Moving the MCP mangrove nursery

Harsh habitat and adaptations
Mangroves are halophytic plants, meaning they can tolerate growing in saline soils. They live in a harsh environment, a wet, saline habitat along tropical coastlines. Living in this habitat means they have had to adapt to salinity exposure, tidal fluctuations, high temperatures, and the heavy impact of the anthropogenic activity that coastlines are typically subject to.

Salinity
Even though mangroves are most commonly found in saltwater, they can also live in freshwater environments. The main reason they live in saltwater is reduced competition, since few other plants can tolerate the saline habitat, leaving mangroves room to thrive. They have developed different ways to cope with the salinity, and a common one is through their leaves. Some species sacrifice a few leaves, concentrating excess salt in them until the leaves turn yellow and die. Others excrete salt as the sun evaporates the water; you can recognize those species by feeling or seeing the salt crystals on the leaves. In some species, the roots have developed a membrane that prevents salt from entering but allows water to pass through.

Tidal fluctuations
Tidal fluctuations mean mangroves have to cope with being submerged in salt water for part of each day. On top of the salinity, this makes it an uninhabitable environment for most other plants, reducing the competition even further. Root adaptations have made it possible for mangroves to live in soft sediment that shifts with the tides. When walking in a mangrove forest, you need to be careful not to trip over roots, since they can extend from both the trunk and the branches to increase stability.

These tidal swamp trees are remarkable: many have found a way to air their roots even when most of the tree is covered in water. This is their “snorkel,” or pneumatophore, which allows the tree to take in oxygen. Mangroves are often exposed to nutrient deficiency in their harsh environment, but incoming tides deliver nutrients and carry away excreted waste as they recede. Furthermore, propagules, or mangrove seedlings, which have adapted to the environment and remain buoyant in water, are transported out with the tides.

Why mangrove nurseries?
You can find mangrove nurseries in many places around the world, mainly due to the high level of anthropogenic pressure mangroves face. Mangroves have adapted well to their harsh natural environment, but they have not had enough time to adapt to rapidly increasing anthropogenic pressures. Today they are exposed to dredging, deforestation, water pollution from oil spills and herbicides, and urban development. Destruction of their habitat eventually leads to the death of mangrove trees. Because of these pressures, many places have set up nurseries. Survival isn’t guaranteed just because propagules are planted, but the evidence consistently shows that survival improves substantially with structured post-planting care and technical guidance. In one study of Sri Lankan restoration sites, average survival reached 46% where technical guidance was used, compared to 0% where it was not, exactly the kind of hands-on tending a nursery provides.

A nursery is typically a closed-off system, either on land or in water, suitable for propagules to grow in without too many outside factors influencing them. There, they can be given the right amount of nutrients and kept safe from anthropogenic effects.

Mangrove importance
Mangroves protect shorelines from hurricanes, storms, and erosion. Research on storm surge reduction shows that a single kilometer of healthy mangrove forest can reduce wave energy by more than 75% and lower storm surge height by 5 to 50 centimeters for every kilometer of forest width. Their root systems also stabilize sediment to prevent erosion. Additionally, mangroves maintain water quality by filtering pollutants and trapping sediment coming from land. Beyond stabilizing sediment and keeping water clean, these forests act as nurseries for marine life such as shrimp, crustaceans, mollusks, and fish, offering a rich source of food and shelter from predators. In many parts of the world, mangroves are also a renewable resource, used for boats, houses, furniture, medicine, or even tea. They are thus important not only for the ecosystem, but for the valuable functions they can offer people too. This is why mangrove nurseries matter: they help maintain the mangrove population, both for the ecosystem and as a valuable resource.

At Marine Conservation Philippines we had a mangrove nursery ground on base where mangrove seedlings had been growing for some time. When they had grown big enough, we replanted them into their natural habitat.

Volunteers brought the seedlings to nearby Tambobo Bay, where we know mangroves are thriving. Selecting the site takes careful consideration. Questions to ask before choosing a location include: how exposed will the seedlings be to tidal fluctuations, are there seagrass beds or other intertidal habitats nearby (since mangroves should not be planted there), how soft is the substrate, and how exposed is the site to anthropogenic pressures?

Their size needs to be considered too, when finding the right spot for each seedling. We had to walk out into the mudflat to find the best location for them.

We brought the mangrove propagules out into the water and dug a small hole in the mud, one deep enough to hold each propagule without letting it be submerged. Volunteers built a small fence around the not-yet-mature plants to protect them from dogs and other human activity. Signs and posters were also put up to inform the community about what mangroves are and why we built a fence there. We continue to keep an eye on the new mangroves to make sure they are growing as expected. Hopefully, there will be even more mature mangroves to bring out into the “big forest” soon.