In many areas worldwide, coastal wetlands are threatened by saltwater intrusion caused by rising sea levels and increases in severity of hurricanes and storms. It is well known that elevated salinity generally has negative effects on marsh integrity, however, how and whether some plant communities may confer resilience to those effects has been severely understudied. For example, two mechanisms that can potentially confer resilience to environmental stressors are high diversity and a tolerant dominant species. Understanding how these processes contribute to ecosystem stability is essential to successful coastal management and restoration. Here we assess how plant communities are impacted by salinity, how they contribute to ecosystem function, and whether mechanisms conferring resilience vary across wetland types in coastal Louisiana. Using data from the Coastwide Reference Monitoring System (CRMS), we analyze plant composition and accretion data from 269 sites during the period 2007-2016. For each wetland type, we test whether salinity impacts plant diversity and whether changes in plant community composition affect accretion rates. We hypothesize that increased salinity will be associated with a decrease in diversity, higher species diversity will be associated with higher accretion rates, and that individual species will differ in their ability to promote accretion.
Results/Conclusions
Results show that salinity has a negative impact on plant diversity across most wetland types (significant for freshwater, intermediate, and saline communities). We find that diversity significantly increases vertical accretion only in intermediate marshes and that dominant species’ impacts on accretion differ across wetland types. In saline wetlands, Juncus roemarinus has a negative effect on accretion. In brackish sites, Spartina alterniflora and Schoenoplectus americanus have a positive effect on accretion. In intermediate sites, Spartina patens negatively impacts accretion, while Vigna luteola and Schoenoplectus americanus positively impact accretion. In freshwater sites, Sagitaria lanceolata negatively affects accretion. These results suggest that saltwater intrusion may result in decreased diversity in all wetland types, but changes in species diversity per se may not impact accretion rates. However, the presence of individual species can either promote or hinder accretion and this pattern varies across wetland type. This underscores the importance of species selection in future coastal restoration planning that aims to mitigate land loss.