Facilitation and competition reduction in tree species mixtures in Central Europe: Consequences for growth modeling and forest management

That tree species mixing may strongly affect tree structure and tree growth is so far hardly considered in tree and stand models. Hence, for a better understanding and design of mixed species stands, the inter-specific facilitation and competition needs better representation in individual tree models. Here, we show for the five most common tree species combinations in Central Europe that mixing causes facilitation and competition reduction in mixed stands and how such effects can be implemented in individual tree stem diameter growth models. This study was based on 62 long-term experimental plots belonging to 10 chronosequences in Germany with repeated spatially explicit stand inventories from 1991 to 2016. They covered monospecific and mixed species stands of Norway spruce (Picea abies [L.] KARST.), Scots pine (Pinus sylvestris L.), European beech (Fagus sylvatica L.), sessile oak (Quercus petraea [MATT.] LIEBL.), European ash (Fraxinus excelsior L.), and sycomore maple (Acer pseudoplatanus L.). The study represented medium and high qualitative site conditions, fully stocked, or only moderately thinned stands of 22-238 years of age, and the mixing patterns ranged from individual-tree to cluster-mixtures. Based on spatially explicit measurements, we quantified for each tree the intra- or interspecific neighborhood, local stand density, and growth. We applied mixed models to analyze how inter-specific neighborhood modified tree growth. First, we showed that the inter-specific neighborhood can increase tree growth significantly beyond the level of open-grown trees in intra-specific neighborhoods (net faciltation). The potential growth rates of the fastest growing trees in mixed stands were 14-78% higher compared to monospecific stands matched for size, crown, and site index. The mixing effect differed between species combinations. Second, we provided evidence that mixing on average reduced competition by 16%, and that this effect increased with the mixing proportion. In four out of the five species combinations we observed a competition reduction of both associated tree species (mutualistic relationship). Third, for the mixtures of Norway spruce/European beech and sessile oak/European beech that covered a broader range of site conditions, we found a mainly positive modulation of facilitation and competition depending on site conditions. We discussed the potential causes for the observed facilitation and competition reduction, their implementation in tree growth models, and the relevance of the findings for the design and management of mixedspecies stands.