Phragmites australis is a tall, perennial reed that forms dense stands in wetland habitats. Although it is an introduced species, it is a critical erosion control mechanism in many southeastern coastal marshes. Recently documented dieback in P. australis stands could have devastating ecological and economic consequences for the Mississippi River Delta. In conjunction with the U.S. Fish and Wildlife Service, USGS scientists used two complementary sources of satellite data to map P. australis changes in the lower Mississippi River Delta of Louisiana from 2016 to 2019: synthetic aperture radar (SAR) data from the Advanced Land Observing Satellite-2 (ALOS-2) and optical data from the Landsat and Sentinel-2 platforms. The information about vegetation structure derived from SAR data enhances optical mapping of P. australis live fractional cover (LFC). Yearly maps from 2016 to 2019 of SAR data indicated broad density changes in P. australis marsh. The research team used the enhanced combination of density and LFC measurements to develop a tool to track P. australis marsh condition. Major findings from the application of the density-LFC trajectory tool were the high decrease in marsh density from 2016 to 2017, identification of severely degraded P. australis marsh and European P. australis marsh, and indications of linkage between the density decline from 2016 to 2017 and elephant-ear (Colocasia esculenta) replacement from 2018 to 2019. The trajectory tool application also uncovered an inverse relationship between elephant-ear occurrence and density changes from 2018 to 2019. If substantiated, direct elephant-ear replacement detection based on canopy structure would be a major development in mapping and management. Relationships uncovered by the trajectory tool demonstrate enhanced detection of P. australis marsh condition change and possibly even future change, differentiation at the taxa level, and a means for early detection of P. australis marsh replacement by elephant-ear and other unwanted plant species.
Advanced Land Observing Satellite-2 synthetic aperture radar (SAR) color composite indicates variation in yearly marsh density by displaying density as red (2016), green (2017), and blue (2018). Higher densities are most often represented by oranges, pinks, and off-whites.
The 2019 minus 2018 elephant-ear difference compared to the 2019 minus 2018 HV-amplitude. Green site symbols were used in the regression and red symbols were not. [RMSE, root-mean-square error]