Understanding the States and Dynamics of Mangrove Forests in Land Cover Transitions of The Gambia Using a Fourier Transformation of Landsat and MODIS Time Series in Google Earth Engine
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Date
2023
Authors
Harou, Issoufou Liman
Inyele, Julliet
Minang, Peter
Minang, Peter
Duguma, Lalisa
Journal Title
Journal ISSN
Volume Title
Publisher
Frontiers in Forests and Global Change
Abstract
Introduction: Mangroves are resilient forests of transitional zones between
land, ocean and freshwater for they are tolerant to salinity. In The Gambia,
mangrove forests are found along the coast of Atlantic Ocean and River
Gambia where they support the livelihoods of millions through multiple
ecosystems services. Despite their importance in The Gambia, consistent
country-wide information on their coverage, dynamics and change hotspots
are lacking. Thus far, it remains unclear whether the coverage of mangroves
has decreased or increased over the last few decades. Often, the existing
estimates vary strongly across sources and the methodologies in the available
literature are not always reproducible. This study attempts to fill these gaps by
providing up-to-date spatial information on mangrove forests in The Gambia.
Methods: To provide a reproducible workflow and a comprehensive
assessment, we used continuous time series of freely available data to study
the extent and dynamics of mangrove forests in The Gambia. To construct
gap-free image time series, we used statistical models to describe land surface
phenology based on monthly composites derived from Landsat and MODIS
land surface reflectance acquired between 2000 and 2020. We used 1212
Landsat and 407 MODIS tiles spread across multiple spectral indices along with
a calibrated set of training and validation data to train and validate a random
forest classifier for accurate land cover classifications.
Results and discussion: The overall accuracy and Kappa statistics of
the classifications range between 0.96 and 0.98. Our findings suggest a
net increase of 4,000 ha in mangrove forests over the last 2 decades,
corresponding to an annual rate of 200 ha. The net increase is largely
due to strong policy making which results in participative forest resource
management through the national forest action plan. The net increase in
mangrove forests should not mask the substantial degradation in some places
across the country. We estimated a total loss of nearly 5,670 ha, most of which
appears to have taken place during the last decade in favor of wetlands. The
Gambian mangroves are amongst the most promising green business in Gulf of
Guinea, deserving an integrated governance - policy framework that address
the key requirements for ecological sustainability.
Description
Article
Keywords
satellite image time series, landsat observations, MODIS collection, harmonic modeling, gap-filling, land cover classification, mangrove, change detection
Citation
Liman Harou, I., Inyele, J., Minang, P., & Duguma, L. (2023). Understanding the states and dynamics of mangrove forests in land cover transitions of The Gambia using a Fourier transformation of Landsat and MODIS time series in Google Earth Engine. Frontiers in Forests and Global Change, 5, 934019.