Plant-plant interactions are driven by environmental conditions, evolutionary relationships (ER) and the functional traits of the plants involved. However, studies addressing the relative importance of these drivers are rare, but crucial to improve our predictions of the effects of plant-plant interactions on plant communities and of how they respond to differing environmental conditions. To analyze the relative importance of -and interrelationships among- these factors as drivers of plant-plant interactions, we analyzed perennial plant co-occurrence at 106 dryland plant communities established across rainfall gradients in nine countries. We used structural equation modeling to disentangle the relationships between environmental conditions (aridity and soil fertility), functional traits extracted from the literature, and ER, and to assess their relative importance as drivers of the 929 pairwise plant-plant co-occurrence levels measured. Functional traits, specifically facilitated plants' height and nurse growth form, were of primary importance, and modulated the effect of the environment and ER on plant-plant interactions. Environmental conditions and ER were important mainly for those interactions involving woody and graminoid nurses, respectively. The relative importance of different plant-plant interaction drivers (ER, functional traits, and the environment) varied depending on the region considered, illustrating the difficulty of predicting the outcome of plant-plant interactions at broader spatial scales. In our global-scale study on drylands, plant-plant interactions were more strongly related to functional traits of the species involved than to the environmental variables considered. Thus, moving to a trait-based facilitation/competition approach help to predict that: 1) positive plant-plant interactions are more likely to occur for taller facilitated species in drylands, and 2) plant-plant interactions within woody-dominated ecosystems might be more sensitive to changing environmental conditions than those within grasslands. By providing insights on which species are likely to better perform beneath a given neighbour, our results will also help to succeed in restoration practices involving the use of nurse plants.

中文翻译：

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在全球旱地，功能性状比环境或进化相关性更能决定植物的共生。

植物-植物相互作用是由环境条件、进化关系 (ER) 和所涉及植物的功能特征驱动的。然而，解决这些驱动因素相对重要性的研究很少，但对于改进我们对植物-植物相互作用对植物群落的影响以及它们如何响应不同环境条件的预测至关重要。为了分析这些因素作为植物-植物相互作用驱动因素的相对重要性和相互关系，我们分析了 9 个国家的 106 个跨降雨梯度建立的旱地植物群落的多年生植物共生。我们使用结构方程模型来解开环境条件（干旱和土壤肥力）、从文献中提取的功能特征和 ER 之间的关系，并评估它们作为所测量的 929 个成对植物-植物共现水平的驱动因素的相对重要性。功能性状，特别是促进植物的高度和护士生长形式，是最重要的，并调节环境和 ER 对植物-植物相互作用的影响。环境条件和 ER 分别对涉及木质和禾本科植物护士的相互作用很重要。不同植物-植物相互作用驱动因素（ER、功能性状和环境）的相对重要性因所考虑的区域而异，这说明了在更广泛的空间尺度上预测植物-植物相互作用结果的难度。在我们对旱地的全球规模研究中，与所考虑的环境变量相比，植物-植物相互作用与所涉及物种的功能性状更密切相关。因此，转向基于性状的促进/竞争方法有助于预测：1) 旱地中较高的促进物种更有可能发生积极的植物-植物相互作用，以及 2) 以木本为主的生态系统中的植物-植物相互作用可能是对变化的环境条件比草原内的环境条件更敏感。通过提供有关哪些物种可能在给定的邻居下表现更好的见解，我们的结果也将有助于在涉及使用苗木的恢复实践中取得成功。1) 旱地中较高的促进物种更有可能发生积极的植物-植物相互作用，2) 以木本为主的生态系统中的植物-植物相互作用可能比草原内的环境条件对变化的环境条件更敏感。通过提供有关哪些物种可能在给定的邻居下表现更好的见解，我们的结果也将有助于在涉及使用苗木的恢复实践中取得成功。1) 旱地中较高的促进物种更有可能发生积极的植物-植物相互作用，2) 以木本为主的生态系统中的植物-植物相互作用可能比草原内的环境条件对变化的环境条件更敏感。通过提供有关哪些物种可能在给定的邻居下表现更好的见解，我们的结果也将有助于在涉及使用苗木的恢复实践中取得成功。