Tree-level differences in Norway spruce and Scots pine growth after extreme thinning treatments

We studied tree-level dynamics (stem slenderness and growth) in spruce- and pine-dominated stands after intermediate commercial thinning of different type (low, crown normal, and crown strict) and intensity (standard and extreme), for two 5-years growth periods. Thinning treatments were included in modeling as numerical variables based on how they changed the stand- and tree-level conditions (in terms of mean diameter of remaining trees and basal area of larger trees removed). Stem slenderness significantly decreased with time for both species and all types only in the extreme intensity (excluding low thinning for pine). Regarding basal area growth, for both species it was slightly higher in low than crown thinning, and much higher in extreme intensity for all thinning types. Pine had a lower basal area growth in the second period after thinning compared with the first one. Height growth differences were not found across treatments. Concluding, extreme thinning increased individual tree basal area growth and decreased stem slenderness for both species compared with thinning carried out according to the standard guidelines. Across types, there were only small differences, hence crown thinning seems a viable option to the widely used low thinning in Fennoscandia.