As mangrove trees grow, they store carbon from the atmosphere in their wood. More carbon builds up in mangrove soils with the accumulation of organic matter, such as dead leaves and branches. We wanted to know whether the ecological conditions that control mangrove growth in different settings could also indicate how much blue carbon these ecosystems accumulate over their lifetime, or how much carbon they sequester each year.
Annual carbon sequestration by mangroves
There is no question that mangrove wood and soils around the world have accumulated significant quantities of carbon since they started to grow. Mean values range from 50 metric tons of carbon per acre (125 metric tons per hectare) in delta settings to as much as 220 metric tons per acre (550 metric tons per hectare) in carbonate coastlines.
For the purpose of curbing climate change, the most important question is how much carbon an ecosystem sequesters each year, mitigating carbon emissions produced from human activities such as burning fossil fuels. Carbon sequestration by mangrove forests is the amount of carbon that accumulates in wood or soils each year and remains stockpiled there, isolated from the atmosphere. In total, the world’s mangroves sequester approximately 24 million metric tons of carbon in soil per year. A mangrove forest on the Pacific island of Kosrae, in Micronesia, can store as much carbon annually as a tropical rain forest in Panama.
Our review found very significant differences in the rate of carbon uptake by different types of mangrove forests. Mangrove soils in the intertidal zone gain elevation as sediments are deposited each year. Coastal zones with major rivers have some of the highest rates of soil formation, so they also have high annual carbon sequestration rates. Since mangrove trees grow taller in these more benign delta environments, they also sequester more carbon in their wood each year.
Overall, mangroves in deltaic coasts such as the Mississippi River delta, the Amazon in Brazil and the Sundarbans in India and Bangladesh can sequester more carbon yearly than any other aquatic or terrestrial ecosystem on the globe. These are the world’s blue carbon hot spots.
However it is important to distinguish between carbon density in mangrove soils – the amount of carbon stored per unit area – and carbon sequestration, which is the amount of carbon that accumulates in that same area per year. Carbon sequestration is enhanced in deltaic environments because rivers continuously deposit sediment in mangrove soils. This leads to higher burial rates of carbon, both from the trees themselves and from carbon carried by the rivers.
Delta soils also contain a lot of mineral sediment (rock), so they have a relatively low carbon content by volume. In contrast, in carbonate settings such as Caribbean islands, most of the soil volume is made up of plant roots, so it has a higher carbon content. But soil builds up in deltas more quickly, since rivers are always depositing new sediment.
This distinction is important for conservation strategies. Destroying mangrove forests to build roads, shrimp ponds or other commercial projects can release significant quantities of carbon to the atmosphere – even in carbonate settings, since so much of the soil there consists of plant material. This makes it important to conserve mangroves in all types of settings.
It also is very important to restore mangroves, particularly in deltaic regions, where they can increase those ecosystems’ capacity to clean the atmosphere of existing carbon dioxide. By improving our understanding of how mangroves function under different conditions, we can safeguard and increase these valuable blue carbon stores.