Mangrove Forests

Trees of mangrove trees

Many of the tree and plant species that are found in mangrove forests are endemic to that habitat. They are not generally found outside of mangrove forest ecosystems.  Few plants and trees can tolerate the wet, salty soil and harsh coastal conditions as well as mangroves species can. There are four species of trees that are included in mangrove forests. While similar in many respects there are some interesting differences between them.

Red mangroves grow farthest out in the tidal zone where their roots may be flooded by water all or most of the day. Because they grow in soft, wet mud and their roots have more difficulty getting enough oxygen from underwater, red mangroves produce long external roots that hang down from the trunk and branches of the trees. These odd-looking prop roots help to stabilize the trees from high winds and strong waves while they draw in extra oxygen for the tree.  The submerged roots have special layers that filter out salt while allowing fresh water to enter. This allows the trees to survive in either fresh or salt water.

Red mangroves have developed seeds that begin to germinate while they are still on the trees. This provides a competitive advantage by allowing them to get a head start on other species that may try to establish. The seed propogules are long, thin sprouts that may float in the water or lie dormant in the mud for many months until conditions are right to establish and grow.

Black mangroves also grow in the low tidal zone and are flooded by incoming tides for much of each day. However, these trees are usually found growing farther inland where the land is dry at low tide. Due to the daily inundation by sea water black mangroves have also developed extra roots for oxygen intake. Thin, pencil-like tubes protrude out of the ground around the base of the trees and draw in oxygen to the underground roots. These are called cable roots or pneumatophores. These tubes grow above the water level at high tide and provide a constant source of oxygen.

Black mangroves do not filter salt out at the roots as red mangroves do but they expel salt through the leaves as they transpire. Tiny salt crystals may often be seen on the leaf surface. This allows them to use the salt water without harming the trees. Like red mangroves, black mangroves have seeds that begin to germinate while still on the trees.

As we move farther up the tidal zone we find slightly upland areas where flooding is common but longer dry periods occur. This is where white mangroves grow. While these trees still benefit from frequent inundation and flushing, they need to have dry periods every day. White mangroves do not have aerial roots however, the lower trunk of the trees have tiny lenticels along the bark to help bring in extra oxygen when water levels are high. White mangroves are generally smaller than the other species and may be gnarled by high winds and poor growing conditions. White mangrove trees have two salt glands at the base of each leaf that expel excess salt. Salt crystals may often be seen on the lower leaf surface near the base.

Buttonwood, sometimes called button mangrove, is found growing on higher ground and farther inland along the tidal zone than the other mangrove species.  Named for their round, button-like flowers and fruits, these trees are extremely dense and durable. Old stumps and downed branches of buttonwood trees may lie along coastal beaches for decades before they decay.  Buttonwood trees grow just below the salt line and are often seen bordering tropical hardwood hammocks. The trees are favorite hosts for many epiphytes.  They are exposed to frequent fluctuations in water levels, especially during stormy weather but they are on relatively dry land much of the time. Buttonwood trees, like white mangroves, have salt glands on the leaves to expel salt and lenticels on the lower trunk to bring in extra oxygen when it is needed. Of all of the mangrove species, buttonwood is the only one that does not have oppositely-arranged leaves. The pointed leaves are alternately-arranged along the branches.

 Since buttonwood grows in the exposed upland terrain along coastal beaches it is especially vulnerable to strong tropical winds. Many of the buttonwood forests of South Florida have been destroyed by hurricanes. The remnants of these old forests may still be seen as gnarled snags or large stumps along tidal beaches.