Development of crop trees after different thinning methods in mixed Norway spruce (Picea abies L.) and Birch (Betula spp.) forests in Southern Sweden

In Northern Europe, novel thinning methods of small diameter trees are being tested to promote early competition release in young tree -dense forests at a low cost. These methods involve harvesting the whole aboveground tree biomass in 1 - 2 m wide strips between extraction racks (i.e. strip roads). Before recommending such methods to practical forestry, we need to know how the growth of remaining stand and individual trees perform at different stages of stand rotation to avoid compromising future forest production and the overall economy. Therefore, we analysed data from three Norway spruce dominated experimental sites in Southern Sweden with conventional selective biomass thinning (BT) versus geometrical BT in strips, where trees between extraction racks are cut in 1 - 2 m wide, ca. 10 m long strips (i.e. boom -corridor thinning) including unthinned reference treatment. To assess the thinning effect on the remaining stand, we analysed data from the remaining trees five to six years after thinning. We compared stand properties such as standing volume, diameter, height, basal area, volume increment, basal area increment and damages for different cohorts of the remaining trees. We found that BT in 1 - 2 m wide strips do not affect the growth and yield of the most important crop trees (i.e. the 500 -largest crop trees and the next 400 -largest crop trees) and to some extent protect the remaining stand from damages. Although not statistically significant, we observed that for the cohort of the 500 -largest crop trees, the proportions of new damages were lower in unthinned treatment (4.8 %), and strict BT in 1 - 2 m wide strips (1.7 % and 5.8 % respectively) than in conventional BT (14.4 %) and semi -selective BT in 1 m wide strips (7.9 %). A similar pattern was seen for the next 400 -largest crop trees. Thus, these short-term results suggests that BT in narrow strips of 1 - 2 m can be used in young tree -dense forests in Southern Sweden without compromising the development of the remaining crop trees. However, more long-term research on a broader geographical scale is required before we can fully recommend these novel -thinning methods to forest owners across the Northern Europe on a large scale.