Extreme salinity fluctuations and invasion by Phragmites australis in wetland ecosystems

Extreme salinity fluctuations and invasion by Phragmites australis in wetland ecosystems

Background/Question/Methods
Ongoing climate change is producing extreme weather events that are likely to modify local ecosystems. For example, increased hurricane intensity combined with sea level rise is expected to have a deleterious effect on coastal plant communities, which in turn causes an increase in wave energy and land loss. Coastal land loss rate equates to about 56km2 per year in Louisiana alone. Plants play pivotal role in stabilizing wetlands ecosystems and yet understanding of what specific environmental and biological factors are facilitating invasion of alien plants species and support native wetland plant communities is often missing.

We focus on the model invasive plant, Phragmites australis, to understand what drives plant invasion in Gulf Coast and what its long-term effect on native plant communities is. Using long-term environmental data (314 sites, over 2009–2016 period) from Coastal Protection and Restoration Authority (CRMS) in Louisiana, we sought to answer the following questions: 1) What is the interannual variability in water salinity and water table level on Louisiana coast? 2) How does the variability in water salinity and water table affect local plant communities and occurrence of P. australis? 3) How does the presence of P. australis affect local plant communities?

Results/Conclusions

Mean surface water salinity in Louisiana coastal communities showed a slight decline and water table increased over 2009–2016. Variability in salinity in research sites ranged from a 94.2% decrease to a 113% increase compared to the previous year, while variability in water table ranged from 4458% decrease to 2657% increase. Despite negative association with mean salinity, invasive P. australis was most likely to occur in intermediate wetland communities that recorded the broadest oscillations in mean water salinity (0.171–30.7 PPT) and mean water table (-37.2–35.2 cm). Similarly, presence of P. australis showed the highest impact on plant composition in intermediate communities explaining 7.6 % of variation (P < 0.01).

The observed extreme fluctuations in salinity and water table may destabilize local plant communities and open an invasion window for non-native plants such as P. australis. P australis likely performs well in areas of variable, especially high, water levels because of its adaptive traits for inundation and anaerobic conditions (e.g., aerenchyma). Looking to the future, if variability in water tables and salinity continues to rise, this invasive plant may be well-suited to these anthropogenic conditions and may actually help to stabilize marshes and prevent land loss.

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