Abstract Calcareous grasslands are rich in biodiversity and thus receive much attention in nature conservation. In such grasslands, the formation of moss layers is perceived as a management problem. However, its impacts on the community level are complex, as not only inhibition but also facilitation of vascular plant recruitment occur. Possible filters of recruitment are shading by mosses, isolation from soil resources and the resulting desiccation. To understand how seed size and shape moderate moss effects, a combined glasshouse and field experiment was conducted in southern Germany. Seeds of 14 species from calcareous grasslands were sown either on top or underneath a moss layer, or on bare soil without moss. We determined the total number of emerged and established seedlings, i.e. the ones that survived until the end of the experiment. Both measures were reduced for most species sown on moss, while mortality was slightly enhanced by moss presence. Seed size explained a significant proportion of the moss effects on plant recruitment. Inhibitive effects on recruitment increased with seed size when seeds were placed on top of moss. When germinating underneath moss, the effect on emergence changed from negative in small-seeded plant species to positive in large-seeded species, but this effect was insignificant for establishment. The positive response in large seeds was probably due to their higher moisture requirements for imbibition, and lower dependency on light for germination. However, moss-seed-interactions were not affected by seed shape. Seedling establishment of plants with large seeds strongly depended on their initial position in moss, while small-seeded species showed no effect. Hence, moss layers constitute a selective recruitment filter by modifying seed penetration and hence germination conditions. This effect could increase beta diversity of vascular plant species in calcareous grasslands, and thus conservation should aim at intermediate and patchy moss abundance.

中文翻译：

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苔藓层的补充过滤不利于大种子草原物种

摘要 钙质草地生物多样性丰富，因而受到自然保护的重视。在这样的草原上，苔藓层的形成被认为是一个管理问题。然而，它对群落水平的影响是复杂的，因为不仅抑制而且促进了维管植物的募集。可能的补充过滤器是苔藓遮荫、与土壤资源隔离以及由此导致的干燥。为了了解种子大小和形状如何减轻苔藓的影响，在德国南部进行了温室和田间联合试验。将石灰质草地的 14 种种子播种在苔藓层的顶部或之下，或在没有苔藓的裸露土壤上播种。我们确定了出苗和建立的幼苗的总数，即存活到实验结束的幼苗。对于播种在苔藓上的大多数物种，这两项措施都减少了，而苔藓的存在则略微提高了死亡率。种子大小解释了苔藓对植物补充影响的很大一部分。当种子放在苔藓上时，对招募的抑制作用随着种子大小的增加而增加。在苔藓下萌发时，对出苗的影响从小种子植物物种的负面影响变为大种子物种的积极影响，但这种影响对建立不显着。大种子的积极反应可能是由于它们对吸水的水分要求较高，并且发芽对光的依赖性较低。然而，苔藓种子相互作用不受种子形状的影响。大种子植物的幼苗建立强烈依赖于它们在苔藓中的初始位置，而小种子种则没有影响。因此，苔藓层通过改变种子渗透和发芽条件构成了选择性补充过滤器。这种效应可以增加钙质草原维管植物物种的β多样性，因此保护应以中等和斑驳的苔藓丰度为目标。