Salt Water Exposure Exacerbates the Negative Response of Phragmites australis Haplotypes to Sea-Level Rise

Abstract

The response of coastal wetlands to sea-level rise (SLR) largely depends on the tolerance of individual plant species to inundation stress and, in brackish and freshwater wetlands, exposure to higher salinities. Phragmites australis is a cosmopolitan wetland reed that grows in saline to freshwater marshes. P. australis has many genetically distinct haplotypes, some of which are invasive and the focus of considerable research and management. However, the relative response of P. australis haplotypes to SLR is not well known, despite the importance of predicting future distribution changes and understanding its role in marsh response and resilience to SLR. Here, we use a marsh organ experiment to test how factors associated with sea level rise-inundation and seawater exposure-affect the porewater chemistry and growth response of three P. australis haplotypes along the northern Gulf of Mexico coast. We planted three P. australis lineages (Delta, European, and Gulf) into marsh organs at five different elevations in channels at two locations, representing a low (Mississippi River Birdsfoot delta; 0-13 ppt) and high exposure to salinity (Mermentau basin; 6-18 ppt) for two growing seasons. Haplotypes responded differently to flooding and site conditions; the Delta haplotype was more resilient to high salinity, while the Gulf type was less susceptible to flood stress in the freshwater site. Survivorship across haplotypes after two growing seasons was 42% lower at the brackish site than at the freshwater site, associated with high salinity and sulfide concentrations. Flooding greater than 19% of the time led to lower survival across both sites linked to high concentrations of acetic acid in the porewater. Increased flood duration was negatively correlated with live aboveground biomass in the high-salinity site (χ² = 10.37, p = 0.001), while no such relationship was detected in the low-salinity site, indicating that flood tolerance is greater under freshwater conditions. These results show that the vulnerability of all haplotypes of P. australis to rising sea levels depends on exposure to saline water and that a combination of flooding and salinity may help control invasive haplotypes.

Keywords: dieback; ecosystem services; habitat loss; invasion; restoration; sea-level rise; stress; wetlands.

Figure 1

Marsh organ percent time flooded, and water depth across both sites. Differences in percent time flooded between sites were due to the influence of river (MRD) versus coastal bay with limited exchange (Rockefeller) hydrodynamics.

Figure 2

Flooding and salinity levels in each site during the marsh organ study period in the Mississippi River Delta (MRD) and Rockefeller Wildlife Refuge. Some data are missing at Rockefeller due to errors with the Coastwide Reference Monitoring System data loggers.

Figure 3

(a) Logistic regression results showing how acetic acid concentration predicts the probability of survival on Phragmites australis in the marsh organs from both the Mississippi River Delta (red circles) and Rockefeller Wildlife Refuge (blue triangles); (b) scatterplot displaying the relationship between percent time flooded and porewater acetic acid concentration in the first growing season.

Figure 4

Boxplots displaying the first growing season (a,b) and second growing season (c,d) relationship between percent time flooded and porewater sulfide concentration in the experimental marsh organs placed in the MRD and in Rockefeller Wildlife Refuge.

Figure 5

Boxplots displaying the relationship between percent time flooding and redox potential recorded at the end of the second growing season in the Mississippi River Delta (a) and Rockefeller Wildlife Refuge (b).

Figure 6

Phragmites transplant survivorship following the first (a,b) and second (c,d) growing season in the marsh organs at the Mississippi River Delta (MRD; a,c), and Rockefeller Wildlife Refuge (b,d).

Figure 7

Plots displaying the relationships between percent time flooded and either aboveground or belowground biomass. Plots (a,b) show results from Delta Phragmites, plots (c,d) show European Phragmites, and (e,f) display Gulf Phragmites. The Mississippi River Delta data is represented by red circles, while Rockefeller Wildlife Refuge is represented by blue triangles.

Figure 8

Scatterplots displaying the relationship between sulfide concentration and biomass (g), separated into above (a) and belowground biomass (b).