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

Andreas Brunner,David I Forrester

doi:10.1016/j.foreco.2020.118334

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.

Highlights

Introduction data sourcesIndividual-tree-based approaches for the analyses of growth records and experimental designs have been proposed (Kelty and Cameron, 1995; Vanclay, 2006a) and been applied frequently for other objectives (e.g., Canham et al, 2004)
We only found two studies that have used tree growth models to analyze the interaction between stand density and mixture effects
Growth model predictions for beech growing in pure beech neighborhoods show that potential growth only declines little even for the largest trees, as can be expected for basal area growth models (Fig. 5)

Summary

Introduction

Introduction data sourcesIndividual-tree-based approaches for the analyses of growth records and experimental designs have been proposed (Kelty and Cameron, 1995; Vanclay, 2006a) and been applied frequently for other objectives (e.g., Canham et al, 2004). Stand density effects on productivity of mixed stands have not been addressed explicitly with these approaches, except for a few examples presented below. We are still lacking a quantitative empirical understanding of these effects, which are essential for managing mixed species forests. Young stands are most often established at densities far below maximum density and intraspecific as well as interspecific competition between trees only starts as the canopy closes and intensifies as the density increases. Few stands are managed to develop along the self-thinning line, which would indicate maximum competition and significant interspecific effects on total productivity. Maximum stand density for mixed species stands varies with site quality (Condes et al, 2017; Ducey and Knapp, 2010) and stand structure and is most often unknown

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