Weiterentwicklung der Dichte- und Mischungsregulierung forstwirtschaftlich wichtiger Baumarten. Äquivalenz-Koeffizienten und Dichte-Steigerungs-Koeffizienten für generische waldbauliche Behandlungsalgorithmen

The study developed the basics for a generic quantitative regulation of the density of mono-specific and mixed-species stands for common tree species. For that purpose, we introduced density-equivalence-coefficients for conversion between species in mono-specific stands and density-increase-coefficients for considering the higher packing density of mixed compared to mono-specific stands. These coefficients enable the density regulation of stands of various tree species and mixtures according to an overarching concept. In this way, the silvicultural regulation of mono-specific stands and mixed-species stands becomes a continuum; mono-specific stands represent a borderline case of mixed-species stands. The introduced concept is suitable for design of silvicultural guidelines and the algorithms for their implementation in simulators for scenario analyses. Based on fully stocked long-term experimental plots in Germany we in a first step derived species-specific relative density values, mean growing areas per tree and mean tree to tree distances for Douglas-fir, European larch, Norway spruce, Scots pine and silver fir as well as for black alder, black walnut, cherry tree, sessile oak, European beech, European chestnut, European walnut, lime tree and red oak (Table 1). The species-specific densities and growing areas enabled the calculation of density-equivalence-coefficients (Table 2) which can be used for comparison and conversion of the densities and growing area requirements of different tree species. We used the density of European beech as reference and show how to convert and compare the density of other species. The density-equivalence-coefficients range between 0.34 for black walnut and 1.74 for Norway spruce and show that black walnut has just a third of trees per hectare compared with beech whereas Norway spruce can have almost double the number of trees per hectare. The density-equivalence-coefficients support the development of density guidelines for different tree species and admixtures. In a second step, the density-equivalence-coefficients were applied for calculating the standardized stand density for mixed stands of European beech/sessile oak, European beech/Douglas-fir, European beech/Scots pine, European beech/ European larch, European beech/Norway spruce, and European beech/Norway spruce/silver fir. The results were used for a comparison of the density of mixed versus mono-specific stands. The ratio between the (on European beech) standardized density of the mixed stands with the density of the European beech monocultures amount to 1.11–1.43 (Table 3) and indicate a higher packing density for all considered mixtures compared with the pure beech stands. The ratios were used as density-increase-coefficients in the subsequent evaluation. In a third step, we used the density-equivalence-coefficients and density-increase-coefficients for the derivation of stand density and thinning guidelines for pure and mixed stands. Starting point is the site-specific maximum stand density (SDI_max) for pure European beech stands. Depending on the species assemblage in the mixed stand (e.g. European beech/Scots pine, European beech /sessile oak, European beech/ European larch), the density-increase-coefficients were applied to estimate the maximum stand density in the respective mixed stand (SDI_maxmix) considering their higher tree packing density. This mixture-specific maximum stand density is used to calculate the potential tree number for European beech or any mixture of interest. Based on the density-equivalence-coefficients it becomes possible to calculate the tree numbers and area proportions of any mixing combination of interest. In addition we show how to define different scenarios of stand density reduction (e.g. early thinning from above, staggered thinning, accretion thinning) and how to calculate the respective possible species-specific tree number and species area proportions. In the discussion, we stressed the conceptual character of this paper: Future works should extend the tree species numbers in pure stands and analyse how the equivalence-coefficients are affected by age, silvicultural treatment, and site conditions. Furthermore, the analyses of the mixed stands density should be developed beyond mixtures with European beech. The implementation of the approach and algorithms in stand simulators will make their application for mixed-species stands more flexible and easy.