Wetlands are landscape sinks particularly susceptible to invasions. Wetland invaders often form monotypic stands which lower species diversity, increase productivity and change ecosystem properties. The focus of this study was Phragmites australis which is widely distributed and expanding in the US states located along the Gulf Coast of Mexico. This is the first study to report both above-and belowground effects of P. australis along a salinity gradient in coastal Louisiana wetlands. We assessed the effects of P. australis invasion by establishing 8 sites and 168 plots across a salinity gradient. At each site, we established 3 transects (native, transition and invaded). In each plot, we measured species richness, productivity and soil porewater salinity at 2 depths (15 and 45 cm). For a subset of sites and dominant species along a salinity gradient, we cultured root endophytic fungal communities in the laboratory. We hypothesised that P. australis will exert strong effects both above- and belowground. Specifically, we predicted that species richness will be lowest in P. australis (i.e. invaded plots) and highest in native plots, whereas ecosystem productivity, on the contrary, will be highest in invaded and lowest in native plots. We also predicted that soil porewater salinity will vary more in P. australis plots as it has been reported to have a higher tolerance for fluctuations in salinity. Finally, we expected that P. australis will harbour different fungal endophytes as compared to other species, which may play an important role in its salinity tolerance in these wetlands.
Results/Conclusions
Our results showed that plant species richness was significantly higher in transition plots that contained ~ 50% P. australis, whereas species richness in invaded and native plots was surprisingly similar. The aboveground biomass was similar across all transects; whereas litter amount was highest in P. australis plots. Soil porewater salinity at 15 and 45 cm was consistently lowest in P. australis plots, contrary to our expectations. Fungal taxa cultured from roots of P. australis were different than the fungi cultured from roots of native Spartina sp. grasses. Our results suggest that in these species poor (i.e. on average 3-4 species) wetlands P. australis invasion has negligible effects on species richness and ecosystem productivity. We found that the strongest ecosystem impacts of P. australis were evident belowground, both on abiotic (salinity) and biotic (fungal communities) conditions suggesting that P. australis expansion in these wetlands may drastically alter these ecosystems in the future.