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基础过程之一。尽管已有大量研究探讨了植物间相互作用（即促进作用（facilitation）与竞争作用（competition））与多样性之间的关联，但研究者们往往忽略了第三种被称为化感作用（allelopathy）的相互作用类型。本研究选取西班牙东北部半干旱生态系统中的3个优势物种，探究其植物间相互作用对植物多样性与物种组成的影响：多年生草本Lygeum spartum、具化感作用的矮灌木Artemisia herba-alba以及保育灌木（nurse shrub）Salsola vermiculata。具体而言，本研究基于物种共现量化相互作用结果（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，并通过计算蔡斯-雅卡德（Chao-Jaccard）相似性指数探究物种组成变化。研究发现，S. vermiculata的IO值较L. spartum更高，而A. herba-alba的IO值介于二者之间。Lygeum spartum与A. herba-alba均发挥多样性排斥因子的作用，而S. vermiculata则为多样性富集因子。随着干旱度升高，A. herba-alba的作用从多样性排斥因子转向中性，S. vermiculata则从中性转向多样性富集因子，而Lygeum spartum始终为多样性排斥因子。Artemisia herba-alba局域邻域内的多年生草本物种数量较零模型（null model）预期更高，这表明该草本类群对其“化学邻体”存在一定耐受性。因此，不同Artemisia herba-alba个体邻域内与之共存的物种组成极为相似。本研究结果凸显了保育灌木S. vermiculata作为生态系统工程师（ecosystem engineer）的作用——其可营造并维持多样性斑块，同时也揭示了化感植物对群落多样性与物种组配可能存在的复杂调控机制。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的整体种间干扰效应中的相对重要性。