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It is already well known that the effects of tree diversity on aboveground wood productivity depend on tree species identity and site conditions and thus can vary among different tree mixtures and forest sites. The effects of species diversity, specifically on belowground productivity and dynamics, have rarely been studied, so potential interactions with species identity and/or site conditions are largely unknown. Sequential soil coring was applied in mature pure and mixed stands of European beech, Norway spruce, and Douglas-fir in two regions of northern Germany to investigate the potential site, species, and mixing effects on fine root productivity, mortality, and turnover rates. In these ecologically and economically important species mixtures, a strong effect of site conditions on all fine root variables was found. In particular, more fine root growth was observed at the less favorable site conditions than at the more favorable ones. Species identity and interaction between site conditions and species identity were additional explanatory factors, whereat beech showed the strongest effect by site conditions. No overyielding was observed when fine root productivity was plotted against area potentially available (APA), nor were mortality or turnover. However, at specific soil depths, a mixing effect, caused mainly by beech, was observed for all variables and for both species mixtures. This study suggests that site conditions and species identity rather than species mixture are essential in explaining fine root dynamics and that increasing tree species diversity may not guarantee higher belowground productivity. However, in the face of climate change and the demand for wood as a natural and renewable resource, an admixture of Douglas-fir to pure European beech stands seems to be a reasonable alternative to an admixture of spruce, a species that is already seriously suffering by drought and bark beetle attacks.
Lwila et al. (Tue,) studied this question.