Effect of mixture of Norway spruce and European beech on the total above ground dry mass production MEAW in dependence on mixture proportion of beech (m_be) and site index (h₀) of spruce and beech. (a) Occurrence of over- and underyielding of the mixed stand depending on the site index of the involved species and (b) growing overyielding of the mixed stand with decreasing site index of Norway spruce.