Résumé
1. Deciphering the mechanisms that drive variation in biomass production
across plant communities of contrasting species composition and diversity
is a main challenge of biodiversity-ecosystem functioning research. Niche
complementarity and selection effect have been widely investigated to
address biodiversity-productivity relationships. However, the overlooking
of the specific role played by key species have limited so far our
capacity to comprehensively assess the relative importance of other
potential drivers of biodiversity effects. 2. Here, we conducted a
grassland diversity-productivity experiment to test how four potential
facets of biodiversity effects, namely species richness, functional
diversity, species identity and the relaxation of intraspecific
competition, account for variations in above and root biomass production.
3. We grew six plant species in monoculture, as well as in every
combinations of two, three and six species. Plant density was kept
constant across the richness gradient but we additionally grew each
species in half-density monoculture to estimate the strength of
intraspecific competition for each studied species. We characterized eight
functional traits, including root traits, related to nutrient and light
acquisition and computed both the functional dissimilarity and the
community weighted mean (CWM) of each trait. We further partitioned
aboveground biodiversity effect into complementarity and selection
effects. 4. We observed strong positive biodiversity effects on both
aboveground and root biomass as well as strong positive complementarity
effect. These arose largely from the presence of a particular species
(Plantago lanceolata) and from CWM trait values more than from a higher
functional dissimilarity in plant mixtures. P. lanceolata displayed the
highest intraspecific competition, which was strongly relaxed in species
mixtures. By contrast, the presence of Sanguisorba minor negatively
affected the productivity of plant mixtures, this species suffering more
from interspecific than intraspecific competition. 5. This study provides
strong evidences that the search for key species is critical to understand
the role of species diversity on ecosystem functioning and demonstrates
the major role that the balance between intraspecific and interspecific
competition plays in biodiversity-ecosystem functioning relationships.
Developing more integrative approaches in community and ecosystem ecology
can offer opportunities to better understand the role that species
diversity plays on ecosystem functioning.