The term mangrove loosely describes those tropical trees or shrubs that are specially adapted to grow in salty, wet, and muddy environments. This includes the shallow waters of calm bays, and the periphery of salt ponds, marshes, and wetlands that are exposed to flooding and saltwater intrusion.
These are three of the types of mangroves that you can find on the island of St John.
The red mangrove proliferates along the shorelines of shallow calm bays, both on the muddy shore and in the water itself. The red is the classic mangrove characterized by its numerous arch-shaped roots that start at the base of the tree and arch out and down into the water and mud.
It also has distinctive seeds that, at maturity, look something like foot-long red pencils, which emerge prominently from the center of the mangrove’s leaf clusters.
The black mangrove, Avicennia germinans, is easily identified by little sticks, called pneumatophores, coming out of the mud around its trunk. These are actually part of the black mangrove’s roots and serve two purposes. Most importantly, they act like snorkels bringing fresh air to the majority of the root that exists in the oxygen-depleted environment underwater and underground.
Secondly, the pneumatophores help to anchor the black mangrove to its tenuous foundation of loose mud. The red mangrove’s lenticles and the black’s pneumatophores are extremely sensitive to greasy contaminants, which can clog up the openings. They are, therefore, at particular risk from oil spills.
The black mangrove is less tolerant of salt than the red mangrove and cannot live its entire life in salt water. It’s, therefore, usually found behind the red mangrove or on the shoreside of salt ponds or marshes.
Like the red, this mangrove excludes salt at the roots but at a 90% efficiency instead of the 99% capability of the red. The salt that enters the black mangrove tissues is excreted by salt glands on the upper surface of the leaves. If you hold a black mangrove leaf up to the sunlight, you will see salt crystals on the leaf.
The white mangrove, Laguncularia racemosa, is the least salt-tolerant of the three mangrove varieties and cannot withstand prolonged periods in flooded conditions. For this reason, white mangroves are usually found on drier land than are its red and black cousins. On St. John, white mangroves are particularly plentiful around the edge of salt ponds and along guts where they open to the sea.
The white mangrove, like the black, excretes salt from its leaves. It does so through salt glands that occur in pairs on the stem just below each leaf. These glands look like two raised bumps and provide a good way to identify the white mangrove.