Invasive plants often successfully occupy large areas encompassing broad environmental gradients in their invaded range, yet how the effects of invasion on ecological communities vary across space has rarely been explored. Furthermore, while the impacts of invasion on plant communities is often well studied, it is not well understood whether aboveground plant communities and belowground soil microbial communities exhibit coordinated responses to plant invasions. Here we test how the invasive grass Phragmites australis alters plant and microbial communities 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 community composition and used metagenomic methods to assess soil fungal and bacterial composition in plots within the dense Phragmites stand, in a transition zone of 50:50 Phragmites and native plants, and in native-only areas (21 plots per site). We hypothesized that Phragmites’ impact on above and belowground communities would vary across the salinity gradient and that above and belowground communities would respond similarly to invasion.
Results/Conclusions
We found that Phragmites consistently decreased plant species richness to the same extent at all sites across the salinity gradient. However, soil fungal and bacterial richness did not respond consistently to Phragmites, rather microbial richness tended to decrease with invasion in fresh areas and increase with invasion in brackish areas but there was a high degree of variation among sites. Phragmites significantly affected plant community composition at all but one site, and the effect was stronger at the more saline end of the gradient. Soil fungal and bacterial composition was less consistently affected by Phragmites with significant effects at six and four sites, respectively. Effects on microbial composition tended to be stronger in brackish areas, and they were coordinated such that sites with large fungal response also had large bacterial response. Overall, results suggest that salinity influences the effect of Phragmites on biotic communities, but there is also a high degree of site-level variation in response to invasion, making broad generalizations and prediction difficult. Furthermore, above and belowground communities do not show synchrony in their response to Phragmites; thus understanding plant community responses to invasion does not give insight into impacts occurring belowground.