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

Abstract Background The establishment of economically valuable conifers into a matrix of native broadleaved tree species may serve as model systems that combine economic interests and nature conservation. However, it is not clear which effects the enrichment by conifers has on soil properties. Methods Our study analyzed 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 along a soil and climate gradient in Northern Germany. We determined chemical soil properties of the O-horizon and upper mineral soil horizons. Soil pH, concentrations and storage of exchangeable cations, base saturation (BS) as well total P contents were analyzed. Results We observed lowest pH and BS in spruce stands while beech showed higher BS. The impact of Douglas fir on soil chemistry varied depending on the site. Under Douglas fir-beech mixtures, mineral soil pH and BS were higher than under the respective Douglas-fir stands at nutrient-poor sandy soils. While spruce and its admixture deplete soil exchangeable Ca and Mg more than Douglas fir mixed with. Total soil exchangeable K under mixed stands were among the highest, independent of the site condition. Conclusions 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.