While there are a number of trees that tolerate some soil salinity and salt over-spray, there is just one species, the mangrove, that actually grows submerged in salt water for much of its life. The mangrove is specifically adapted not only to survive the dehydrating effects of salt, but to thrive and spread. Trees besides the mangrove that have high tolerance for salinity include, but are not limited to, horse chestnut, ash, honeylocust, sycamore and hedge maples, sweet gum and American holly.
Able to filter out some salt at root level and some through its leaves, mangrove is also able to tolerate a much higher internal level of salinity. Its sap may be up to 10 percent as salty as seawater. They are also able to "breathe," absorbing oxygen through pore-like lenticels on their above-ground roots. This allows them to thrive in anaerobic soil, where there is a lack of oxygen. Their aerial roots -- which, while above-ground, spend part of their time submerged by high tide -- not only absorb oxygen, but are also able to transport it throughout the rest of the tree. Even though the mangrove can tolerate salinity, it also relies on freshwater to flush excess salt out of its system. Without freshwater flushing, the trees would die. Rain provides the freshwater needed for their survival.
With the most exposure to saltwater, the red mangrove grows off tropical coastlines as well as off the coast of Florida. It is an important player in the local ecosystem, providing food and shelter for many sea animals, as well as preventing erosion of the coastline. In tropical regions, it may reach 80 feet in height, but Florida's mangrove is a shrubby tree that barely reaches 20 feet. It flowers in the spring, and produces seeds that germinate while still on the mother tree, sending a root out from the seed base. Once it falls, it lodges and begins growing as soon as it contacts soil.
White mangroves appear not only along coastlines but also grow in lagoons, and can be somewhat larger than the red versions. They are unique in that they produce small glands at the base of the leaf that emit a sugary nectar. A variety of insects and birds feed on the nectar. The prop roots of these trees arc out of the water and provide oxygen at high tide, and may originate from either the trunk of the tree or the branches. The roots of the tree help to build "islands" by trapping sand and sediment, allowing it to form a landing for additional trees to root.
Growing inland in low-lying coastal areas, the black mangrove is only exposed to saltwater during the highest of tides. It grows along coastal estuaries, and holds off erosion of coastal land. The black, hard wood of the tree has been used in building and carpentry, and the tannins in its leaves are often utilized for preparing leather hide. Beekeepers prize the nectar of the white flower for honey production, as it yields high-quality honey. Black mangrove breathes through tube-like pnuematophores rather than prop roots. Its 50-foot height decreases the farther north the tree grows.
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