BACKGROUND AND AIMS Facilitation is an important ecological process for plant community structure and functional composition. Although direct facilitation has accrued most of the evidence so far, indirect facilitation is ubiquitous in nature and it has an enormous potential to explain community structuring. In this study, we assess the effect of direct and indirect facilitation on community productivity via taxonomic and functional diversity. METHODS In an alpine community on the Tibetan Plateau, we manipulated the presence of the shrub Dasiphora fruticosa and graminoids in a fenced meadow and a grazed meadow to quantify the effects of direct and indirect facilitation. We measured four plant traits: height, lateral spread, specific leaf area (SLA) and leaf dry matter content (LDMC) of forbs; calculated two metrics of functional diversity (range of trait and community-weighted mean (CWM) of trait); and assessed the responses of functional diversity to shrub facilitation. We used structural equation modelling to explore how shrubs directly and indirectly drove community productivity via taxonomic diversity and functional diversity. KEY RESULTS We found stronger effects from herbivore-mediated indirect facilitation than direct facilitation on productivity and taxonomic diversity, regardless of the presence of graminoids. For functional diversity, the range and CWM of height and SLA, rather than lateral spread and LDMC, generally increased due to direct and indirect facilitation. Moreover, we found that the range of traits played a primary role over taxonomic diversity and CWM of traits in terms of shrub effects on community productivity. CONCLUSIONS Our study reveals that the mechanism of shrub direct and indirect facilitation of community productivity in this alpine community is expanding the realized niche (i.e., expanding range of traits). Our findings indicate that facilitators might increase trait dispersion in the local community, which could alleviate the effect of environmental filters on trait values in harsh environments, thereby contributing to ecosystem functioning.

中文翻译：

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直接和间接促进通过高山系统功能多样性的变化影响社区生产力

背景与目的 易化是植物群落结构和功能组成的重要生态过程。尽管到目前为止，直接促进已经积累了大部分证据，但间接促进在自然界中无处不在，并且具有解释社区结构的巨大潜力。在这项研究中，我们通过分类和功能多样性评估了直接和间接促进对社区生产力的影响。方法在青藏高原的一个高山群落中，我们操纵了围栏草甸和放牧草甸中灌木 Dasiphora fruticosa 和禾本科植物的存在，以量化直接和间接促进的影响。我们测量了四种植物性状：杂草的高度、横向蔓延、比叶面积 (SLA) 和叶干物质含量 (LDMC)；计算功能多样性的两个指标（性状范围和性状的社区加权平均值（CWM））；并评估了功能多样性对灌木促进的反应。我们使用结构方程模型来探索灌木如何通过分类多样性和功能多样性直接和间接推动群落生产力。主要结果我们发现，无论是否存在禾本科植物，草食动物介导的间接促进作用比直接促进作用对生产力和分类多样性的影响更大。对于功能多样性，高度和 SLA 的范围和 CWM，而不是横向扩展和 LDMC，通常由于直接和间接促进而增加。此外，我们发现，就灌木对群落生产力的影响而言，性状范围对分类多样性和性状的 CWM 起着主要作用。结论 我们的研究表明，在这个高山群落中，灌木直接和间接促进群落生产力的机制正在扩大已实现的生态位（即扩大性状范围）。我们的研究结果表明，促进者可能会增加当地社区的特征分散，这可以减轻环境过滤器对恶劣环境中特征值的影响，从而有助于生态系统功能。