Data from: Plant–plant interactions as a mechanism structuring plant diversity in a Mediterranean semi-arid ecosystem

Plant–plant interactions are among the fundamental processes that shape structure and functioning of arid and semi-arid plant communities. Despite the large amount of studies that have assessed the relationship between plant–plant interactions (i.e., facilitation and competition) and diversity, often researchers forget a third kind of interaction, known as allelopathy. We examined the effect of plant–plant interactions of three dominant species: the perennial grass Lygeum spartum, the allelopathic dwarf shrub Artemisia herba-alba, and the nurse shrub Salsola vermiculata, on plant diversity and species composition in a semi-arid ecosystem in NE Spain. Specifically, we quantified the interaction outcome (IO) based on species co-occurrence, we analyzed diversity by calculation of the individual species–area relationship (ISAR), and compositional changes by calculation of the Chao-Jaccard similarity index. We found that S. vermiculata had more positive IO values than L. spartum, and A. herba-alba had values between them. Lygeum spartum and A. herba-alba acted as diversity repellers, whereas S. vermiculata acted as a diversity accumulator. As aridity increased, A. herba-alba transitioned from diversity repeller to neutral and S. vermiculata transitioned from neutral to diversity accumulator, while L. spartum remained as diversity repeller. Artemisia herba-alba had more perennial grass species in its local neighborhood than expected by the null model, suggesting some tolerance of this group to its “chemical neighbor”. Consequently, species that coexist with A. herba-alba were very similar among different A. herba-alba individuals. Our findings highlight the role of the nurse shrub S. vermiculata as ecosystem engineer, creating and maintaining patches of diversity, as well as the complex mechanism that an allelopathic plant may have on diversity and species assemblage. Further research is needed to determine the relative importance of allelopathy and competition in the overall interference of allelopathic plants.植物间相互作用（plant–plant interactions）是塑造干旱与半干旱植物群落结构与功能的核心过程之一。尽管已有大量研究探讨了植物间相互作用（即促进作用与竞争作用）与群落多样性之间的关联，但研究者往往忽略了第三种互作类型——化感作用（allelopathy）。本研究针对西班牙东北部半干旱生态系统中的3个优势物种展开了植物间相互作用效应分析：多年生草本Lygeum spartum、具有化感作用的矮灌木Artemisia herba-alba，以及护生灌木（nurse shrub）Salsola vermiculata，旨在探究其对植物群落多样性与物种组成的影响。具体而言，本研究基于物种共现性量化了互作结果（interaction outcome, IO），通过计算单种-面积关系（individual species–area relationship, ISAR）分析群落多样性，并通过蔡斯-雅卡尔相似度指数（Chao-Jaccard similarity index）分析物种组成变化。研究结果显示，S. vermiculata的互作结果值显著高于L. spartum，而A. herba-alba的互作结果值介于二者之间。Lygeum spartum与A. herba-alba对群落多样性具有抑制作用，而S. vermiculata则为多样性促进者。随着干旱度升高，A. herba-alba从多样性抑制者转变为中性作用类型，S. vermiculata则从中性转变为多样性促进者，而Lygeum spartum始终保持多样性抑制作用。Artemisia herba-alba所在局部群落中的多年生草本物种数量高于零模型（null model）的预期值，表明该类群对其“化学邻株”具有一定耐受性。因此，与A. herba-alba共存的物种在不同A. herba-alba个体间的组成极为相似。本研究结果凸显了护生灌木S. vermiculata作为生态系统工程师（ecosystem engineer）的作用——其能够构建并维持多样性斑块，同时揭示了化感植物对群落多样性与物种组合的复杂调控机制。未来仍需开展进一步研究，以明确化感作用与竞争作用在化感植物整体干扰效应中的相对重要性。