Douglas fir and Norway spruce admixtures to beech forests along a site gradient 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; B), Douglas fir (Pseudotsuga menziesii; D) and Norway spruce (Picea abies; S) stands as well as mixtures of beech with either Douglas fir (DB) or spruce (SB), i.e. 5 forest stands per site along Northern Germany with two regionally clearly differing sites conditions (i.e., 3 loess-influenced, loamy sites in the Solling region, southern Lower Saxony and 4 sandy lowland sites further north). In order to determine possible influences of the tree species and their mixtures on soil properties, the organic layer and the mineral topsoil were first chemically characterized for all 35 plots down to a depth of 30 cm (pH, C, N, P, CEC, exchangeable nutrient cation contents and stocks, base saturation-BS). Our results indicated, independent of sites condition, that pure S stands showed the lowest pH and BS, meanwhile B the highest BS. The impact of D varied depending on site condition. On sandy soils, pure D showed higher pH and BS than under pure S, while on loamy soils the pH under D and B was lower than under S. Regarding cations stocks under sandy soils conditions, S stands and its admixture SB depleted soil Ca and Mg stocks more than pure D and B. In contrast, under loamy soil conditions B showed depleted (lowest) soil exchangeable Ca and Mg stocks more than under S stands. Soil exchangeable K under mixed stands were among the highest compared to pure stands, independent of the site condition. Thus, mixed species stands generally decreased soil base cation depletion compared to pure conifer stands. Admixtures of Douglas-fir (DB) seem to lead to smaller changes in pH, CEC and BS than those of Norway spruce, this effect become more important at sandy soil sites. Therefore, forest management may consider mixtures of European beech and Douglas fir as a reasonable management option without apprehending negative effects on soil chemistry.