Influence of tree species composition on soil and soil solution properties in two mixed spruce-beech stands with contrasting history in Southern Germany

The properties of the soil and soil solution of two mixed spruce-beech forests in Southern Germany were investigated in order to identify non-additive effects of tree species compared to the monocultures. At five subplots in each of the two monocultures and at 10 subplots in each of the two mixed stands humus morphology, topsoil acidity as well as nitrate and sulphate concentrations in seepage water below the rooting zone were measured. At the Höglwald site, an 8×8 m sampling grid was also installed. Tree species composition within a 10 m circle surrounding each sampling point was correlated with the soil properties and the soil solution properties using the method of breakpoint estimation as well as a linear and a cubic model. At both sites, thickness and acidity of the forest floor as well as sulphate and nitrate concentrations in seepage water were significantly higher in the spruce monocultures than in the beech monocultures. Non-linear patterns in the correlation between both the thickness and the acidity of the forest floor and tree species composition occurred at both sites. In addition, nitrate concentration in the seepage water showed a non-linear correlation pattern at the Höglwald site. The correlation patterns were site specific and depended on stand history. While the influence of spruce at the Höglwald site was greater than expected from spruce monocultures, the opposite was found at the Schongau site. Interaction effects on nitrate concentration were absent at the Schongau site. Interaction effects on sulphate concentrations were absent at both sites. Current knowledge about the complex processes and patterns in mixed species stands is still limited. At least for certain properties, mixed species stands cannot be treated as a summation of the corresponding monocultures.