The rapidly declining coastal wetlands of Louisiana are experiencing threats from climate change (abiotic factors) as well as species invasions (biotic factors), yet these wetlands are important for coastal communities by providing buffering from storm surge, stabilizing shorelines, reducing wave energy and supporting the fisheries. We focus on the model invasive plant, Phragmites australis (common reed), which is a widespread wetland invader across North America, including in the Gulf Coast, and its effects on co-occurring native vegetation and wetland abiotic conditions across a wetland salinity gradient (i.e., freshwater, intermediate, brackish and saline). Understanding short- and long-term vegetation dynamics of these coastal wetlands is critical to better predict the vegetation patterns and dynamics in the future. Using the vegetation and environmental data from Coastal Protection and Restoration Authority (CRMS) in Louisiana, we analysed vegetation data from 197 CRMS stations across 1968 plots and 84803 observations for the period 2006 -2016. We tested whether wetland types differ in their susceptibility to vegetation change over 10 years and how P. australis affects plant richness along a temporal scale and salinity gradient.
Preliminary results suggest that total mean species richness is higher in freshwater as compared to intermediate and brackish wetlands and lowest in saline wetlands. P. australis was most likely to invade the open intermediate communities. In intermediate wetlands, P. australis mean percent cover per plot was 27.4% (27% natives) compared to 8.8% (66.6 % natives) in freshwater, 7.1 % (66.4 % natives) and 2.7% ( 65.1 % natives) in brackish and saline, respectively, suggesting that P. australis is more abundant in intermediate wetlands. Soil organic matter decreased across the salinity gradient. Organic matter (-0.03, p < 0.0001), soil pH (-0.17, p < 0.0001) and salinity (-0.12, p < 0.0001) were negatively associated with P. australis presence across all wetland types. These results suggest that P. australis may reduce native species richness only in specific wetland niches along the coastal Louisiana with intermediate wetlands being more likely invaded by P. australis.