Mixed forests with native species mitigate impacts of introduced Douglas fir on soil decomposers (Collembola)

Forest ecosystem management requires the conservation of associated biodiversity. Enriching native forests with economically valuable conifer species provides economic gains and meets the increasing societal demand for timber but may threaten biodiversity. Soil sustains most of forest biodiversity, but the impact of changes in tree species composition, including native and non-native species, on soil invertebrates remains little studied. We investigated the impact of different forest types on the taxonomic and functional composition of springtail communities (Collembola, Insecta), an abundant and diverse microarthropod group inhabiting litter and soil. Using native Fagus sylvatica (European beech) as reference, we compared Collembola communities with native but range-expanding Picea abies (Norway spruce) and non-native Pseudotsuga menziesii (Douglas fir) as well as beech-conifer mixtures. The abundance of Collembola was higher in Norway spruce than in European beech, with little difference among the other forest types. Further, the taxonomic and functional composition of Collembola was shifted to more parthenogenetic species at sandy sites, stressing the importance of regional factors such as soil type and climate in structuring Collembola communities. Collembola communities in Douglas fir were more pigmented and distributed to the surface, resulting in a lower proportion of euedaphic Collembola compared to European beech forests. In mixed forests, the impacts of Douglas fir on euedaphic Collembola were reduced, suggesting that negative effects of introduced tree species on soil animal communities might be alleviated by limiting Douglas fir to enrichment plantings only. Overall, the results indicate that vertical distribution in soil and morphological traits of Collembola help to better understand the changes in decomposer communities due to planting non-native tree species.