased any more than volume production by any of the treatments except for the very heavy thinning from below. Generally, thinning decreased the juvenile wood content of the stand. As a consequence of selection, thinning from above will decrease the juvenile wood content more compared with thinning from below, provided that trees reach the same diameter in the final stand. Branch diameter in the lower part of the stem was found to be higher in the naturally thinned stands than in the thinned ones. This appeared to be due exclusively to selection since the live crown probably had already started receding upwards prior to the first thinning. Differences in stem taper between thinning regimes could be attributed to changes in growth allocation and effects of selection between trees.