In our last installment of Habitat Focus we explored our Coastal Wetlands and briefly touched on mangrove stands as part of this vast habitat. This time we will explore the mangrove habitat in more detail.
Mangroves are a group of salt tolerant trees that are commonly found in the coastal zone of tropical and subtropical latitudes worldwide. Like some of us, their tolerance to freezing is low and this appears to limit their geographic range. A recent summary estimates their abundance at 39.3 million acres worldwide. This is equivalent to about 1.7% of all land in the US or 0.1% of all land worldwide. The taxonomic group of plants that includes mangroves reaches its greatest biodiversity in the South Pacific region with Asia having the largest number of the world’s mangroves. The Caribbean area is inhabited by only 4 species – red mangrove (Rhizophora mangle), black mangrove (Avicennia germinans), white mangrove (Laguncularia racemosa), and buttonwood (Conocarpus erectus), with each species typically occupying a specific zone, based on tidal height, along the shoreline.
Where they occur, mangroves are typically the dominant plant in the ecosystem, structurally and functionally. Consequently the term mangroves has also been used to refer to the ecosystem; other terms you may encounter are mangrove swamp or mangal.
As you may know from houseplant ‘failures’, waterlogged soil is a difficult environment for plants. One of the major reasons for this is the lack of, or reduced amount of, oxygen in waterlogged soil. The diffusion of oxygen from the atmosphere through water is much slower than that through air. Mangroves have several really interesting adaptations that allow them to grow in these low oxygen environments. For example, the red mangrove has large prop roots that sprout and grow outward and downward from the trunk increasing the area above water for oxygen absorption through pores in the bark. These roots also create a network that hang in the water allowing the development of a unique and diverse community of algae and animals. In contrast, black mangroves have pneumatophores, root-like structures also covered with pores that grow upward from the soil projecting into the atmosphere.
Waterlogged soil that is salty is even more stressful. Several mangrove species have salt glands at the base of the leaves that allow the tree to secrete salt that has been taken up along with the water. Just like marsh grasses, if you look closely at the leaves of the white or black mangrove, you can actually see the salt crystals. Plants that can grow in salt water are called halophytes (noun) or halophytic (adjective).
Until relatively recently, the occurrence of mangroves in the United States was limited to Florida. This is where the story gets interesting. Back in the 1980’s, one of the professors at LSU used to take his students for a trip just to see a solitary black mangrove that had managed to grow on Grand Isle, a barrier island on the Louisiana coast. Since that time, however, black mangroves, have been observed in many locations in the northern Gulf of Mexico, including Texas, Louisiana, and the barrier islands of Mississippi. This range expansion for black mangroves, the most cold tolerant species in the group, is occurring due to milder winters resulting from climate change. The expected coastal habitat in the northern Gulf is salt marsh. Now we are seeing these habitats with a significant number of black mangroves. Does this matter? Do the mangroves change the function of the habitat? How are the ecosystem services of the marshes changed? ACER scientists are investigating how the type of coastal habitat (salt marsh only, mangrove only, or a mix of salt marsh and mangroves) affects the resilience of the coastal wetland ecosystem to disturbances such as hurricanes or oil spills.
 Osland, MJ, N. Enwright, RH Day, and TW Doyle. 2013. Winter climate change and coastal wetland foundation species: salt marshes vs. mangrove forests in the southeastern United States. Global Change Biology, 19 (5): 1482-1494. http://onlinelibrary.wiley.com/doi/10.1111/gcb.12126/abstract
Check out the Smithsonian Marine Station at Ft. Pierce webpage for even more interesting information about mangroves.