Mixing effect between Norway spruce and European beech in dependence on site conditions indicated by site index. (a) Relative mixing effect MERW for Norway spruce depending on the admixture of beech (m_be) and site index of Norway spruce (h_o), and (b) Relative mixing effect MERW for European beech depending on the admixture of spruce (m_sp) and site index of European beech (h_o). MERW = 1.0 indicates equal growth for mixed and pure stands, MERW > 1.0 and MERW < 1.0 over- and underyielding, respectively.