Why do mangroves flourish in some
environments, but are totally absent in others? Some environmental conditions
like cold and high wave energy prevent them from growing, while other conditions
that mangroves are adapted to, like salt and flooded soils, prevent competing
plants from growing.
Mangroves are adapted to many harsh environmental conditions, but don’t like cold weather. Specifically, they don’t grow well in climates with an annual average temperature of less than 66 degrees Fahrenheit, and freezing temperatures will damage or kill them. The range of Florida’s mangroves fluctuates with invasions of severe cold weather. For example, the series of devastating freezes of the 1980s that wiped out the citrus industry in north central Florida also killed mangroves as far south as Naples on the Gulf coast and West Palm Beach on the Atlantic coast. Although mangroves have been reported as far north as St. Augustine and Cedar Key, the northern mangroves are stunted and may only be present as a shrub form that sprouts from roots after freeze damage.
Hurricanes are an important factor controlling the mangrove ecosystem of south Florida. Ninety-six tropical storms have affected Florida Bay since 1916, with a major hurricane hitting about every 30 years. Wind damage to foliage and severe erosion of sediments prevents mangroves from reaching their growth potential in areas with frequent, severe hurricanes. For example, in 1960 Hurricane Donna’s 180mph winds and 12ft storm surge ripped away the mangrove fringes of the keys in Florida Bay. Some scientists believe that mangroves in hurricane prone areas have evolved to reach their maximum productivity in about 30 years.
Mangroves grow best in protected environments with low wave energies. Areas with heavy wave action are not suitable for mangroves because waves destroy their shallow root system, prevent seedling establishment, and prevent the build up of fine sediments. Mangroves are found in areas protected from waves by reefs, shell islands, and the leeward side (side away from the wind) of peninsulas and islands, bays and tidal rivers.
On the other hand, the movement of tidal water in and out of mangroves is key to their survival. Tides remove toxic compounds like hydrogen sulfide (H2S) that build up in flooded soils. Hydrogen sulfide is responsible for a rotten egg like smell that can sometimes be detected at low tide in mangroves. Tides also prevent the build up of salt and re-supply the system with oxygenated water.
Fine sediment brought into mangroves with the tide can be trapped by the mangroves root system. Over time, this may result in the build-up of land islands around the trees. For this reason some people describe mangroves as land builders. Mangrove trees depend on these sediments to provide a place to reproduce and regenerate new trees within the community.
Mangroves are halophytes or salt-tolerant plants. Their adaptations to saltwater allows them to flourish where no other trees can survive. They can grow quite well in fresh water, but their special ability to regulate salt allows them to out compete most other trees in the tropical and subtropical tidal environment. Mangroves prevent excessive amounts of salt from damaging their tissues by restricting the amount of salt that can enter through the roots, expelling salt crystals through transpiration, growing a thick cuticle to restrict salt absorption, or by salt excreting glands.
Scientists studying the pollen and spores from sediment buried in the past in Florida Bay have discovered that mangroves expanded their range when natural and manmade events decreased freshwater flow from the Everglades and increased salinities.
Water on top of soil blocks oxygen from entering. Tree roots and many of the soil organisms need oxygen for respiration. Under flooded conditions they will use up the soil oxygen and it will become anaerobic (without oxygen). Wetland plants, including mangroves, have adaptations for supplying roots in wet soils with oxygen. Red mangroves have prop roots that grow above the water from the base stem and drop roots that that grow down from the branches. Both prop and drop roots have small, above ground pores called lenticels and spongy passages called aerenchyma, that transport oxygen from the air to the root below the ground. The roots of black mangroves have special structures called pneumatophores that stick out of the soil like hundreds of fingers. At low tide air travels through the aerenchyma of the pneumatophore into the below ground roots. When mangrove forests are removed, for example by hurricanes, soils return to a less productive anaerobic condition.
Environmental factors and adaptations
