Abstract: | Abstract: Mixed-species forests often enhance the provision of ecosystem functions, both above- and below-ground. Several of these effects are mediated by the amount and spatial distribution of tree tissues. However, previous studies on tree diversity effects on fine-root biomass (FRB) have returned inconsistent results and did not distinguish between absorptive and transport fine roots. Furthermore, owing to the lack of species-specific data, it is not well understood whether complementarity or selection effects contribute more to these mixing effects.<br>Here, we analysed tree species mixing effects on fine-root traits while considering the respective tree species contributions and root functional types. Specifically, we tested whether tree species mixing increases FRB and root length density (RLD) and results in vertical root stratification. We quantified FRB and RLD in 30-cm deep soil profiles for 13 tree species in mixed and pure stands across four widespread European forest types. The differentiation of different fine-root species in mixtures allowed us to disentangle complementarity and selection effects.<br>Across all sites, mixtures supported on average less FRB than pure stands, which was reflected in negative complementarity and selection effects. RLD of absorptive fine roots did not change across the soil profile and even increased in the topsoil, which was associated with positive complementarity effects. There was no evidence for vertical root stratification. Conifer proportion, which was mainly associated with selection effects, dampened net diversity effects. Root functional type further influenced tree species mixing effects.<br>Synthesis. Despite the underyielding of FRB in mixtures, overall soil occupation by absorptive fine roots (RLD) did not decrease in mixtures, pointing to morphological root trait adaptations associated with higher resource-use efficiency. Increased RLD in the most nutrient-rich layer in mixtures further indicates complementary interactions among species and a greater resource uptake capacity. This work illustrates that considering only one aspect of trait-functioning relationships, for example, root biomass, may not capture the full effect of plant diversity on ecosystem functioning. The integration of a larger range of relevant traits is required. Moreover, traditional classification of fine roots based on the 2-mm diameter cut-off may obscure responses of roots to environmental changes |
