A previous study (https://doi.org/10.3133/ofr20171098) demonstrated that satellite-derived Normalized Difference Vegetation Index (NDVI) could detect Phragmites australis (marsh grass) dieback caused by Asian scale insect infestation. However, NDVI is a relative indicator when quantitative mapping is needed. Quantification required field spectral reflectance calibration to live fractional cover (LFC) of Phragmites canopies nearly 4 meters tall. This study designed and implemented a satellite NDVI mapping tool that not only detected Phragmites change, it quantified the LFC change magnitude. The key to development of the quantitative LFC mapping was the field-to-satellite calibration design. The calibration of Phragmites marsh LFC to optical satellite imagery combined specialized field and near-real time satellite data collections in the fall of 2018 and summer of 2019. Basing the field NDVI to field LFC and the satellite NDVI to field NDVI calibrations on combined near senescence and peak growth data offers year-round LFC mapping. The utility of the developed Phragmites marsh LFC mapping tool was demonstrated by creating a yearly suite of Mississippi River Delta LFC status and change maps extending from 2009 to 2019. Phragmites marsh LFC mapping relies on Sentinel-2 for current to future mapping and Landsat for historical mapping.
The graph shows the fall to summer field Phragmites australis live fractional cover (LFC) to Normalized Difference Vegetation Index (NDVI) calibration. The LFC calibration was used to calibrate satellite NDVI maps to Phragmites australis LFC maps. The figure on the right shows a Phragmites australis LFC difference map. Yellow to pink represents LFC decrease and green to blue LFC increase from the early to later date.