Improved productivity and modified tree morphology of mixed versus pure stands of European beech (Fagus sylvatica) and Douglas-fir (Pseudotsuga menziesii) with increasing precipitation and age

The mixture of Douglas-fir and European beech produced more biomass compared to what would have been expected from a weighted average of pure stands. Overyielding of the mixed stands improved with increasing stand age and under better site conditions. The mixture of Douglas-fir and European beech has the intrinsic potential to be one of the most productive forest types in Central Europe. The study investigated how the structure and productivity of mixed stands changed in comparison to pure ones. It analyzed whether there is overyielding in mixed stands and if it was modified due to stand development or along an ecological gradient. Throughout Germany, 18 research plot triplets with 1987 trees were established in seven different ecological regions from dry to moist site conditions at ages 30 to 120 years. To investigate the growth of the stands, tree cores were collected from 1293 stems. The study revealed significant overyielding of biomass in mixed stands of 6 % or 0.81 Mg ha(-1) year(-1). It was found that: (i) Overyielding in mixed stands was driven by an increase in Douglas-fir growth. (ii) Both species modified their morphology in mixture. Compared to the species in pure stands, Douglas-fir diameters in mixed stands were significantly larger, whereas European beech had a smaller diameter at breast height in the mixture. The effect increased with the age. (iii) The analyses revealed more pronounced overyielding in older stands and on better sites. The findings show that overyielding of Douglas-fir and European beech in mixed stands results from a higher light interception by complementary space occupation.