Studies of facilitative interactions as drivers of plant richness along environmental gradients often assume the existence of an overarching stress gradient equally affecting the performance of all the species in a given community. However, co-existing species differ in their ecophysiological adaptations, and do not experience the same stress level under particular environmental conditions. Moreover, these studies assume a unimodal richness-biomass curve, which is not as general as previously thought. We ignored these assumptions to assess changes in plant-plant interactions, and their effect on local species richness, across environmental gradients in semi-arid areas of Spain and Australia. We aimed to understand the relative importance of direct (microhabitat amelioration) and indirect (changes in the competitive relationships among the understorey species: niche segregation, competitive exclusion or intransitivity) mechanisms that might underlie the effects of nurse plants on local species richness. By jointly studying these direct and indirect mechanisms using a unifying framework, we were able to see how our nurse plants (trees, shrubs and tussock grasses) not only increased local richness by expanding the niche of neighbouring species, but also by increasing niche segregation among them, though the latter was not important in all cases. The outcome of the competition-facilitation continuum changed depending on the study area, likely because the different types of stress gradient considered. When driven by both rainfall and temperature, or rainfall alone, the community-wide importance of nurse plants remained constant (Spanish sites), or showed a unimodal relationship along the gradient (Australian sites). This study expands our understanding of the relative roles of plant-plant interactions and environmental conditions as drivers of local species richness in semi-arid environments. These results can also be used to refine predictions about the response of plant communities to environmental change, and to clarify the relative importance of biotic interactions as a driver of such responses.

中文翻译：

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改善微生境并减少林下植物之间的竞争作为促进环境梯度的驱动因素：迈向统一框架。

促进相互作用作为沿环境梯度的植物丰富度驱动因素的研究通常假设存在一个总体压力梯度，同样影响给定群落中所有物种的表现。然而，共存物种的生态生理适应不同，并且在特定环境条件下不会经历相同的压力水平。此外，这些研究假设单峰丰富度 - 生物量曲线，这并不像以前认为的那样普遍。我们忽略了这些假设，以评估植物-植物相互作用的变化，以及它们对西班牙和澳大利亚半干旱地区环境梯度的当地物种丰富度的影响。我们旨在了解直接（改善微生境）和间接（下层物种之间竞争关系的变化：生态位隔离、竞争排斥或不传递性）机制的相对重要性，这些机制可能是苗木对当地物种丰富度影响的基础。通过使用统一的框架共同研究这些直接和间接机制，我们能够看到我们的护生植物（树木、灌木和草丛）不仅通过扩大邻近物种的生态位来增加当地的丰富度，而且通过增加之间的生态位隔离。它们，尽管后者在所有情况下都不重要。竞争促进连续体的结果因研究区域而异，可能是因为考虑了不同类型的应力梯度。当降雨和温度，或仅由降雨驱动时，苗木在整个社区范围内的重要性保持不变（西班牙地点），或沿梯度显示出单峰关系（澳大利亚地点）。这项研究扩展了我们对植物-植物相互作用和环境条件作为半干旱环境中当地物种丰富度驱动因素的相对作用的理解。这些结果还可用于改进关于植物群落对环境变化反应的预测，并阐明生物相互作用作为这种反应驱动因素的相对重要性。这项研究扩展了我们对植物-植物相互作用和环境条件作为半干旱环境中当地物种丰富度驱动因素的相对作用的理解。这些结果还可用于改进关于植物群落对环境变化反应的预测，并阐明生物相互作用作为这种反应驱动因素的相对重要性。这项研究扩展了我们对植物-植物相互作用和环境条件作为半干旱环境中当地物种丰富度驱动因素的相对作用的理解。这些结果还可用于改进关于植物群落对环境变化反应的预测，并阐明生物相互作用作为这种反应驱动因素的相对重要性。