Previous syntheses on the effects of environmental conditions on the outcome of plant-plant interactions summarize results from pairwise studies. However, the upscaling to the community-level of such studies is problematic because of the existence of multiple species assemblages and species-specific responses to both the environmental conditions and the presence of neighbors. We conducted the first global synthesis of community-level studies from harsh environments, which included data from 71 alpine and 137 dryland communities. Here we: i) test how important are facilitative interactions as a driver of community structure, ii) evaluate whether the frequency of positive plant-plant interactions across differing environmental conditions and habitats is predictable, and iii) assess whether thresholds in the response of plant-plant interactions to environmental gradients exists between "moderate" and "extreme" stress levels. We also used those community-level studies performed across gradients of at least three points to evaluate how the average environmental conditions, the length of the gradient studied, and the number of points sampled across such gradient affect the form and strength of the facilitation-environment relationship. Over 25% of the species present were more spatially associated to nurse plants than expected by chance in both alpine and dryland areas, illustrating the high importance of positive plant-plant interactions for the maintenance of plant diversity. Facilitative interactions were more frequent, and more related to environmental conditions, in alpine than in dryland areas, perhaps because drylands are generally characterized by a larger variety of environmental stress factors and plant functional traits. The frequency of facilitative interactions in alpine communities peaked at 1000 mm of annual rainfall, and globally decreased with elevation. The frequency of positive interactions in dryland communities decreased globally with water scarcity or temperature annual range. Positive facilitation-drought stress relationships are more likely in shorter regional gradients, but these relationships are obscured in regions with a greater species turnover or with complex environmental gradients. By showing the different climatic drivers and behaviors of plant-plant interactions in dryland and alpine areas, our results will improve predictions regarding the effect of facilitation on the assembly of plant communities and their response to changes in environmental conditions.

中文翻译：

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植物-植物相互作用、环境梯度和植物多样性：社区水平研究的全球综合。

先前关于环境条件对植物-植物相互作用结果的影响的综合总结了成对研究的结果。然而，由于存在多种物种组合以及对环境条件和邻居存在的物种特异性反应，将此类研究升级到社区水平是有问题的。我们首次对来自恶劣环境的社区级研究进行了全球综合，其中包括来自 71 个高山和 137 个旱地社区的数据。在这里，我们：i）测试促进相互作用作为群落结构驱动因素的重要性，ii）评估不同环境条件和栖息地之间植物-植物积极相互作用的频率是否可预测，iii) 评估植物-植物相互作用对环境梯度的响应阈值是否存在于“中等”和“极端”压力水平之间。我们还使用在至少三个点的梯度上进行的社区级研究来评估平均环境条件、所研究的梯度长度以及在这种梯度上采样的点数如何影响便利化环境的形式和强度关系。在高山和旱地地区，超过 25% 的物种在空间上与护理植物的关联性比偶然预期的要高，这说明植物与植物之间的积极相互作用对于维持植物多样性非常重要。促进互动更频繁，与环境条件更相关，高山地区比旱地地区多，这可能是因为旱地的环境胁迫因素和植物功能性状的种类较多。高山群落中促进相互作用的频率在年降雨量 1000 毫米时达到峰值，并且随着海拔的升高而在全球范围内下降。全球干旱地区社区积极互动的频率随着缺水或年温度变化而下降。在较短的区域梯度中，正促进-干旱胁迫关系更有可能出现，但在物种更替或环境梯度复杂的区域，这些关系变得模糊。通过展示旱地和高山地区植物-植物相互作用的不同气候驱动因素和行为，