The diversity of plants in
mangroves is low compared to other ecosystems. Few plants can tolerate the harsh
conditions of coastal tidal zones and the mangrove's thick canopies can block most of
the light from other plants. There are four species of trees that are included
in Florida’s mangrove forests. While similar in many respects, each has special
adaptations for their watery environment.
Red Mangrove -(Rhizophora mangle L.)
Red mangroves are easily identified by their tall arching roots, called prop roots that give them the appearance of walking on water. These roots are adapted to supply air to the underground roots, as well as adding to the stability of the trees. The roots also have special cells which allow them to absorb mostly fresh water from salt water.
Leaves of the red mangrove are opposite, egg shaped to elliptical, leathery and shiny dark green above and paler underneath. Leaf edges are smooth and somewhat thickened. Clusters of 3 to 4 pale yellow flowers can be seen in the spring.
Red mangroves grow further out in the water than the other mangroves and are usually flooded at high tide. They can root on intertidal surfaces such as oyster beds and sand bars forming "mangrove islands".
New trees establish themselves further away from the original trees by a well-adapted reproductive process. In a process called vivipary, seeds sprout into torpedo-shaped seedlings called propagules, while still attached to the parent tree. Sprouts then drop to the ground or into the water. When a seedling lands in the water it may either take root among the other mangrove roots or float with the current until it drifts on to suitable ground. Seedlings remain viable for long periods of time and can become established after floating as long as 12 months .
Black Mangrove -Avicennia germinans
Black mangroves can be easily identified by the numerous pencil-like breathing tubes, called pneumatophores, which grow vertically from the mud to just above the highest sustained water level. Like the prop roots of the red mangrove, these provide air to the flooded roots. The roots of some black mangroves can also filter out salt water, but much of their salt regulation is accomplished by salt excreting glands in the leaves. Black mangroves also tend to have dark bark, where as the bark of white mangroves is light. The drop roots and inner flesh of red mangrove may have a reddish tint.
As a result of this salt excretion, the upper surfaces of black mangrove leaves are frequently coated with salt crystals. The leaves are simple, oppositely arranged, persistent, and 2" to 4" long by ¾" to 1 ½" wide. The oblong shaped leaves usually have shiny upper surfaces while the underneath surface is hairy. The leaf base is wedged and the leaf tip is rounded. The leaf margin is smooth and sometimes slightly rolled down along the side edges.
Black mangroves bloom heavily in June and July with white flowers. During black mangrove bloom, beekeepers set up their hives to collect the nectar for production of "mangrove honey". This honey is of very high quality. Considerable quantities were made in the United States until about 1895. Hurricane destruction of the best forests decreased honey production. In recent years there has been renewed interest in this product. The lima bean shaped propagule is green and 1½" long by 1" wide and has splits along two edges.
Black mangroves grow closer to the shore where they are reached only by high tides. At the northern edge of their range, (St. Augustine on the east coast and Cedar Key on the Gulf coast), these trees are small and shrub-like. Larger trees up to 50 feet tall with a branch spread of 35 feet are found further south around Sanibel-Captiva Islands.
White Mangrove -Laguncularia racemosa
White mangroves typically grow at the highest and driest part of the tidal zone. They may have peg and/or prop roots depending on habitat conditions, but most have neither. Peg roots are similar to the pneumatophores of black mangrove except they are shorter and stouter. The lower trunk has tiny lenticels along the bark to help bring in extra oxygen when water levels are high.
The bark and leaves of “white” mangroves are lighter than other mangroves. They are best differentiated from other mangroves by succulent, light green leaves which are rounded at the base and tip and smooth underneath (Figure?). Glands at the base of each leaf called nectarines, excrete sugar and salt (Figure?). Some insects feed on the sugar. Its flowers are greenish-white and produce good honey. The fruit is small, dry, leathery and ribbed. It contains a dark red seed, and like other mangrove propapagules, it’s buoyant allowing it to float to new growing sites.
Buttonwood -Conocarpus erectus
Buttonwood is in the same family as the white mangrove, but it’s often considered only an associate of mangroves. Like white mangrove, it has salt glands on the leaves to expel salt and lenticels on the lower trunk to bring in extra oxygen. However, buttonwood is less tolerant of salty conditions than the “true” mangroves and therefore it grows further inland. Frequently it occupies a transition zone between the tidal mangroves and the tropical hammock.
The "button" part of the name comes from the button-like appearance of the dense, rounded flower heads that grow in a branched cluster, and the purplish-green, round, cone-like fruit. The other three mangroves have leaves located opposite one another. Button mangrove leaves are alternate, leathery, pointed at the tips, have smooth edges, and two glands at the base of each leaf (Figure ?). It is shrubby along the shore, but takes on a tree form further inland. The wood of these trees is extremely dense and durable and was used for firewood, cabinets, and making charcoal. Old stumps and downed branches of buttonwood trees may lie along coastal beaches for decades before they decay.
Mangroves frequently border salt tolerant marshes that may include salt grass, black needle rush, spike rush, gulf cordgrass and glasswort. The landward fringes of mangroves can interface with seaside mahoe (a “naturalized” exotic), fan palm, coin vine, and nickerbean. Where sufficient light filters through the mangrove canopy, salt tolerant herbaceous plants like the leather ferns, Spanish bayonet, sea blite, rubber vine and air plants.can be found. The Powdery Catopsis.(Catopis berteroniana)is an endangered air plant of the Florida Everglades that has a slippery powder to attracts insects. They slide into a water reservoir at the base of the plant and their decaying body supplies nutrients. Most invasive exotic species find mangroves too inhospitable to invade. However, Brazilian pepper and Australian pine can sometimes penetrate into black mangrove basin communities.
Environmental factors and adaptations
