Plant and microbial impacts of an invasive species vary across an environmental gradient

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. 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. 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. 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