1. Invasive plants often successfully occupy large areas encompassing broad environmental gradients in their invaded range, yet how invader dominance and effects on ecological communities vary across the landscape has rarely been explored. Furthermore, while the impacts of invasion on plant communities are well studied, it is not well understood whether responses of above‐ground (plant) and below‐ground (microbial) communities are coupled.
2. Here we test patterns in Phragmites australis (common reed) invasion in a field survey of eight sites situated across a salinity gradient, ranging from freshwater to saline marsh, in Southeast Louisiana. At each site, we surveyed plant composition and used metabarcoding methods to assess soil fungal and bacterial composition in plots within the dense Phragmites stand, in a transition zone of ~50:50 Phragmites:native plants, and in native‐only areas. We hypothesized that Phragmites' abundance and impact on above‐ and below‐ground communities would vary across the salinity gradient and that the responses of above‐ and below‐ground communities to invasion would be coupled.
3. We found weak evidence that invasion varied across the gradient: Phragmites stem densities increased slightly with salinity, and Phragmites increased above‐ground litter accumulation more in fresh and saline areas compared to brackish. We found stronger evidence that plant and microbial responses to invasion varied with salinity. Phragmites strongly reduced native plant density across the gradient, with slightly greater reductions in fresh and saline areas. Plant species richness displayed consistent decreases with invasion across the salinity gradient; however, fungal and bacterial richness increased sharply with invasion only in brackish sites. Furthermore, the effect of Phragmites on plant and microbial community composition became stronger as salinity increased. Plants and microbes exhibited coupled responses to invasion in the magnitude of compositional shifts brought on by Phragmites, but Phragmites' effects on richness were not coupled.
4. Synthesis. Overall, the variability in Phragmites impacts across the gradient, particularly soil microbial impacts, suggests that it may be difficult to generalize invader effects from single‐site or single‐ecosystem studies. However, above‐ and below‐ground communities showed some coupled responses to Phragmites; thus understanding plant community responses to invasion gives insight into impacts occurring below‐ground.

中文翻译：

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入侵物种对植物和微生物的影响随环境梯度的变化而变化

1. 入侵植物通常成功地占据了其入侵范围内大范围环境梯度的大面积区域，但很少探索入侵者在生态系统中的优势地位和对生态群落的影响如何变化。此外，尽管已经深入研究了入侵对植物群落的影响，但对地上（植物）群落和地下（微生物）群落的响应是否耦合还没有很好的了解。
2. 在这里，我们通过对路易斯安那州东南部盐度梯度（从淡水到盐碱沼泽）不等的八个地点进行实地调查，测试了芦苇（普通芦苇）入侵的模式。在每个站点，我们调查了植物组成，并使用元条形码方法评估了密集芦苇林中，芦苇：原生植物约50:50过渡区以及仅本地地区中的土壤真菌和细菌的组成。我们假设芦苇的丰度和对地上和地下社区的影响在整个盐度梯度上会有所不同，并且地上和地下社区对入侵的反应将是耦合的。
3. 我们发现微弱的证据表明入侵随梯度的变化而变化：与咸淡相比，芦苇的茎密度随盐度的增加而略有增加，而芦苇在新鲜和盐渍区的地上凋落物的积累更多。我们发现更有力的证据表明植物和微生物对入侵的反应随盐度的变化而变化。芦苇在整个梯度上都大大降低了原生植物的密度，新鲜和盐渍区的降低幅度更大。随着盐度梯度的入侵，植物物种丰富度不断降低。然而，真菌和细菌的丰富性仅在微咸的地方随入侵而急剧增加。此外，芦荟的功效随着盐度的增加，植物和微生物群落的组成变得更强。植物和微生物在芦苇引起的成分变化幅度上表现出对入侵的耦合反应，但是芦苇对丰富度的影响没有耦合。
4. 综合。总体而言，芦苇在整个梯度上的影响具有变化性，尤其是土壤微生物的影响，表明可能难以从单点或单生态系统研究中概括入侵者的影响。然而，地上和地下社区对芦苇表现出一些耦合反应。因此，了解植物群落对入侵的反应可以洞悉地下发生的影响。