Douglas fir and Norway spruce admixtures to beech forests along in Northern Germany – Are soil nutrient conditions affected?

The establishment of mixed forest stands can be seen as an option to improve soil nutrient conditions and to protect forest ecosystems from various impacts of climate change. Our study analyzed groups of pure mature European beech (Fagus sylvatica), Douglas fir (Pseudotsuga menziesii) and Norway spruce (Picea abies) stands as well as mixtures of beech with either Douglas fir or spruce at long Northern Germany. As a first comparative approach, we determined chemical background conditions of the O-horizon (L, F and H layer) and upper mineral soil horizons (0-30 cm) to gain insights into possible species-specific impacts of trees on chemical site conditions. Soil pH, concentrations and storage of exchangeable cation, base saturation (BS) as well as total P contents were analyzed. Spruce forest had lowest pH and BS, meanwhile beech showed highest BS. The impact of Douglas fir on soils varied depending on the site. Under Douglas fir-beech mixture, mineral soil pH and BS were higher than under the respective pure conifer stands at nutrient-poor sandy soils. Meanwhile spruce and its admixture deplete soil exchangeable Ca and Mg more than Douglas fir and Beech under sandy soils, total soil exchangeable K under mixed stands were among the highest, independent of the site condition. Thus, mixed species stands decreased soil base cation depletion compared to pure conifer stands. Thereby, this effect seems to become all the more important at sites with sandy soils. Overall, our study suggest that the enrichment of beech stands by Douglas fir does not cause unexpected and detrimental changes of soil acidity and does not strongly affect soil exchangeable base cation reserves when compared to European beech. Instead, admixtures of Douglas-fir seem to lead to smaller changes in pH, CEC and BS than those of Norway spruce. Therefore, forest management may consider mixtures of European beech and Douglas fir as a reasonable management option without apprehending negative effects on soil chemistry.