Mixing degree, stand density, and water supply can increase the overyielding of mixed versus monospecific stands in Central Europe

Tree species mixtures can overyield monospecific stands and provide many other advantageous regulating and provisioning ecosystem services. So far, the effect of mixing on growth were mostly described at the individual tree level or cumulatively at the stand level. How overyielding emerges from the mixing pattern, how it is modulated by the current environmental conditions, and how overyielding develops with progressing stand age is largely unexplored. However, such knowledge might promote the silvicultural design of mixed stands. Here, we use 50 long-term experimental plots in Germany with repeated spatially explicit stand inventories. They cover monospecific and mixed species stands of Norway spruce (Picea abies [L.] Karst.) and European beech (Fagus sylvatica L.), sessile oak (Quercus petraea [Matt.] Liebl.) and European beech, and Scots pine (Pinus sylvestris L.) and European beech. The stands represent medium to very good site conditions, fully to moderately thinned 15–238 years-old stands with mixing patterns ranging from single-tree and group-mixture to monospecific stands. The main hypothesis is that mixing pattern, environmental conditions, and stand age modulate the stand growth and overyielding of mixed versus monospecific stands. Based on metrics of mixing the mixing pattern and indices of environmental conditions we show that mixed stands of European beech were on averaged by 12–36% more productive than respective monocultures. Overyielding can increase with stand density, mixing degree, regularity of horizontal tree distribution, tree size variation, site index, and water supply. Overyielding continued throughout the whole rotation time. The found overyielding suggests general facilitative effects of mixing in terms of bioclimate or soil conditions, independent of the mixing pattern. In addition to this general effect, overyielding was increased by stand density and mixing degree. This suggests a strong competition reduction due to structural complementarity. That the overyielding increases in moist years, points to the competition reduction as dominant cause. We hypothesize that in moist years competition for light and nutrient becomes stronger, so that species complementarity is more important and competition reduction may operate stronger and more beneficial for growth than in years with growth limitation by water. The findings stress the relevance of spatially explicit approaches of analyzing and modelling facilitation and competition at the stand and tree level. Silvicultural prescriptions may incorporate the findings that the analyzed mixtures with European beech in general overyield monospecific stands and that the benefit can be further increased by stand density, mixing degree, and tree size variation. Beneficial effects of mixtures can be expected on medium to high quality sites, in dry years, and even more in moist periods.