Mangroves, carbon and sea levels rise

  Carbon storage in mangrove forests may be released due to rising seas levels 

Mangroves, such as in Cispata Bay, Colombia effectively produce, trap and store carbon-rich soil, but the future of this carbon storage is uncertain because of rising sea levels and climate change. Locally, mangroves can sometimes adapt to rising seas, but global trends look troubling. Mangrove forests straddle the edge of land and sea along some tropical and subtropical coastlines. These trees and shrubs have distinctive tangles of roots which trap sediment and produce organic matter, forming dense soils and efficiently storing carbon. Though mangroves cover only 1% of Earth’s surface, they store a whopping 15% of global ocean carbon in their trapped soils. Read about mangroves and climate change and you’ll encounter a reassuring idea: as sea levels rise, these coastal forests will actually store more carbon. A new study from the University of Exeter says that story has been built on measurements taken at the wrong scale. When you model what happens not at individual sampling points but across an entire mangrove forest the conclusion reverses. Rising seas are likely to reduce carbon storage over the coming century. In worse-case scenarios, forests that have been absorbing carbon for decades could start releasing it.

Their location along coastlines means mangroves are at the mercy of changing sea levels and sediment availability. Rising sea levels can drown mangroves or push them landward. At the same time, sediment supplies, belowground root growth and organic matter accumulation can help build up mangrove soils, allowing forests to keep pace with sea level rise. So over time, will mangroves keep locking carbon into their soils, or will they start losing it? Field studies measuring carbon in mangrove soils are doing good science. At specific locations, rising seas can increase carbon storage. More flooding delivers more sediment, more sediment means more organic accumulation. The carbon content of the soil at that spot goes up. But a forest isn’t a point. It’s an interconnected and dynamic system, with processes in one part affecting what happens everywhere else. Field observations capture a sample; they don’t capture the system. And the system, it turns out, behaves differently from the sum of the samples. “Research about carbon storage in mangroves is usually based on field observations, and such studies have found that carbon storage can increase as sea levels rise,” said Arya Iwantoro, who conducted the research at Exeter and is now at the University of Plymouth. “But this may not reveal the wider picture of what is happening across the forest as a whole.”

To find that wider picture, the research team built a model that links three interacting processes, water flow and sediment movement, mangrove growth and dieback, and carbon storage, and runs them forward together over a century. The researchers modeled a simplified tidal embayment to explore how different rates of sea level rise and sediment supplies would affect the mangroves. In these experiments, they found that carbon accumulation can increase at specific locations as waters rise because the increased water can lead to more mangrove growth, a result that matches existing data. However, when looking at landscape scales, they found sea level rise generally reduces total carbon sequestration through mangrove loss and soil erosion. The results showed that rising sea levels can alter mangroves from carbon storage sinks to carbon emitters. It would be a mistake to reduce this to a carbon accounting problem. Mangroves are doing several things simultaneously which coastal communities, fisheries and ecosystems depend on.

They absorb the energy of storms before they reach inhabited coastlines, a function which engineer defenses struggle to replicate and can’t sustain indefinitely. At the same time, they provide nursery habitat for the fish and shellfish which millions of people across the tropics eat and sell, while their root systems stabilize sediment and protect shorelines from erosion. As well as being vital carbon stores, mangroves protect coasts from storms, provide livelihoods to coastal communities and habitats for a wide range of species,” said Barend van Maanen, who leads the project at Exeter. “Our findings emphasize that understanding the coastal landscape as a whole is crucial when predicting how mangroves might respond to climate change, and how we can protect them.”

“Mangrove plants are highly specialized, and they require a certain duration of flooding with each tide,” said Luisa Fernanda Gómez Vargas. “If this period is exceeded, a location will no longer be suitable, the plants will ‘drown’ and mangroves will die back.” As seas rise, tidal floods grow longer. Parts of the forest which were previously within the tolerable range cross the threshold and the plants there die. When mangroves die, the carbon-rich soils beneath them become vulnerable to erosion. Carbon which accumulated over decades or even centuries can then be released back into coastal waters and the atmosphere. The model found that some parts of a forest can indeed gain carbon storage as conditions change. These are the locations where flooding stays tolerable and sediment keeps accumulating. But those gains don’t offset the losses across the rest of the system. At the whole-forest scale, over a hundred years, the direction of travel is consistently downward. The researchers ran the model against multiple IPCC sea-level rise scenarios. The relationship was clear every time: greater rises produce larger losses.

The landscape as a whole is precisely what makes this modelling approach different from what came before. Mangroves need rivers delivering sediment. They need tidal dynamics which keep flooding within survivable limits. And as seas rise, they need space to retreat, land behind them to migrate into as the intertidal zone shifts. Where that land has been occupied by buildings, roads and seawalls, the mangroves have nowhere to go. They drown in place. None of that complexity shows up in measurements at individual field sites. You only see it when you model the whole system. The reassuring story about rising seas and mangrove carbon was never baseless, it was just built on evidence from the wrong vantage point. Seen from above, across whole forests over a century, the picture looks considerably less hopeful. The findings demonstrate that local trends in carbon sequestration may not be representative of larger-scale outcomes in mangrove forests. The study shows that understanding coastal landscapes as an interconnected system is crucial to understanding how mangroves can respond to climate and human-induced pressures, the researchers say. However, new assessments and approaches are needed to better understand future mangrove vulnerabilities for benefit of all.