Factors influencing the distribution and abundance of earthworm communities in pure and converted Scots pine stands

Earthworm populations were sampled in 19 coniferous or mixed forests in Bavaria. The stands are dominated by Scots pine (Pinus sylvestris L.), or had been pure Scots pine stands before conversion to mixed stands with the broad-leaved tree species European beech (Fagus sylvatica L.), and/or pedunculate oak (Quercus robur L.) and sessile oak (Quercus petraea (Matt.) Liebl.) was started 14–66 years ago. Based on this survey, we tested whether the introduction of beech and oak into pure pine stands resulted in an altered earthworm community. In addition, we tried to explain the high observed variance in earthworm abundance and biomass by regressing them against the factors extracted by principal component analysis (PCA). The variables included in the PCA comprised soil nutrient concentrations in the Of-layer, stand parameters, annual mean precipitation and temperature, the number of diplopods as potential competitors for food and habitat, the number of invertebrate predators, and the distance to agricultural land and roadside as potential sources of earthworm immigration, respectively.Earthworms were found in each stand; however, the variance in earthworm abundance and biomass both among and within the stands was high. At two out of seven sites where pairwise comparisons between pure and converted stands were possible, significantly higher earthworm abundance and biomass were found in the converted stands. Three additional sites showed a non-significant trend towards more numerous earthworms in stands with introduced broad-leaved tree species. Moreover, at stands with broadleaves there was a shift towards an increased earthworm species diversity.The results of the PCA and subsequent regression analyses showed that the mitigation of soil acidity is the key factor for the enhancement of the epigeic earthworm fauna in Scots pine stands. We concluded that on poor sites the introduction of broadleaves influences the earthworm community positively only if it creates improved habitat conditions by, e.g., altering soil chemical properties. This requires the presence of nutrient-rich horizons in the subsoil, and means that only deep rooting and sufficiently old broad-leaved trees which are able to reach these horizons will result in improved chemical properties of the organic layer. On soils with more favourable conditions where even Scots pine stands contain considerable earthworm densities, the introduction of broad-leaved tree species may lead to a relatively prompt increase in the earthworm community and change in its species composition.