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.

中文翻译：

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多样性对植物生产力的多方面影响：物种丰富度、功能多样性、物种特性和种内竞争

1. 破解不同物种组成和多样性的植物群落生物量产生变化的机制是生物多样性-生态系统功能研究的主要挑战。生态位互补性和选择效应已被广泛研究以解决生物多样性与生产力的关系。然而，迄今为止，对关键物种所发挥的具体作用的忽视限制了我们全面评估生物多样性影响的其他潜在驱动因素的相对重要性的能力。2. 在这里，我们进行了草原多样性生产力实验，以测试生物多样性影响的四个潜在方面，即物种丰富度、功能多样性、物种特性和种内竞争的放松，如何解释上面和根生物量生产的变化。3. 我们在单一栽培中种植了六种植物，以及两种、三种和六种的每种组合。植物密度在整个丰富度梯度上保持不变，但我们还在半密度单一栽培中种植每个物种，以估计每个研究物种的种内竞争强度。我们描述了八个功能性状，包括根性状，与养分和光照相关，并计算了每个性状的功能差异和群落加权平均值 (CWM)。我们进一步将地上生物多样性效应划分为互补效应和选择效应。4. 我们观察到对地上生物量和根生物量都有很强的正生物多样性效应以及很强的正互补效应。这些主要源于特定物种（Plantago lanceolata）的存在和 CWM 性状值，而不是植物混合物中更高的功能差异。P. lanceolata 表现出最高的种内竞争，在物种混合物中强烈放松。相比之下，Sanguisorba minor 的存在对植物混合物的生产力产生负面影响，该物种更多地遭受种间竞争而不是种内竞争。5. 本研究提供了强有力的证据，表明寻找关键物种对于了解物种多样性对生态系统功能的作用至关重要，并证明了种内和种间竞争之间的平衡在生物多样性-生态系统功能关系中的主要作用。