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）与植物多样性之间的关联，但研究者们往往忽略了第三种相互作用类型——化感作用（allelopathy）。本研究聚焦西班牙东北部的半干旱生态系统，探究三种优势植物的种间相互作用对植物多样性及物种组成的影响：多年生草本植物Lygeum spartum、具有化感作用的矮灌木Artemisia herba-alba，以及护养灌木Salsola vermiculata。具体而言，本研究基于物种共现情况量化了相互作用结果（interaction outcome, IO），通过计算单种-面积关系（individual species–area relationship, ISAR）分析物种多样性，并通过计算Chao-Jaccard相似性指数分析物种组成变化。研究结果显示，S. vermiculata的相互作用结果得分较L. spartum更趋正向，而A. herba-alba的得分介于二者之间。Lygeum spartum与A. herba-alba均表现为多样性抑制者，而S. vermiculata则为多样性促进者。随着干旱程度加剧，A. herba-alba的作用从多样性抑制转向中性，S. vermiculata则从中性转向多样性促进，而L. spartum始终维持多样性抑制作用。Artemisia herba-alba的局部群落中，多年生草本物种的数量较零模型预期更多，这表明该草本类群对其‘化学邻株’具有一定耐受性。因此，不同A. herba-alba个体的共存物种组成极为相似。本研究结果凸显了护养灌木S. vermiculata作为生态系统工程师的作用——其可构建并维持多样性斑块，同时也揭示了化感植物对植物多样性及物种集合的复杂调控机制。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的整体干扰效应中的相对重要性。