Why are mangroves essential ecosystems for climate change mitigation?

Wetland restoration projects, also referred to as Blue Carbon projects, operate at the nexus of land and sea, integrating complex and interdependent factors to function effectively. These projects focus on restoring seagrass meadows, mangroves, coastal marshes, and peatlands. This article highlights the concept of mangroves and the key factors to consider in managing a mangrove project. 

WHAT ARE MANGROVES ?

Mangroves are unique coastal ecosystems characterized by salt-tolerant trees and shrubs that thrive in tropical and subtropical tidal zones near the equator (between 30 degrees north and 30 degrees south), where the climate fosters mangrove growth (with an average water temperature of 20 degrees Celsius). These environments are typically found along coastlines, estuaries, and river deltas where they provide a critical buffer between land and sea. According to the Global Mangrove Watch, the there are 64 species of mangroves worldwide.

Contrary to common belief, mangroves comprise more than just the biomass they consist of. They represent an entire ecosystem composed of trees, including their roots (mangroves), vegetation, characteristic organic soil, and biodiversity. Mangroves boast unique biodiversity, with over 450 vertebrate and invertebrate species living within this system and even more invertebrate species depending on it.

These areas are influenced by the sea, with soil often saturated or submerged. Mangroves do not require saltwater for growth; instead, they are among the plant species with metabolisms that actively combat it (water filtration through roots).

Mangroves have extraordinary migration capabilities, with the sea carrying their seeds (propagules via viviparity) to other estuaries (up to 200 km away), enabling unique colonization capacities. Combined with viviparity, each propagule can massively colonize new areas within months, with canopy heights reaching 2-3 meters in a year.

Mangroves cover approximately 14.7 million hectares (147,000 km²) globally and could entirely disappear by the end of the 21st century due to aquaculture (shrimp farming), agriculture (deforestation), submersion, and fires if no conservation action is taken. South-East Asia is home to almost a third of all mangroves (mainly in Indonesia), but it is also in this region that the greatest losses have occured due to intensive acquaculture.

Their disappearance could lead to irreversible damage to our planet and local populations in various ways:

• Mass extinction of species living in the ecosystem, as well as the reproductive cycles of many fish species.

• Exposure of local populations to natural disasters (tsunamis, hurricanes).

• Increased erosion of coastal habitats leading to populations forced displacement.

• Thousands of small-scale fishermen would lose their jobs.

• Loss of an energy source (wood) and food for local populations.
  

• Release of heavy pollutants into the sea - mangroves act as a water filter and buffer zone with hydrological connections.

• And of course, the loss of one of the most effective carbon sinks on our planet, which would drastically accelerate global warming.

MANGROVES CONSTITUTE AN EXCEPTIONAL NATURAL CARBON SINK

In the decarbonization field, mangroves are part of the so-called "blue carbon" technologies, with carbon being stored in the hydrosphere or in soils under the influence of the sea.

Coastal blue carbon ecosystems cover only 2% of the ocean's surface but represent 50% of carbon absorption by the ocean. It is estimated that the lithosphere (ocean, sea, river, lake, etc.) accounts for 83% of the natural carbon cycle. See the dedicated article on the carbon cycle.

Mangroves stock carbon in two ways: (i) carbon is stocked in their biomass (mangroves and vegetation in the area) and (ii) carbon is stored in the soil (SOC - Soil Organic Carbon). 

Mangroves have a high CO2 absorption capacity, with an average of 44 tons of CO2 per hectare absorbed annually, with an additional 7 tons sequestered in the soil. They have a leaf-level absorption capacity of 12 μmol CO₂/m²/s compared to 0.6 μmol CO₂/m²/s for respiration, representing an impressive photosynthetic ratio. The world’s mangrove’s (United Nation) study has estimated that the net CO2 absorption benefit of mangroves is four times higher than that of rainforests. 

However, the largest carbon stock is in the soil, with microorganisms decomposing a large amount of organic matter from dead biomass, as well as deposits related to the influence of the sea (sediment or other organic matter). 

An experiment conducted in New Caledonia demonstrated that mangroves in an environment with more carbon dioxide in the atmosphere increase their absorption capacity, making mangroves an ideal and resilient natural solution for decarbonization. 

Carbon stored in mangrove soils can remain sequestered for remarkably long periods due to the anoxic conditions that slow down decomposition rates. Studies have shown that carbon in mangrove soils can be stored for several hundred years, and in some cases, even thousands of years.

A FRAGILE ECOSYSTEM

To thrive, a mangrove must strike a balance between several influencing factors: 

• Tidal influence: defined as the influence of tides oxygenating mangrove waters and bringing sediment layers. Excessive salinity can lead to mangrove eutrophication and sulfurous conditions that are unsuitable for mangrove growth and can kill the tree. 

• Erosion: primarily linked to tidal influence, erosion leads to land subsidence, converting wetland into open water zones. 

• Wetland migration: wetland migration can be horizontal (along coasts) or vertical (inland). 

• Soil nature: the natural activity of organic soils can lead to subsidence if not sufficiently enriched with organic matter, i.e., complete decomposition of organic soil until it becomes mainly mineral. Organic soil is essential for mangrove system balance. 

• Sea level rise: mangrove areas, being directly or indirectly connected to the sea, are susceptible to partial or complete submersion due to rising sea levels combined with tidal fluctuations. Submerged underwater, the mangrove trees cannot engage in photosynthesis and deteriorate. 

• Oxygenation: stagnant water or water-saturated soil can lead to anaerobic conditions, preventing the natural oxidation of organic matter and resulting in nitrification or denitrification (emission of methane and nitrous oxide, greenhouse gases more impactful than carbon dioxide). 

• Species diversity: a mangrove ecosystem composed of multiple mangrove species, often endemic, exhibits greater resilience. This biodiversity enhances the ecosystem's ability to withstand and recover from environmental stresses such as storms, rising sea levels, and pollution. Additionally, a diverse mangrove forest provides a wider range of habitats for various marine and terrestrial species, contributing to overall ecological health and stability. 

ANTHROPOGENIC INFLUENCING FACTORS

The direct consequences of human activity are responsible for more than 60% of mangrove destruction.

• Aquaculture: the expansion of aquaculture, particularly shrimp farming, encroaches on mangrove habitats, leading to habitat destruction and loss of biodiversity. 

• Agriculture: conversion of mangrove areas into agricultural land results in deforestation and habitat loss, disrupting the delicate balance of these ecosystems. 

• Fire: human-induced fires, whether intentional or accidental, can devastate mangrove forests, causing biomass loss and soil damage through methane combustion. These fires destroy vegetation, reduce biodiversity, and weaken ecosystem resilience. 

• Harvesting: overharvesting of mangrove wood for fuel, construction, and other purposes depletes the forest and diminishes its ability to provide essential ecological services such as coastal protection and carbon sequestration. 

• Maritime transport: activities associated with maritime transport, including the creation of shipping channels and vessel movement, significantly impact mangroves. Ship-generated waves cause erosion, and dredging for channel creation leads to habitat destruction and alterations in water flow patterns. 

• Water temperature: climate change and industrial activities can elevate water temperatures, stressing mangrove ecosystems. Higher temperatures affect the growth and survival of mangrove species, disrupt marine life reproductive cycles, and increase vulnerability to diseases and pests. 

Out of the 1,100,000 hectares (ha) of mangroves lost since 1996, approximately 818,300 ha are considered "restorable." 

THE APOLOWNIA METHODOLOGY

At Apolownia, we are a qualified partner dedicated to blue carbon projects. We have developed unique expertise with a methodology based on the latest scientific publications regarding wetlands and VERRA's VCM-0033 methodology, to closely align with the reality of mangroves, their evolution, and regulatory context. 

Our methodology relies on a technological foundation combining scientific (field measurement and laboratory analysis), spatial (remote sensing), and operational (implementation of restoration project actions) approaches. This approach allows us to fully internalize a mangrove project, eliminating intermediaries and ensuring maximum transparency and integrity of carbon credits for our clients. 

ABOUT APOLOWNIA

Apolownia is a mission-driven company committed to making a significant impact in the climate sector.   

We support businesses and funds willing to engage in long-term and impactful decarbonization strategies - within and beyond their own value chain - by designing, implementing and monitoring science-based carbon reduction projects that restore natural ecosystems.  

Through technology and innovative solutions, we aim at shaping a resilient and environmentally friendly world, by encouraging the decarbonization of the economy and supporting social and environmental initiatives.  

You can drive positive change for the climate, biodiversity and local communities.  

Contact us to engage or for more information. Find us on www.apolownia.com.

Sources:

• Mangrove – une forêt dans la mer – CNRS 

• Global Mangrove Watch data 

• VERRA – VCM0033 

• The World’s Mangroves 2000-2020, Food and Agriculture Organization of the United Nations (2023) 

• Best practice guidelines for mangrove restoration, GMA, 2023. 

• Leal and Spalding, eds., 2022