Shifts in Soil Testate Amoeba Communities Associated with Forest Diversification

We studied changes of testate amoeba communities associated with the conversion of spruce monocultures into mixed beech-fir-spruce forests in the Southern Black Forest Mountains (Germany). In this region, forest conversion is characterized by a gradual development of beech undergrowth within thinned spruce tree stands leading to multiple age continuous cover forests with a diversified litter layer. Strong shifts in the abundance of testate amoeba observed in intermediate stages levelled off to monoculture conditions again after the final stage of the conversion process had been reached. The average number of species per conversion stage (i.e., local richness) did not respond strongly to forest conversion, but the total number of species (i.e., regional richness) was considerably higher in the initial stage than in the mixed forests, due to the large number of hygrophilous species inhabiting spruce monocultures. Functional diversity of the testate amoeba community, however, significantly increased during the conversion process. This shift was closely associated with improved C and N availability as well as higher niche diversity in the continuous cover stands. Lower soil acidity in these forests coincided with a higher relative abundance of eurytopic species. Our results suggest that testate amoeba communities are much more affected by physicochemical properties of the soil than directly by litter diversity.