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A more potent CO2 sink than the Amazon, Brazil’s mangroves remain overlooked

  • A new study shows that the mangrove forests along Brazil’s coast store up to 4.3 times more carbon in the top 1 meter (3 feet) of soil than any other biome in the country, including the Amazon rainforest.
  • Mangroves have long been known as a more effective carbon sink than other types of tropical forest, but this study is the first that tries to quantify the extent and carbon stock of Brazil’s mangroves.
  • It found that Brazil’s mangroves hold 8.5% of all carbon stocks stored in mangroves worldwide, and that they sequester 13.5% of the carbon sequestered in the world’s mangroves annually.
  • Despite this carbon stock potential, Brazilian mangroves aren’t included in protected areas deemed a conservation priority under Brazil’s emissions reduction commitments to the Paris Agreement, and there’s a general lack of funding for research on the ecosystem.

Tropical forests play a fundamental role in storing carbon dioxide, the main greenhouse gas responsible for global warming. And while the Amazon rainforest is often top of mind when it comes to stores of CO2, there’s another ecosystem in Brazil that stashes away a much larger amount of carbon per hectare: mangroves.

Scientists have long known about the carbon-sequestration abilities of these coastal ecosystems. But few countries have an accurate idea of their potential or how they can be used in the carbon trading market.

A new study of mangroves in Brazil aims to fill this gap. “For the first time, we have tried to quantify, in an integrated way, the carbon stocks and sequestration in the Brazilian mangroves,” said study co-author Pablo Riul, a biology professor at the Federal University of Paraíba (UFPB). “This way, it will be possible to convert these estimates into carbon credits and know how much each hectare is worth in this market, generating subsidies for their valuation.”

Mangrove on the coast of Espírito Santo state. Image by Leonardo Merçon.

According to the study, published in January in Frontiers in Forests and Global Change, the mangroves along Brazil’s coastline hold 8.5% of global carbon stocks of these ecosystems. (Indonesia is in first place, having the biggest total area of mangrove forests in the world.) Much of this carbon is held in the soil and sediment; compared to other vegetated biomes in Brazil, mangroves store up to 4.3 times more carbon in the uppermost 1-meter (3-foot) layer of soil. The mangrove trees themselves store less carbon than Amazon rainforest trees.

“The scientific community has no doubt about the amount of carbon stored in mangroves and their role in removing this gas from the atmosphere,” said first author Andre Rovai, a senior researcher in coastal sciences at Louisiana State University. “What is lacking in Brazil is the political will to allocate adequate funds for the development of science, especially in the biodiversity field.

“Thus, more than bringing new estimates, our study points to gaps to be filled, indicating areas with little or no information available that need to be prioritized in future research,” he said.

The amount of carbon stock held in biomass, in green, and in the top 1 meter (3 feet) of the soil layer for different ecosystem types. Image from Rovai et al. (2022).

Large but poorly protected carbon reservoirs

Mangroves are found along practically the entire Brazilian coastline, from Amapá state in the north to Santa Catarina in the south. The largest expanses occur in the north and northeast, and it’s in these regions where the carbon sequestration is greatest. By state, Maranhão has the largest area of mangroves, at nearly 298,000 hectares (736,000 acres), followed by Pará (186,000 hectares, or 460,000 acres), and Amapá (141,000 hectares, or 348,000 acres). Maranhão and Pará combined are home to 60% of Brazil’s mangrove area.

To generate numbers for the total carbon stored, the researchers analyzed existing data on carbon stocks and sequestration. Rovai said the estimates are summed up from the three main areas where carbon is held in any type of forest, including mangroves.

The first is the aboveground biomass: where the visible structure of the mangrove trees, such as the trunk, branches, leaves, fruits and flowers. “We make an estimate because you never measure all the trees in an environment, but a part of the population. To estimate the whole, you multiply the weight of the trees within known areas (plots, in hectares) by the total area of that environment,” Rovai said.

Below ground, there are two other carbon stores: the roots of the trees, and the soil, which in these coastal environments has formed over the course of thousands of years.

“The amount of carbon removed from the atmosphere annually and added to the soils and trees of the mangroves [in Brazil] is also important, which together equals 13.5% of all the carbon sequestered by mangroves worldwide,” Rovai said.

The study authors say this high capacity for sequestration is due to the fact that mangroves store carbon both from the environment itself (through the mangrove trees) and from nearby ecosystems, such as the restinga broadleaf coastal forests, or from further upstream, such as the Atlantic Forest or the Amazon rainforest, in addition to what comes from the waters of the estuaries, like phytoplankton and zooplankton.

A red aratu (Goniopsis cruentata), one of several crustacean species that inhabit the mangroves of the western Atlantic. Image by Leonardo Merçon.

Overlooked as a climate tool

Despite this massive potential to capture and store carbon, Brazil’s mangroves are largely overlooked as a climate tool. For instance, none of the protected areas deemed conservation priorities in Brazil’s voluntary emissions reduction targets under the Paris Agreement have mangroves.

The country also has no national plans or regional guidelines for the inclusion of mangroves in carbon trading schemes.

“One hectare of mangrove has much more carbon than the same area in the Amazon rainforest,” Riul said. “But it is neglected. And it always has been. People see it as a dirty place, with a lot of garbage and sewage, because it has historically served as a sewage disposal site for many coastal cities.”

The study authors also note that, under Brazilian law, the transition zone between mangrove forests and dry land, commonly known as apicum, isn’t protected.

“These areas, situated in higher elevations, usually in the middle of the mangroves or behind them, are extremely saline, so the typical mangrove vegetation doesn’t develop there,” Rovai said. “But as the sea level starts to rise, that’s where the mangrove advances and gradually colonizes this soil.

“But the law differentiates these areas and establishes that apicum is not mangrove because it is based only on the presence of vegetation today, which is a mistake, since the successional dynamics of vegetation in these environments occurs on a scale of decades and hundreds of years.”

The mangrove rail, known locally as saracura-matraca (Rallus longirostris), is found in mangroves stretching from Pará state in northern Brazil to Santa Catarina in the south. Image by Leonardo Merçon.

Among the main threats to mangrove conservation are shrimp farming, other forms of agriculture, erosion, real estate speculation along the Brazilian coast, and climate change.

But in 2020, the biggest threat to mangrove preservation in recent years came from the federal government itself. That year, the environment minister, Ricardo Salles, tried to end two resolutions that protected mangroves and sandbanks. After examining the case, the Federal Supreme Court declared the measure unconstitutional and reinstated the resolutions. (Salles resigned in 2021 amid an unrelated ongoing probe into illegal logging in the Amazon.)

To ensure the conservation of mangroves, these transitional ecosystems between land and marine environments — which are a very important nursery for several species and also a source of income for local communities — need to be better monitored and better studied, the authors say.

But investment in mangrove research remains very small, Rovai said.

“Fieldwork in these places involves trips, sample collection, equipment, and money for publication, which in Brazil becomes even more challenging when faced with the colossal cuts in the budget allocated to the biodiversity sector,” he said.

One of the most common refrains justifying the lack of research funding is that “Brazil is a country of continental dimensions,” and therefore research is more difficult, Riul said.

“This is an excuse. The coast of Australia has been monitored for more than 30 years. In Brazil we don’t have good historical information that can be used as a reference, only occasional data in a few locations,” Riul said. “So when there’s a disaster, we have no information to accurately assess the impact because there’s nothing to compare it with.

“We have to invest in environmental monitoring policies on a national and long-term scale,” Riul said.

Mangrove inside the Mandira Extractive Reserve in Cananeia municipality, São Paulo state. Image by Xavier Bartaburu/Mongabay.

Citation:

Rovai, A. S., Twilley, R. R., Worthington, T. A., & Riul, P. (2022). Brazilian mangroves: Blue carbon hotspots of national and global relevance to natural climate solutions. Frontiers in Forests and Global Change, 4. doi:10.3389/ffgc.2021.787533

Banner image of a young black-crowned night heron (Nyctanassa violacea) at the Itacorubi Municipal Mangrove Park in Florianópolis municipality, Santa Catarina state, by Sovernigo (CC BY-SA 4.0) .

This story was reported by Mongabay’s Brazil team and first published here on our Brazil site on Feb. 15, 2022.