- Mangroves are a group of trees and shrubs that live in the coastal intertidal zone.
- It is usually found within the tropic or subtropic latitudes. In fact, the various species of mangroves aren’t necessarily closely related to one another, but they do share the unique capability of growing within reach of the tides in salty soil.
- Some mangrove species live so close to the shoreline that they are flooded with salt water every day as the tide comes in and submerges their roots. All mangroves have evolved special adaptations that enable them to live in salty, oxygen-poor soil.
- Rising temperatures and sea level due to climate change are allowing mangroves to expand their ranges farther away from the equator and encroach on temperate wetlands, like salt marshes.
Adaptation and Distribution
- To differentiate species that use different methods for dealing with salt, scientists categorize mangroves as either secretors—those that actively rid their tissue of salt—and non-secretors—those that block the salt from entering their tissue.
- In species from the genera Rhizophora (the red mangrove) and Bruguiera, the plants create a barrier and can almost completely exclude the salt from entering their vascular system—over 90 percent of the salt from seawater is excluded.
- This barrier acts against osmosis, a process where water moves from areas low in salt concentration to areas high in salt concentration. If the mangrove didn’t have such a barrier, the salty ocean water would suck the mangrove dry.
- For many mangroves, however, the salt is dealt with after it enters the plant.
- Mangroves categorized as secretors, including species in the black mangrove genus Avicennia, push salt from the ocean water out through special pores or salt glands within their leaves. As the salty water evaporates, noticeable salt crystals often form on the surface of the leaves. The leaves of some mangrove can also store unwanted salt. Since leaf cells can hold a large volume of water when compared to all other cells, salt is drawn to the leaves as a mechanism to balance the salt concentration. As the leaves age, the cells grow in size since more water is needed to dilute the accumulating salt. This hoarding of water creates thick and fleshy leaves, a characteristic called succulence. Eventually, the leaves age and fall off the tree, taking the salt with them.
A mangrove forest is categorized into five types of forest-based upon its surrounding geography.
- Mangrove forests along open bays and lagoons that experience full sun are considered to be mangrove fringe. These forests are dependent upon the regular tides that flush leaves, twigs, and mangrove propagules out into the open ocean.
- An overwash forest is similar to a fringe forest except the entire forest is an island that becomes flooded at high tide. Isolated from the main land and terrestrial predators, it is a popular place for birds to nest.
- Riverine mangrove forests are within river floodplains by the coast and are heavily influenced by the changing seasons. Sometimes they are inundated with fresh river water, while during summer droughts the soil can become exceptionally salty when the fresh river water is almost nonexistent.
- Basin mangrove forests extend far inland and occur in inlets, deep bays, and coves.
- Dwarf, or scrub, mangrove forests only attain canopy heights of less than 5 feet (1.5 meters) although they contain the same species as the other types of forest. The stunted growth is often attributed to a lack of nutrients, high salinity, and rocky soils.
Importance of Mangroves
- Carbon storage. Mangroves “sequester carbon at a rate two to four times greater than mature tropical forests and store three to five times more carbon per equivalent area than tropical forests” like the Amazon rainforest. This means that conserving and restoring mangroves is essential to fighting climate change, the warming of the global climate fueled by increased carbon emissions, that is already having disastrous effects on communities worldwide. At the same time, mangroves are vulnerable to climate change as sea level rise pushes ecosystems inland.
- Water. Mangroves are essential to maintaining water quality. With their dense network of roots and surrounding vegetation, they filter and trap sediments, heavy metals, and other pollutants. This ability to retain sediments flowing from upstream prevents contamination of downstream waterways and protects sensitive habitat like coral reefs and seagrass beds below.
- Fisheries: Mangrove forests are home to a large variety of fish, crab, shrimp, and mollusk species. These fisheries form an essential source of food for thousands of coastal communities around the world. The forests also serve as nurseries for many fish species, including coral reef fish. A study on the Mesoamerican reef, for example, showed that there are as many as 25 times more fish of some species on reefs close to mangrove areas than in areas where mangroves have been cut down. This makes mangrove forests vitally important to coral reef and commercial fisheries as well.
- Timber and plant products: Mangrove wood is resistant to rot and insects, making it extremely valuable. Many coastal and indigenous communities rely on this wood for construction material as well as for fuel. These communities also collect medicinal plants from mangrove ecosystems and use mangrove leaves as animal fodder. Recently, the forests have also been commercially harvested for pulp, wood chip, and charcoal production.
- Biodiversity. Home to an incredible array of species, mangroves are biodiversity hotspots. They provide nesting and breeding habitat for fish and shellfish, migratory birds, and sea turtles.
- Coastal protection: The dense root systems of mangrove forests trap sediments flowing down rivers and off the land. This helps stabilizes the coastline and prevents erosion from waves and storms. In areas where mangroves have been cleared, coastal damage from hurricanes and typhoons is much more severe. By filtering out sediments, the forests also protect coral reefs and seagrass meadows from being smothered in sediment.
- Tourism: Given the diversity of life inhabiting mangrove systems, and their proximity in many cases to other tourist attractions such as coral reefs and sandy beaches, it is perhaps surprising that only a few countries have started to tap into the tourism potential of their mangrove forests. Places as diverse as Bonaire and offer snorkelling expeditions in and around mangroves to witness a marvellous variety of baby fish, jellyfish, and urchins against a magical background of interwoven roots delving deep into the sandy substrate. Great potential exists elsewhere for revenue generation in this manner, which values the mangroves intact and as they stand.