Aboveground productivity of pure and mixed Norway spruce and European beech stands

The aim of this study was to compare increment in mixed stands to the respective pure stands adopting, as an example, mixed stands comprising Norway spruce (Picea abies [L.] Karst.) and European beech (Fagus sylvatica L.). Using data from two mixed stand age series, in the Solling and in Harz Mountains, first we investigated whether the growth reduction effect arising from competing trees is dependent on the species of the competitors. We than explored whether increment predictions from the forest growth simulation package TreeGrOSS revealed differences between pure and mixed stands and, if so, what factors play a role and what may cause these increment differences. The analysis was based on a simulated trial series in which the parameters such as percentage mixture, age, site quality class, and stocking level were varied. The results indicate that neighboring Norway spruce restrict the basal area increment of both European beech and Norway spruce more than European beech competitors with the same crown cross-sectional area. In the majority of cases, the simulation produced an increment gain in mixed stands compared with pure stands. The amount of increment gain in a mixed stand depended on the percentage mixture, age, site quality class, and stocking level and could be related to changes in stand structure and their effects on aboveground competition.