Thinning of Scots pine and Norway spruce monocultures in Sweden

The effect of thinning intensity, thinning interval, thinning form and timing of the first thinning on stand level gross- andnet stem volume production in Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L. Karst) was investigatedin a nationwide field experiment across Sweden. In total, 35 Scots pine sites distributed from the south to the north ofSweden and 13 Norway spruce sites located in the south and central parts of Sweden were investigated. Thinning treatments ranged from unthinned control, to light and moderate repeated thinnings, to a treatmentwhere 60-70% of the basal area was removed in a single thinning. In addition, thinning from above was compared tothinning from below and delayed first thinning was compared to early first thinning. The average measurement period was31 years for Scots pine and 30 years for Norway spruce. All Scots pine thinning treatments reduced the total gross stemvolume production compared to the unthinned control, whereas only the heaviest thinning treatments, in which a largeproportion of the basal area was removed, reduced the total gross stem volume production for Norway spruce. Thinningfrom above did not affect total gross stem volume production of Scots pine, but there was a tendency towards lowerproduction in Norway spruce. For Norway spruce, thinning from above resulted in lower net stem volume production thanthinning from below. Delaying the first thinning did not affect gross stem volume production for either Scots pine orNorway spruce. Net volume production and volume production in trees with diameter at breast height > 8 cm was higher forthe light thinning treatment than for the unthinned control in Norway spruce. In Scots pine, there was no differencebetween the light thinning treatment and unthinned control in net volume production. For cots pine, the heavy thinningtreatments decreased net volume production compared to the unthinned threatment whereas there were no differences innet volume production between the heavy thinning treatments and unthinned control in Norway spruce.