Data from: A portfolio effect of shrub canopy height on species richness in both stressful and competitive environments

Facilitating effects of benefactor plants on plant species richness have been commonly tested in stressful habitats because competitive effects are assumed to predominate in more productive habitats. Here, we examine this assumption by testing whether benefactor plants can nonetheless be facilitating in competitive environments. We provide a conceptual framework describing how a trait of benefactor plants, canopy height of shrubs, can have a portfolio of facilitative effects on species richness in more competitive environments, and we provide an empirical assessment of this portfolio effect in tundra plant communities. Across tundra plant communities representing an extensive gradient in aboveground live biomass ranging from 11 to above 800 grams per m2, we found that species richness exhibited a humped-back relationship. Increasing canopy height of shrubs to a maximum height of what defines the dwarf shrub tundra, that is 40 cm, consistently and significantly increased species richness along the entire biomass gradient tested. The positive effect of shrub canopy height was not confounded with herbivore intensity, competitive interference or abiotic factors such as bedrock-weathered mineral availability, moisture availability or temperature. However, we cannot rule out that the general presence of large mammalian herbivory may have been central to the positive effect of shrub canopy height in reducing herbivore impacts on species richness. In this study, conceptual and empirical evidence support that increasing canopy height of shrubs facilitates species richness regardless of relative abiotic stress levels within tundra ecosystems. We propose that positive interactions can play an important ecological role in systems where competitive effects are observed or assumed. For tundra plant communities where climate change is currently causing encroachment of shrub species, the effects of increasing canopy height may have unprecedented effects on plant species richness.

护助植物（benefactor plants）对植物物种丰富度（species richness）的促进作用，此前多在胁迫生境（stressful habitats）中得到验证，这是因为学界普遍认为竞争作用（competitive effects）在高生产力生境（productive habitats）中占据主导地位。本研究通过检验护助植物在竞争环境中是否仍能发挥促进作用，对这一假设展开验证。我们构建了一套概念框架，用以阐述护助植物的某一性状——灌木冠层高度——如何在竞争更强的生境中对物种丰富度产生一系列促进效应，并在苔原植物群落中对该系列效应开展实证评估。我们的研究覆盖了地上活生物量梯度跨度极大的苔原植物群落（生物量范围为11 g/m²至800 g/m²以上），结果显示物种丰富度呈现出驼峰型曲线关系。当灌木冠层高度升高至矮灌丛苔原（dwarf shrub tundra）的界定上限——即40 cm——时，在本次测试的全部生物量梯度上，物种丰富度均持续且显著提升。灌木冠层高度的正向效应并未与植食动物强度（herbivore intensity）、竞争干扰（competitive interference）以及基岩风化矿物可获得性（bedrock-weathered mineral availability）、水分可获得性（moisture availability）、温度等非生物因子相混淆。但我们无法排除大型哺乳动物植食作用（large mammalian herbivory）的普遍存在，或许是灌木冠层高度通过缓解植食动物对物种丰富度的负面影响而发挥正向效应的核心前提。本研究通过概念与实证证据表明，在苔原生态系统中，无论非生物胁迫程度如何，灌木冠层高度的增加均能促进物种丰富度。我们提出，正相互作用在那些已被观测到或被假定存在竞争作用的生态系统中，或许可发挥重要的生态功能。当前气候变化正导致苔原地区灌木物种扩张（encroachment of shrub species），在此类苔原植物群落中，灌木冠层高度增加所带来的效应，可能会对植物物种丰富度产生前所未有的影响。