Abstract Biodiversity eVects on ecosystem functioning in forests have only recently attracted increasing attention. The vast majority of studies in forests have focused on above-ground responses to diVerences in tree species diver- sity, while systematic analyses of the eVects of biodiversity on root systems are virtually non-existent. By investigating the Wne root systems in 12 temperate deciduous forest stands in Central Europe, we tested the hypotheses that (1) stand Wne root biomass increases with tree diversity, and (2) ‘below-ground overyielding’ of species-rich stands in terms of Wne root biomass is the consequence of spatial niche seg- regation of the roots of diVerent species. The selected stands represent a gradient in tree species diversity on similar bedrock from almost pure beech forests to medium- diverse forests built by beech, ash, and lime, and highly- diverse stands dominated by beech, ash, lime, maple, and hornbeam. We investigated Wne root biomass and necro- mass at 24 proWles per stand and analyzed species diVer- ences in Wne root morphology by microscopic analysis. Fine root biomass ranged from 440 to 480 g m¡2 in the species-poor to species-rich stands, with 63–77% being concentrated in the upper 20 cm of the soil. In contradiction to our two hypotheses, the diVerences in tree species diver- sity aVected neither stand Wne root biomass nor vertical root distribution patterns. Fine root morphology showed marked
distinctions between species, but these root morphological diVerences did not lead to signiWcant diVerences in Wne root surface area or root tip number on a stand area basis. More- over, diVerences in species composition of the stands did not alter Wne root morphology of the species. We conclude that ‘below-ground overyielding’ in terms of Wne root bio- mass does not occur in the species-rich stands, which is most likely caused by the absence of signiWcant spatial segregation of the root systems of these late-successional species.