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 aboveground (plant) and belowground (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 belowground communities would vary across the salinity
gradient and that the responses of above and belowground 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 aboveground 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 belowground communities show
some coupled responses to Phragmites; thus understanding plant community
responses to invasion gives insight into impacts occurring belowground.