Direct and indirect effects of climate change in coastal wetlands: will climate change influence wetlands by affecting plant invasion?
Introduced species and climate change can have direct impacts on wetland communities, but they can also produce indirect effects such that climate change (i.e., effects of flooding and salinity) can affect native plants by exacerbating or reducing invasion. We assessed the direct and indirect effects of flooding, salinity, and introduced species on wetland communities across a salinity gradient by analyzing existing monitoring data from over 390 Coastwide Reference and Monitoring Sites (CRMS) using path analysis. As expected, we found that introduced species cover and richness was highest in fresh marshes and decreased across the salinity gradient. In fresh marshes, introduced cover and salinity separately negatively affected native cover and richness, but there were no indirect effects. In intermediate marshes, introduced cover and salinity reduced native cover and richness, and indirect effects were weakly positive because salinity negatively affected introduced cover. In brackish and saline marshes increasing salinity and flooding reduced native cover and richness. Our results suggest that climate change will negatively affect all wetland plant communities, and invasion will negatively affect fresher wetlands; however, climate change will not exacerbate invasions in wetlands and could reduce introduced species effects.