The soil fauna community in pure and mixed stands of beech and spruce of different age: trophic structure and structuring forces

This study investigates the response of the soil fauna community to replacement of beech by spruce or by mixed stands of beech and spruce. Stands of different age were investigated in a factorial design with the factors tree species (beech and spruce) and stand age (30 and 120 yr). The input of leaf/needle litter did not differ significantly between the study sites. By contrast, the amount of organic matter in upper soil layers (L/F, H/Ah) of spruce forests strongly exceeded that of beech forests particularly in mature stands. The increase in organic matter in spruce stands was not associated by an increase in the amount of microbial biomass. Biomass of eight (bacterivorous, fungivorous and omnivorous nematodes, enchytraeids, earthworms, isopodes, mycetophilid and cecidomyiid Diptera) of the twelve microbi‐detritivorous soil animal groups studied was significantly increased in beech forests; only that of one group (elaterid beetles) was increased in spruce forests and three groups did not respond significantly (collembolans, oribatid mites, sciarid Diptera). This indicates that in the forests studied neither habitat space (amount of organic matter in L/F and H/Ah layers) nor the amount of microbial biomass controlled microbi‐detritivores. Rather, the quality of litter materials and the concentration of microbial biomass therein appeared to be most important. Herbivores and predators also were favoured by beech: the biomass of one (rhizophagous nematodes) of the three herbivorous groups studied were significantly increased in beech stands and none in spruce stands; the biomass of four (predatory nematodes, centipedes, carabid and cantharid beetles) of the seven carnivorous groups studied were increased in beech stands, none in spruce stands. Generally, the biomass ratio between prey and predators was at a minimum in mature beech and mixed stands indicating more intense top‐down control in these forests. Overall, the study documents that replacement of beech by spruce strongly alters the soil food web. Mixed stands were more similar to spruce stands in respect to the biomass of soil animal groups but predator–prey interactions appeared to be more similar in mature beech and mixed stands. Differences between tree species usually were more pronounced in 120 compared to 30 yr old stands indicating that the development of stand characteristics is slow.