Mapping Mangrove Zonation Changes in Senegal with Landsat Imagery Using an OBIA Approach Combined with Linear Spectral Unmixing

Florent Lombard,Julien Andrieu

doi:10.3390/rs13101961

Abstract

The mangrove areas in Senegal have fluctuated considerably over the last few decades, and it is therefore important to monitor the evolution of forest cover in order to orient and optimise forestry policies. This study presents a method for mapping plant formations to monitor and study changes in zonation within the mangroves of Senegal. Using Landsat ETM+ and Landsat 8 OLI images merged to a 15-m resolution with a pansharpening method, a processing chain that combines an OBIA approach and linear spectral unmixing was developed to detect changes in mangrove zonation through a diachronic analysis. The accuracy of the discriminations was evaluated with kappa indices, which were 0.8 for the Saloum delta and 0.83 for the Casamance estuary. Over the last 20 years, the mangroves of Senegal have increased in surface area. However, the dynamics of zonation differ between the two main mangrove hydrosystems of Senegal. In Casamance, a colonisation process is underway. In the Saloum, Rhizophora mangle is undergoing a process of densification in mangroves and appears to reproduce well in both regions. Furthermore, this study confirms, on a regional scale, observations in the literature noting the lack of Avicennia germinans reproduction on a local scale. In the long term, these regeneration gaps may prevent the mangrove from colonising the upper tidal zones in the Saloum. Therefore, it would be appropriate to redirect conservation policies towards reforestation efforts in the Saloum rather than in Casamance and to focus these actions on the perpetuation of Avicennia germinans rather than Rhizophora mangle, which has no difficulty in reproducing. From this perspective, it is necessary to gain a more in-depth understanding of the specific factors that promote the success of Avicennia germinans seeding.

Highlights

Monitoring mangrove forests is essential for their management and conservation [1,2,3].In particular, knowing the dynamics of a mangrove allows us to estimate the future status of the many ecosystem services that it provides [4,5]
Using an OBIA approach with images of the vegetation fraction resulting from linear demixing, a more detailed quantification of the dynamics at work in Senegalese mangroves was carried out
A clear increase in low and dense mangrove and substantial changes from low and open mangrove to low and dense mangrove demonstrate that the mangrove densification of Rhizophora mangle is due, in part, to the failure of Avicennia germinans to reproduce

Summary

Introduction

Monitoring mangrove forests is essential for their management and conservation [1,2,3].In particular, knowing the dynamics of a mangrove allows us to estimate the future status of the many ecosystem services that it provides [4,5]. Monitoring mangrove forests is essential for their management and conservation [1,2,3]. Mangrove forests can contribute to climate change mitigation by sequestering carbon [6,7] and have become a major focus of conservation efforts on a global scale [8]. Mangroves experienced a significant global decline between 1980 and 2005 [16]. This trend has slowed down significantly in recent years [17,18] and is being reversed in some regions, in West Africa [19]

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Discussion

Conclusion