Can precommercial thinning be used to create more diversity after planting spruce?

Abstract: Mixed forests will harbour other ecosystem services than a planted coniferous monoculture. This study used early results from a long-term experiment to examine if different precommercial thinning treatments can be used to increase diversity in planted stands of Norway spruce (Picea abies) mixed with high densities of natural regeneration of birch (Betula spp.). Precommercial thinning treatments are standard in the Swedish even-aged clear-cut silvicultural system and are used to shape the future commercial thinning stand. Three different precommercial thinning strategies were applied as treatments in planted stands of Norway spruce (Picea abies). Used selection strategies were I) spruce monoculture with an even diameter distribution and the largest stems, II) mixed stand with an uneven diameter distribution and only the largest 1/8 of stems, combined with the smallest trees and III) mixed stand an even diameter distribution and the largest stems. The treatments were tested with densities of 1200 and 1500 stems hectare-1 after treatment. Investigated parameters after treatment were quadratic mean diameter, diameter distribution, mixed species ratio and the development over two years after treatment. The experiment was established on two sites with three blocks on each site. Precommercial thinning was performed in 0.08-hectare sized plots with a buffer zone of 5 m. All trees within the treatment plot were measured directly after precommercial thinning and two years after treatment, as a part of a long-term experiment. Measurements were taken by cross-calipering all trees at 1.3 m and registering tree species. Sample trees were selected from the whole diameter distribution and used for height measurements. Results showed that increased stand diversity could be created through a mixed species strategy in precommercial thinning. The diameter and height distributions were largely unaffected by treatment. Planted spruce had a larger diameter than the naturally regenerated birches, which were more slender and less dominant. Overall, the results were in line with previous studies in southern Sweden. The study was concluded with practical applications and suggestions for future research.