Soil acidity and exchangeable base cation stocks under pure and mixed stands of European beech, Douglas fir and Norway spruce

The establishment of mixed forest stands is nowadays seen as an opportunity to maintain forest services in the course of global climate change. Thus, we determined forest floor and mineral soil pH, base saturation (BS) as well as exchangeable base cation stocks in adjacent groups of pure mature European beech (Fagus sylvatica), Douglas fir (Pseudotsuga menziesii) and Norway spruce (Picea abies) as well as single-tree mixtures of beech with either Douglas fir or spruce at two forest sites in Southern Germany that differ in site and soil properties. Spruce forest floors had lowest pH and BS, while beech favoured less acidic forest floors with higher BS. The impact of Douglas fir on soils varied depending on the site. Under beech-Douglas fir and beech-spruce mixtures, forest floor and mineral soil pH and BS were higher than under the respective pure conifer stands. While beech depletes soil exchangeable Ca and Mg stocks more than Douglas fir and spruce, respectively, total soil exchangeable K stocks under beech were among the highest. Again, beech-conifer mixtures were intermediate. Mixed species stands might maintain forest soil fertility by mitigating soil acidification, nutrient leaching and concomitant soil base cation depletion compared to pure conifer stands.