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Complementary soil exploration by the root systems of coexisting tree species has been hypothesised to result in a higher root biomass of mixed forests than of monocultures but the existing evidence for a belowground diversity effect in forests is scarce and not conclusive.

In a species-rich temperate broad-leaved forest, we analysed the fine root biomass (roots ≤ 2 mm) and necromass in 100 plots differing in tree species diversity (one to three species) and species composition (all possible combinations of five species of the genera Acer, Carpinus, Fagus, Fraxinus and Tilia) which allowed us to separate possible species diversity and species identity effects on fine root biomass.

We found no evidence of a positive diversity effect on standing fine root biomass and thus of overyielding in terms of root biomass. Root necromass decreased with increasing species diversity at marginal significance.

Various lines of evidence indicate significant species identity effects on fine root biomass (10–20% higher fine root biomass in plots with presence of maple and beech than in plots with hornbeam; 100% higher fine root biomass in monospecific beech and ash plots than in hornbeam plots; differences significant). Ash fine roots tended to be over-represented in the 2- and 3-species mixed plots compared to monospecific ash plots pointing at apparent belowground competitive superiority of Fraxinus in this mixed forest.

Our results indicate that belowground overyielding and spatial complementarity of root systems may be the exception rather than the rule in temperate mixed forests.