Tree species mixture effects on stem growth vary with stand density - An analysis based on individual tree responses

Overyielding in mixed species stands is expected to vary with stand density, but only a few studies have quantified this. We used individual-tree growth data from permanent sample plots on 83 sites in Switzerland representing a three-species mixture between Norway spruce, Silver fir, and European beech. Basal area growth models for all three species indicated significant interactions between a competition index (CIs) and species composition in the neighborhood. The spatially-explicit CIs indicates stand density within a 10m radius neighborhood, and the contribution of individual species to the CIs indicates the species composition. Given the rich vertical stand structure in the sample plots, which were often managed by single-tree selection cutting, it is not surprising that interactions between CIs and species proportion varied with relative tree height. We used the individual tree growth models to simulate stand growth with and without species composition effects. A sample plot with its real stand structure in terms of tree size and position was homogenized in terms of species composition at the tree level. By varying the stand density of this plot, we demonstrate how overyielding increases with stand density. In the given mixture, it is mostly beech contributing to the observed overyielding in dense mixed stands. Stand density therefore needs to be considered more explicitly when studying growth and yield of mixed species stands.