Growth–density relationship in mixed stands – Results from long-term experimental plots

Abstract

Forest management mainly controls wood growth through the regulation of stand density. Knowledge of the growth–density relationship is based on numerous studies in pure stands. Currently, in times of more climate plasticity and more nature-oriented forests, silviculture with mixed stands is demanded of forest policy. It is well known that the competition mechanisms in mixed stands are different to those of pure stands. Hence, the question arises for forest managers whether the regulation of density must be different, too.In an attempt to answer this question, we investigated 481 stand surveys from 124 long-term experimental plots of European beech stands, mixed with conifers and also broadleaf trees. With generalized additive models, we estimated the influence of the density on stand volume growth and how it changed with stand development and shade tolerance of the admixed species. The density of the mixed stands was given on a relative scale. 100% density took into account the maximum species-specific density plus a mixed stand specific addition.We found that mixed stands have a constant growth level in the medium- to high-density phase (70–100%). Although we have tried to take into account the higher density in mixed stands, 10% of the stands outperformed the 100% benchmark and exhibited a growth gain in this over-dense phase (>100%). Finally, the growth-density response resembles a cubic function with a saddle or optimum between medium- to high-density phase and an increase of growth in the low-density and the over-density phase. The inclusion of stand development led to the oblation of the growth–density response in younger ages. The admixture of shade-intolerant species with beech caused a more intensified cubic course of the relationship.Finally, we could show that a short-term decrease of the density also has an effect on growth. In terms of growth, weak thinnings are better than severe thinnings. Thinnings from above have a significant positive effect on growth, especially for longer observation periods of up to 10 years.We concluded that mixed stands need an adapted thinning compared to pure stands. Regular, weak thinnings from above with a very high stand density enables maximum volume growth gains in European beech mixed stands. We could show that the growth-density relationship is conceptually different to pure stands, because mixing effect causes density section above 100%. However, the study emphasizes the need for a systematic experimental setup for a better understanding of intraspecific interactions at different densities.