Monitoring UK saltmarsh restoration using earth observation for national greenhouse gas accounting

Saltmarsh habitats are recognised for their role in long-term sequestration and storage of ‘blue carbon’. Historically, saltmarshes have been subject to high levels of drainage and land-use change, resulting in past CO2 emissions. However, reversing these management practices through rewetting and revegetation of saltmarsh at coastal realignment sites presents opportunities to sequester carbon as part of a nature-based mitigation against climate change. We used Google Earth Engine to develop a model based on satellite data that can monitor annual saltmarsh vegetation cover changes following restoration across five UK test sites. Classification of saltmarsh communities was ultimately based on monthly 75th centile Normalized Difference Vegetation Index (NDVI) values, which represents “greenness” of the land surface, to minimise the effect of tidal phases. Model results demonstrate a useful method for historic and continued monitoring of saltmarsh habitat condition, with initial accuracy of 62% for the NDVI classification. However, when classification was simplified into vegetated versus bare soils (i.e. the criteria for inclusion into national GHG inventory reporting) accuracy of over 90% was reported. This method provides a living picture of colonised saltmarsh that could be upscaled for tracking the carbon removals associated with saltmarsh restoration and management in GHG inventories.