Nurse species and indirect facilitation through grazing drive plant community functional traits in tropical alpine peatlands

Facilitation among plants mediated by grazers occurs when an unpalatable plant extends its protection against grazing to another plant. This type of indirect facilitation impacts species coexistence and ecosystem functioning in a large array of ecosystems worldwide. It has nonetheless generally been understudied so far in comparison with the role played by direct facilitation among plants. We aimed at providing original data on indirect facilitation at the community scale to determine the extent to which indirect facilitation mediated by grazers can shape plant communities. Such experimental data are expected to contribute to refining the conceptual framework on plant–plant–herbivore interactions in stressful environments. We set up a 2-year grazing exclusion experiment in tropical alpine peatlands in Bolivia. Those ecosystems depend entirely on a few, structuring cushion-forming plants (hereafter referred to as “nurse” species), in which associated plant communities develop. Fences have been set over two nurse species with different strategies to cope with grazing (direct vs. indirect defenses), which are expected to lead to different intensities of indirect facilitation for the associated communities. We collected functional traits which are known to vary according to grazing pressure (LDMC, leaf thickness, and maximum height), on both the nurse and their associated plant communities in grazed (and therefore indirect facilitation as well) and ungrazed conditions. We found that the effect of indirectly facilitated on the associated plant communities depended on the functional trait considered. Indirect facilitation decreased the effects of grazing on species relative abundance, mean LDMC, and the convergence of the maximum height distribution of the associated communities, but did not affect mean height or cover. The identity of the nurse species and grazing jointly affected the structure of the associated plant community through indirect facilitation. Our results together with the existing literature suggest that the “grazer–nurse–beneficiary” interaction module can be more complex than expected when evaluated in the field.

植食动物介导的植物间易化作用，是指不可食用植物为另一植物提供植食防御保护时发生的间接相互作用。这类间接易化作用会对全球各类生态系统中的物种共存与生态系统功能产生显著影响。然而，迄今为止，相较于植物间直接易化的研究关注度，此类间接易化作用的相关研究仍相对不足。本研究旨在获取群落尺度下间接易化作用的原始数据，以明确植食动物介导的间接易化作用对植物群落的塑造程度。此类实验数据有望助力完善胁迫环境下植物-植物-植食动物互作的概念框架。我们在玻利维亚的热带高山泥炭地开展了为期2年的围栏禁牧实验。该类生态系统完全依赖少数几类具有群落构建作用的垫状植物（下文简称“保育种”），伴生植物群落便依托此类垫状植物生长。实验围栏设置于两种应对植食策略各异的保育种之上：分别采用直接防御与间接防御策略，预计二者可为伴生群落带来不同强度的间接易化作用。我们分别在放牧条件（此时存在间接易化作用）与禁牧条件下，于保育种及其伴生植物群落中采集了已知随植食压力变化的功能性状数据：叶片干物质含量（LDMC）、叶片厚度与最大株高。研究发现，间接易化作用对伴生植物群落的影响取决于所考量的功能性状类型。间接易化作用可降低植食对伴生群落物种相对多度、平均叶片干物质含量以及最大株高分布收敛性的影响，但对群落平均株高与盖度无显著作用。保育种的种类与植食压力二者共同通过间接易化作用调控伴生植物群落的结构。结合现有文献来看，本研究结果表明，若在野外实地评估，“植食动物-保育种-受益种”的互作模块可能比此前预期更为复杂。