Wetland loss in coastal Louisiana outpaces rates of loss in other areas and is an economically important environmental threat. The wetland plant populations that structure these ecosystems are subject to intensifying environmental stressors, including sea level rise, saltwater intrusion and warming temperatures. Mortality in these populations intensifies land loss and stress on neighboring populations, which increases coastal vulnerability to extreme weather events including storm surge and flooding; however, specific drivers of population decline are not well understood. The objective of this study was to test how variation in environmental salinity, water level and water temperature regulates wetland plant population dynamics. To accomplish this, we constructed density based population models of the common wetland grass Phragmites australis, which is a problematic invasive species in many areas of North America but can also contribute to wetland resilience. We studied 43 populations across coastal Louisiana using vegetation surveys and environmental data collected by the Coastal Protection and Restoration Authority (CRMS) over an 8-year period from 2010 to 2017. We hypothesize that Phragmites australis populations will increase over time, but that high levels of growing season environmental stress will negatively affect population growth.
Results/Conclusions
We find that Phragmites australis population growth is negatively affected by mean growing season surface water salinity and water temperature. Additionally, we find that population growth responds positively to growing seasons containing high maximum salinity or low minimum water level events. Finally, we find that populations of Phragmites australis have increased in abundance across coastal Louisiana between 2010 and 2017. Temporal autocorrelation in the data indicates that previous year conditions of Phragmites populations are predictive of present year conditions, but the effect of year is not correlated with any environmental variable. These results suggest that while Phragmites is sensitive to chronically high salinity or temperature throughout the growing season, its populations are robust to short term environmental disturbances. Positive correlation of Phragmites with disturbance events may reflect opportunistic expansion of this model invasive species. Overall, results describe an increasing population that is sensitive to growing season environmental stress, suggesting that Louisiana wetlands dominated by this species will be susceptible to future land loss.