Mangrove ecosystem services under pressure: the history and future of carbon sequestration hotspots

Mangrove forests are often associated with the smell of rotten eggs and swarms of mosquitos. This may be true but at the same time these forests are unique and extremely valuable. Mangrove trees grow in challenging environments surviving hot, muddy and salty conditions as they thrive at the margin of land and sea in the tropics and subtropics. Mangrove ecosystems provide essential habitats for many animal species, they help filtering pollutants and protect the coast against erosion. Moreover, mangroves play a crucial role in combating climate change as they capture and store large amounts of carbon from the atmosphere. In fact, these forests store carbon faster than most land ecosystems. The trees store carbon not only in their wood and leaves, but also in those smelly muddy soils. Despite all these benefits, mangroves are heavily threatened as sea level rise may cause forest drowning and people are increasingly modifying coastal landscapes and interfering with the natural processes on which mangroves depend. The impacts of such pressures on mangrove forests are still unclear, but the consequences may be drastic mangrove loss and reductions in carbon storage.
Mangrove trees flourish under very specific conditions. They grow well under regular inundation by tides, but they cannot survive prolonged flooding. Hence mangroves will need to keep raising the bed on which they grow to cope with rising sea levels. Mangroves may accomplish by trapping sediments from the land and the sea with their roots. In addition, dead roots, leaves and branches accumulate within the muddy soils. This helps mangroves to gain elevation and the build-up of dead plant material creates carbon-rich sediments. Now, essentially two possibilities emerge. If mangroves keep up with sea level rise by accumulating carbon-rich plant material in their soils, then carbon stocks can actually increase. However, if sea level rise outpaces mangrove soil buildup, then tree mortality will reduce carbon storage. Limits to the adaptability of mangrove forests to sea level rise exist and these limits are influenced by human activities. Building of river dams, for example, reduces the delivery of sediment to the coast, while this sediment is needed to help raising mangroves and enable continued carbon storage. Clearly, mangrove environments are highly complex and in order to protect these valuable environments, improved understanding and abilities to predict their future are urgently needed.
In this project, we will unravel the processes that control how and how much carbon is stored in mangrove forests and develop new computer models to investigate the impacts of sea level rise and human activities on future carbon accumulation. We have selected three sites in Colombia (South America) where mangrove trees reach up to 40 meters (!) making these forests true carbon storage hotspots. First, we will obtain soil samples up to a depth of 2 meters. We will estimate their carbon content, how fast that carbon has accumulated during the past, and where the carbon is coming from. We will also use microscopic plant remains preserved in the soil to discover what mangrove species have grown there in the past and whether this has influenced carbon accumulation. Third, we will develop a model capable of simulating how entire deltas and estuaries with mangrove vegetation evolve over tens to hundreds of years. Finally, we will use this new model to investigate the fate of mangrove forests under rising sea levels and varying sediment supply, and impacts on future carbon accumulation. Colombian high-school students and teachers from will participate in fieldwork and will present their work in science fairs for the general public to increase the awareness of the values of mangrove forests. We will also work together with our project partners to use our findings to support the development of sustainable management strategies in order to safeguard mangrove environments.