Scenario analysis of planting density and pre-commercial thinning: will the mixed forest have a chance?

Abstract

In the establishment phase, several management alternatives could be applied that affect the species composition of the future stand. In this study, tree seedling data from experiments with planted Norway spruce (Picea abies L. Karst) and naturally regenerated species, mainly Silver birch (Betula pendula Roth), were used for six simulated pre-commercial thinning (PCT) scenarios. The scenarios included both monocultures and mixed stand alternatives with various initial spacing of the planted Norway spruce (0, 1600 or 2800 seedlings) and selection of main stems based on relative heights. Further stand development and individual tree growth were simulated until final harvest. Potentially, based on findings of the seedling measurements, the stands could be mixed with five to six species, but the browsing pressure from ungulates reduced this possibility since the height growth for all species except Norway spruce was highly affected by browsing. The simulated mixtures had a small variation between the PCT scenarios for the maximum mean annual increment (about 10 %). Although the growth was similar for Norway spruce, the proportion of birch in the final stands differed from 3 to 21 % between treatments and none of the highdensity planting treatments generated a mixed Norway spruce-birch forest at the time of final felling.

Keeping mixtures of Norway spruce and birch in production forests: insights from survey data

“…Secondly, the growth rate of the individual trees is slower than that of the surrounding Norway spruce. These findings indicate that the birch proportion in mixtures demands active management in order to retain the mixture over the full rotation, which is a finding likewise supported by independent results from experiments and scenario analysis (Fahlvik et al 2015;Holmström et al 2015;Holmström et al 2016b;Huuskonen et al 2021). Active management to preserve spruce-birch mixtures may involve heavier thinning in Norway spruce stands, reducing the overall competitive pressures in the stand.…”

“…This is the development stage at which time the land owner decides whether or not to keep and maintain a mixture in the future stand (Agestam et al 2006;Felton et al 2016;Holmström et al 2015). Among the many decisions taken during PCT is not only the choice and proportion of tree species to retain, but also the magnitude of competitive release provided by selecting the size and stem density of the retained tree species (Fahlvik et al 2015;Holmström et al 2016b). Forest experiments with the specific aim to evaluate the growth of spruce-birch mixtures provide strong evidence that density of the stand, after PCT and the height relation between the tree species, will affect the possibility to keep both species in the mixture over a full stand rotation (Fahlvik et al 2005;Fahlvik et al 2011;Fahlvik et al 2015;Holmström et al 2015).…”

Within-Site Variation in Seedling Survival in Norway Spruce Plantations

“…Applying other soil scarification methods could be one solution [18,19]. Alternatively, and perhaps more likely, use of other tree species, either naturally regenerated [24,25] or planted Scots pine or broadleaves, could potentially provide solutions that reduce establishment costs and enhance ecosystem services [26].…”

Importance of the first thinning in young mixed Norway spruce and European beech stands

“…Active management strategy at an early stage is essential also for the creation of mixed stands (Holmstrom et al 2016). Spellmann, Nagel (1996) recommended early, more or less heavy crown thinning for spruce-beech mixtures according to today's tending targets.…”