Mangrove coverage is declining, but there is hope, flags study

Mangroves forests are being threatened at an increasing pace: River dams negatively impact the supply of mud that raises mangrove soils. The space required for their survival is increasingly getting occupied by buildings and seawalls. Tidal barriers have proven to be disastrous for mangrove coverage and can result in species loss. 

But mangrove coverage can increase despite sea-level rise if sediment supply is sufficient and land accommodation space available, according to a recent study published in Environmental Research Letters November 10, 2020. 

Mangroves depend on a steady supply of sediment flowing down from rivers. The delivery of sediment from most rivers reduced over the past three decades.

Coastal mangrove forests are valuable, highly biodiverse ecosystems that protect coastal communities against storms. A narrow mangrove zone is less effective in protecting the coast against storms; in the worst case, it may lose its protective properties altogether.

“Both mangrove coverage loss and diversity loss go hand in hand when that landward retreat is limited by expanding cities, agriculture or flood protection works,” said Barend van Maanen from the University of Exeter and one of the authors of the study. 

The study

Numerical experiments were done as a part of the study to explore changes in mangrove forest extent and diversity under a broad range of sea-level rise rates and sediment supply conditions, both in the absence and presence of a tidal barrier like a dike / seawall that obstructs inland migration.

The study helps to understand mangrove species zonation linked to a dynamic coastal profile, differences in lateral accommodation space (coastal progradation and landward migration) and mangrove properties like root density.

The researchers conducted simulations for a period of 330 years. The first 30 years were used as an adaptation period during which mangroves could settle, allowing the analysis of mangrove assemblage dynamics over the remaining 300 years.

The model tracked key forest characteristics, including tree density, type of species, stem diameter, tree height and associated biomass. Focus was on changes in total forest coverage and species distribution.

Mangroves provide valuable ecosystem services, including carbon sequestration, coastal protection and habitat provision for a plethora of organisms. 

The provision of ecosystem services is highly dependent on the composition of mangrove species assemblages, the loss of species diversity can have dramatic economic and environmental consequences for coastal communities.  

“It is essential to secure or restore mud delivery to coasts to counter negative effects of sea-level rise. For coasts where mud supply remains limited, removal of barriers that obstruct inland migration is of utmost importance to avoid loss of mangrove forests and biodiversity,” Christian Schwarz, co-author of the study, said.

The United Nations Environment Programme (UNEP) guidelines on mangrove restoration for the Western Indian Ocean (WIO) region show that a nature-based solution is needed to address the challenges that the mangrove ecosystem faces. 

Restoring mangroves is possible but challenging, the case studies on mangrove restoration projects from Kenya, United Republic of Tanzania, Mozambique, Madagascar, Mauritius and Seychelles shows.

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