Mangroves are woody plants which are found along almost every coast that has a tropical or subtropical climate. Worldwide, there are approximately 50 species of mangroves. The most diverse location for mangroves is the Indo-Pacific region, containing approximately 40 different species. Somewhat less diverse are the mangrove regions of West Africa, the Caribbean, and the Americas, containing around eight different mangrove species.
Three true species of mangroves comprise the Florida mangrove forests, along with a forth mangrove associate, the buttonwood (Conocarpus erecta). Florida’s mangrove species, which are described in detail below, are the red mangrove (Rhizophora mangle), the black mangrove (Avicennia germinans), and the white mangrove (Laguncularia racemosa).
Mangroves are among our most important trees. They are the backbone and protector of estuaries. They purify water, prevent shoreline erosion, provide habitat for many birds and animals, and sequester carbon. Additionally, mangroves support our recreational and commercial fisheries, and provide tourism opportunities.
The mangrove environment is characterized by unstable and anaerobic (without oxygen) sediments, fluctuating water levels, and waters high in salt concentrations. To survive in these harsh environments, mangroves have evolved adaptations to their root systems to deal with the anaerobic soils, mechanisms for maintaining salt balance, and reproductive dispersal strategies. Interestingly, individual species have evolved different solutions to these same problems.
The red mangrove is generally found closest to the water and is probably best known for its “walking” prop roots. Prop roots support the plant in mucky sediments and have pores called lenticels which allow gas exchange with the buried roots. The red mangrove separates fresh water from salt water by salt exclusion in a process known as nonmetabolic ultra-filtration (similar to reverse osmosis).
Red mangroves produce yellow flowers. After pollination, a small fruit, and a 12- to 18-inch propagule is formed. The propagule is an embryo which begins germination and development while still attached to the tree in a process known as vivipary. Once the propagule breaks away from the tree, it floats freely for up to a year before being washed ashore, where it develops roots and becomes a new mangrove plant.
The black mangrove is most easily recognized by its system of shallow aerial roots, called pneumatophores (say “new-MAT-uh-forz”), which extend like fingers up from the sediments. These pneumatophores provide oxygen to the buried root system. Black mangroves generally occupy a slightly higher elevation than red mangroves. In these higher soils, salt deposits accumulate. Black mangroves excrete salt from salt glands on the leaf surface (if you lick a leaf, it’s very salty).
The leaves of this mangrove are dull green to gray, and the flowers are a creamy white and form clusters at the branch tips. These flowers produce a fruit resembling a lima bean which functions as its propagule. Like the red mangrove, black mangroves utilize vivipary and propagule dispersal reproductive strategies.
The white mangrove usually grows more inland behind the red and black mangroves. White mangroves are the smallest of the three Florida mangrove species, rarely reaching 50 feet in height. White mangroves have yellowish green leaves that contain two small nodules at the leaf stalk, which serve as nectar glands.
White mangroves typically do not exhibit aerial roots. However, in deeper or stagnant waters, some may express roots similar in appearance and function to the black mangrove pneumatophores. White mangroves use the same salt-excreting and reproduction strategies as exhibited by their black mangrove counterparts.
Mangrove soils have a characteristic black color and nose-turning smell. Because mangrove soils are perpetually waterlogged, there is not much free oxygen available. Aerobic bacteria use up any available oxygen quickly. Anaerobic bacteria (which need an low-oxygen environment) perform many important functions. In doing so, they produce sulfur dioxide, which is that rotten-egg smell familiar to most of us. In addition to being particularly pungent, sulfur dioxide also creates conditions hostile to most other plants — giving mangroves a competitive edge in the coastal environment.
Betty Staugler is the Charlotte County extension agent for the Florida Sea Grant Program. She is active in many areas relating to boating, fishing, and watershed/coastal living. The Florida Sea Grant College Program supports research and education activities that help Florida’s shoreline communities, industries and citizens wisely use the state’s coastal and marine resources. Contact her at firstname.lastname@example.org or 941-764-4346.