Long-term effects of different thinning intensity in young Scots pine stands

The paper describes a case study of thinning experiment established in 1976 with a use of random block design in 21-year-old Scots pine stand in Janow Lubelski Forest District (south-eastern Poland). The stand was subjected to no thinning (control variant K), weak thinning with removal of 10% of basal area (variant S1), strong thinning with removal of 30% of basal area (variant S3) and extreme thinning, where only future crop trees and some reserve trees were left (variant TE). S1 and S3 treatments were repeated each 5 years, while in TE treatment the 20-year interval was applied. During the period of 40 years all trees were measured each 5 years and current basal area increment was calculated for the whole stand and for future crop trees population. The relative increment of future crop trees population calculated as a percentage of stand total increment was taken into account too. Data were subjected to ANOVA and differences were compared with Tukey test. The results showed that total stand increment was not influenced by thinning intensity. The future crop trees increment was significantly higher in a case of extreme thinning in comparison to weak thinning and to the control treatment during the first 20 years of experiment (age 21-41). During the second 20 years (age 41-61) only relative basal area increment of future crop trees population was significantly higher on the plots with extreme thinning in comparison to other treatments. The results suggest that a weak thinning brings no silvicultural effect meant as increased growth of previously selected future crop Scots pines. Strong or very strong thinnings give a long-term growth effect and they can be acceptable, but in healthy and well-tended young pine stands only.