Questions: We aim for a better understanding of the different modes of intra- and inter-specific competition in two- and three-species mixed forests. How can the effect of different modes of competitive interactions be detected and integrated in individual tree growth models? Are species interactions in spruce-fir-beech forests more associated with size-symmetric or size-asymmetric competition? Do competitive interactions between two of these species change from two- to three-species mixtures?

Location: Temperate mixed species forests of Norway spruce, silver fir and European beech in Central Europe (Switzerland).

Methods: We used data from the Swiss National Forest Inventory to fit basal area increment models at the individual tree level including the effect of ecological site conditions and indices of size-symmetric and size-asymmetric competition. Interaction terms between species-specific competition indices were used to disentangle significant differences in species interactions from two- to three-species mixtures.

Results: The growth of spruce and fir was positively affected by increasing proportions of the other species in spruce-fir mixtures, but negative effects were detected at increasing presence of beech. We found competitive interactions for spruce and fir to be more related to size-symmetric competition, indicating that species interactions might be more associated to competition for belowground resources. Under constant amounts of stand basal area, the growth of beech clearly benefited from the increasing admixture of spruce and fir. For this species, patterns of size-symmetric and size-asymmetric competitive interactions were similar, indicating that beech is a strong self-competitor for both aboveground and belowground resources. Only for silver fir and beech we found significant changes in species interactions from two- to three-species mixtures, but these were not as prominent as the effects due to differences between intra- and inter-specific competition.

Conclusions: Species interactions in spruce-fir-beech, or other mixed forests, can be characterized depending on the mode of competition, allowing interpretations whether they occur mainly at above- or belowground level. Our outcomes illustrate that species-specific competition indices can be integrated in individual tree growth functions to express the different modes of competition between species, and they highlight the importance of considering the symmetry of competition alongside with competitive interactions in models aiming at depicting growth in mixed-species forests.