The interactive effects of soil fertility and tree mycorrhizal association explain spatial variation of diversity–biomass relationships in a subtropical forest

Observed biodiversity–ecosystem function (BEF) relationships are highly variable, particularly in natural forests. However, our understanding of the factors that generate these often contradictory patterns, especially the role of different mycorrhizal associations, is still limited. By relating tree species richness and above‐ground biomass (AGB) in a fully mapped 24‐ha subtropical forest dynamics plot, we evaluated the impacts of soil fertility and tree mycorrhizal type in mediating BEF relationships at multiple spatial scales. Our results demonstrate a highly positive total richness effect on AGB for arbuscular mycorrhizal (AM) trees but a negative effect on AGB for ectomycorrhizal (EcM) trees, and their relationships were highly spatial scale dependent. However, the observed BEF relationships turned into positive at small spatial scales (i.e. 10 and 20 m) after controlling for other confounding factors (i.e. topography, soil fertility and AM proportion). In addition, we found significant interactions between soil fertility and species richness on AGB. Specifically, the positive effect of total species richness on AGB for major mycorrhizal types gradually weakened with increasing soil fertility, while the positive effect of EcM species richness on AM AGB gradually enhanced at small spatial scales, suggesting the observed diversity effects can be largely attributed to resource niche complementarity and the role of EcM fungi. Synthesis. We conclude that the variable BEF relationships among forest communities could be explained by spatial variation in abiotic environments and community mycorrhizal composition because different types of symbionts perform different nutrient uptake strategies and ability in protection from antagonists. Our findings provide novel insights into the understanding of the variation in the shape of BEF relationships in natural forests, which is critical for forest management, conservation and restoration in a changing world. 摘要 在自然森林群落中，物种多样性与生态系统功能之间的关系通常具有较高的空间变异。但是我们对这些变异产生的原因，特别是植物菌根类型所发挥的作用，仍知之甚少。 本研究以古田山24公顷亚热带常绿阔叶林动态监测样地为研究平台，通过植物多样性与地上生物量关联分析，评估了壤肥力和植物菌根类型对不同空间尺度物种多样性与生态系统功能关系的影响。 研究结果表明，总物种多样性与丛枝菌根植物地上生物量存在较强的正相关关系，而与外生菌根植物生物量呈现显著负相关关系，并且其相互关系具有显著的尺度依赖效应。但是，当我们排除其它干扰因子(如地形、土壤肥力以及丛枝菌根植物比率)影响后，发现总物种多样性与植物地上生物量的相关关系在较小尺度转变为正相关。此外，物种多样性对地上生物量的影响显著依赖于土壤肥力。具体而言，随着土壤肥力的增加，总物种多样性与地上生物量的正相关关系逐渐减弱，而外生菌根植物多样性与丛枝菌根植物地上生物量的正相关关系在较小空间尺度逐渐增强，表明资源生态位互补效应和外生菌根真菌的保护效应可能是影响物种多样性和生物量关系的关键机制。 结论:我们发现由于不同类型菌根植物具有不同的养分利用策略和对天敌的防御能力，物种多样性和生产力的关系可能