Development of Norway spruce dominated stands after single-tree selection and low thinning

The study included 23 stands (at least 2 ha each in size) distributed from southern to northern Finland. These Norway spruce (Picea abies (L.) Karst.) dominated stands grew on fertile (Oxalis-Myrtillus and Myrtillus site types) mineral soils. Each stand contained two substands randomly treated with single-tree selection or low thinning. The harvested volumes (trees > 9 cm) varied greatly but averaged 94 m(3).ha(-1) in the former consisting of mainly medium-sized and larger (>15 cm) trees and 68 m(3).ha(-1) in the latter of mainly medium-sized and smaller (<15 cm) trees. After treatment, mean diameter at breast height (DBH), basal area, and stand volume were 12-17% lower in single-tree selection than in low thinning. The stem distributions were reverse-J shaped and bell shaped, respectively. During the monitoring of a mean of 11 years, about one cutting cycle in single-tree selection, stand volume (trees > 9 cm) increased 38% in single-tree selection and 27% in low thinning. The respective current annual volume and relative increments were 5.4 (3.6%) and 4.6 m(3).ha(-1).year(-1) (2.4%). In 18 (volume) and in 22 (relative) of the 23 plot pairs the increment was higher after single-tree selection than after low thinning (p values 0.013 and <0.001, respectively). Single-tree selection plots additionally included 1300 saplings/ha (from breast height to DBH 9 cm) after cutting, with the transition of 80 saplings/ha into larger trees and with the ingrowth, mainly spruce, of 170 seedlings/ha into saplings during monitoring.