Indicators of Expansion and Retreat of Phragmites Based on Optical and Radar Satellite Remote Sensing: a Case Study on the Danube Delta

Reed is an important wetland species. In some places it provides valuable ecosystem services, while in other places it poses a threat as an invasive species. Thus, monitoring and predicting reed dynamics is crucial. We not only detected changes in reed area using remote sensing, but also developed indicators for the stability of reed wetlands based on remote sensing that would allow to predict its future development. We used satellite imagery to study reed development in the Danube Delta in Romania over a period of 22-years and identified expanding, stable and retreating reedlands. We then compared optical vegetation indices and radar backscatter among those three different reed development categories. We found clear spatial differences in long-term reed dynamics. We also revealed a clear difference in radar backscatter, but no difference in the optical signal of expanding, stable and decreasing reed areas. The radar data showed the largest seasonal variation in locations where reed was expanding and smallest seasonal variation where reed was decreasing. Overall, our study shows that the stability of reed ecosystems, and their services, can be monitored by quantifying seasonal changes in backscatter of reed-lands using radar satellites. This principle looks promising for monitoring other ecosystems as well.