Pure and Mixed Stands of European Beech and Douglas Fir — Effects of Tree Species Composition on Soil and Seepage Water Chemistry.

Because of the climate change, the use of Norway spruce (Picea abies) in German forestry is becoming increasingly problematic. Coastal Douglas fir (Pseudotsuga menziesii menziesii) grows well in Central Europe, and is considered a suitable alternative to spruce, particularly in mixture with beech. However, little information is available about the ecological effects of admixture of Douglas fir to beech in Europe, particularly with respect to soil and seepage water chemistry as well as ecosystem nutrient cycling.

We present results from a study in Germany in which adjacent pure and mixed stands of European beech and Douglas fir at nutrient-poor (sandstone) and nutrient-rich (loess) sites were compared with respect to soil fertility indicators (pH, base saturation, C/N ratio, stocks of organic C, N, and plant-available base cations) as well as seepage water variables (quantity and chemistry, e.g. NO₃^-, Al³⁺, Ca²⁺). We sampled topsoil as well as subsoil down to 90 cm depth at different distances to the tree trunks.

Significant tree species effects on most soil parameters were mainly found in the forest floor and in the uppermost mineral soil. Larger SOC stocks under mixed compared to pure stands of beech and Douglas fir indicate more efficient soil occupation by tree roots. SOC and N stocks as well as other soil parameters change with changing distance of the sampling location to the trunks. Our results underline the importance of investigating forest soils at different distances to the trees as well as including the subsoil in the assessment of soil OC and nutrient stocks. Especially on the N-saturated loess sites, seepage water showed higher nitrate concentrations under pure Douglas fir compared to mixed Douglas fir-beech and pure beech forest. In summary, establishment of mixed Douglas fir-beech forest seems to be a promising option for German forestry with regard to important soil ecological features.