State Key Laboratory of Grassland and Agro-ecosystems, School of Life Sciences, Lanzhou University, Lanzhou, Gansu, China.
University of Bordeaux, UMR CNRS 5805 EPOC, Talence, France.
Ann Bot. 2021 Jan 7;127(2):241-249. doi: 10.1093/aob/mcaa170.
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.
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.
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.
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.
促进作用是植物群落结构和功能组成的一个重要生态过程。尽管到目前为止，直接促进作用已经积累了大部分证据，但间接促进作用在自然界中普遍存在，它具有极大的潜力来解释群落结构。在这项研究中，我们通过分类和功能多样性来评估直接和间接促进作用对群落生产力的影响。
在青藏高原的一个高山群落中，我们在围栏草地和放牧草地中操纵灌木杜香和禾本科植物的存在，以量化直接和间接促进作用的影响。我们测量了四个植物特征：草本植物的高度、横向扩散、比叶面积 (SLA) 和叶片干物质含量 (LDMC)；计算了两个功能多样性指标[性状范围和性状加权平均值 (CWM)]；并评估了功能多样性对灌木促进作用的响应。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地驱动群落生产力。
无论是否存在禾本科植物，食草动物介导的间接促进作用对生产力和分类多样性的影响都强于直接促进作用。对于功能多样性，高度和 SLA 的范围和 CWM，而不是横向扩散和 LDMC，通常由于直接和间接促进作用而增加。此外，我们发现，在灌木对群落生产力的影响方面，性状范围比分类多样性和性状 CWM 更能发挥作用。
我们的研究表明，在这个高山群落中，灌木对群落生产力的直接和间接促进作用的机制是扩大实现的生态位（即扩大性状范围）。我们的研究结果表明，促进者可能会增加局部社区中的性状分散度，这可以减轻环境过滤器对恶劣环境中性状值的影响，从而有助于生态系统功能。
