Abstract 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 10-m 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.

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树种混合对茎生长的影响随林分密度而变化——基于个体树木反应的分析

摘要 混合物种林分的超产预计会随林分密度而变化，但只有少数研究对此进行了量化。我们使用了来自瑞士 83 个地点的永久样本地块的个体树木生长数据，代表挪威云杉、银冷杉和欧洲山毛榉之间的三种混合物。所有三个物种的基础面积增长模型表明，竞争指数 (CI) 与附近物种组成之间存在显着的相互作用。空间显式 CI 表示 10 米半径邻域内的林分密度，单个物种对 CI 的贡献表示物种组成。鉴于样地中丰富的垂直林分结构，通常通过单树选择切割进行管理，CIs 和物种比例之间的相互作用随着相对树高的变化而变化也就不足为奇了。我们使用单个树木生长模型来模拟有和没有物种组成影响的林分生长。就树木大小和位置而言，具有真实林分结构的样地在树木级别的物种组成方面被均质化。通过改变该图的林分密度，我们展示了超产如何随林分密度增加。在给定的混合物中，主要是山毛榉有助于观察到密集混合林中的超产。因此，在研究混合物种林分的生长和产量时，需要更明确地考虑林分密度。就树木大小和位置而言，具有真实林分结构的样地在树木级别的物种组成方面被均质化。通过改变该图的林分密度，我们展示了超产如何随林分密度增加。在给定的混合物中，主要是山毛榉有助于观察到密集混合林中的超产。因此，在研究混合物种林分的生长和产量时，需要更明确地考虑林分密度。就树木大小和位置而言，具有真实林分结构的样地在树木级别的物种组成方面被均质化。通过改变该图的林分密度，我们展示了超产如何随林分密度增加。在给定的混合物中，主要是山毛榉有助于观察到密集混合林中的超产。因此，在研究混合物种林分的生长和产量时，需要更明确地考虑林分密度。