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

Background: 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. Methods: 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 a soil and climate gradient in Northern Germany. As a first comparative approach, we determined chemical background conditions of the O-horizon and upper mineral soil horizons to gain insights into possible species specific impacts of trees on chemical site conditions. Soil pH, concentrations and storage of exchangeable cations, base saturation (BS) as well total P contents were analyzed. Results: Spruce forest had lowest pH and BS, meanwhile beech showed higher 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. While spruce and its admixture deplete soil exchangeable Ca and Mg more than Douglas fir and Beech, total soil exchangeable K under mixed stands were among the highest, independent of the site condition. Conclusions: 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 sand soils.