The divide between Tall and Shrub Mangroves: A case study of Sierra Leone

Mangrove ecosystems are critical to climate change mitigation, biodiversity conservation, and coastal community protection. There is no common methodology of assessing mangrove ecosystems through remote sensing resulting in mangrove cover inaccuracies. One of the recent difficulties is differentiating between shrub and tall mangroves at a fine scale to meet Verra’s fine resolution criteria for baseline deforestation and accurate carbon stocks and GHG removals. We present a preliminary assessment of mapping the mangrove ecosystem in West Africa to differentiate shrub and tall mangrove ecosystems. The objective of this study is to stratify mangrove extent into tall and shrub using local data and estimate aboveground biomass based on this division from random sampling. We present a preliminary analysis of Sierra Leone. We map mangrove extent for the years 2012, 2017 and 2022 covering a 10-year period. We used Landsat 7/8 imagery for 2012 and Sentinel 2 for 2017 and 2022. We then stratify mangroves into tall and shrub mangroves. All analysis was done using the Google Earth Engine Platform. Our analysis shows that in 2022 mangrove extent was 115,021 ha. Independent accuracy assessment of the 2022 classification using drone imagery showed an accuracy of 91% with a user’s accuracy of 84%. Analysis of shrub and tall shows that shrub mangroves had a cover of 56,252 ha, while tall mangroves covered an extent of 58,768 ha in Sierra Leone with an overall accuracy of 75%. Ongoing aboveground biomass survey of random plot selections shows shrub plots (n=22) with a mean AGB of 6.5tC/ha (32% uncertainty) with tall plots (n=22) 81tC/ha (uncertainty 30%). The mean height of shrub plots was 3.7m and tall plots 20.6m. Challenges we encountered include the exclusion of shrub vegetation during mangrove classification and transition zones between shrub and tall and mixed vegetation. We plan on improving our mapping process to tackle these challenges.