Transgressive overyielding in mixed compared with pure stands of Norway spruce and European beech in Central Europe: evidence on stand level and explanation on individual tree level

The temperate, humid climate and nutrient-rich soils in the pre-alpine areas of southern Bavaria represent conditions where European beech and Norway spruce come out with rather equal above ground biomass production when cultivated in pure stands. In order to reveal the effect of mixture we established 37 experimental plots in even-aged pure and mixed stands of Norway spruce and European beech covering an age span of 37-155 years. The site conditions ranged from warm, dry and base-rich to cool, wet and acidic sites. The ratio of above ground biomass growth of Norway spruce in relation to European beech decreases from 1.14: 1 in the monocultures to 1.04: 1 in the mixed stands. The mixing of spruce and beech results in a mutual stimulation of biomass production and acceleration of size growth. Together both species produce up to 59% more above ground biomass than the neighboring pure stands. On average the overyielding amounts to 21% in the case of Norway spruce and 37% in the case of European beech. A total of 67% out of the plots indicate overyielding and 57% transgressive overyielding. In mixed stands both species' tree sizes are significantly ahead of the corresponding pure stands. Facilitation of spruce and competitive reduction of beech yields mutualism with respect to growth on tree and stand level. Consequences for analyzing and modeling interspecific competition and for silvicultural prescriptions are discussed. Ecological implications of the mixing effect on the occurrence and stability of natural and man-made mixed stands of spruce and beech are considered.