I first became the UF/IFAS Extension Sea Grant agent for Flagler and St. Johns Counties in 2001. At that time, I was told that a hard freeze a few years earlier had stunted many of the black mangrove trees that were growing along the intracoastal waterway. Since then, there has not been an extended freeze that has affected these salt-tolerant trees. Black mangroves have been joined in our area by Florida’s other two mangrove species, the white and red mangroves. Scientists recently analyzed 28 years of satellite imagery and temperature records from northeast Florida. They found that the area of mangrove forests has doubled at the northern end of their historic range (St. Augustine) on the east coast of Florida. This expansion is associated with a reduction in the frequency of “extreme” cold events (days colder than −4 °C).
Three species of mangroves
In developed mangrove forests, the three true mangroves found in Florida generally occupy different positions along the shoreline. The red mangrove is characterized by its “prop roots.” These arch out from the trunk and down from lower branches and help stabilize the tree. Red mangroves often grow where their roots are continually submerged by water, and they are typically the mangrove that is growing closest to the water. Just landward of the red mangrove is the black mangrove. Black mangroves also have unusual roots. Their roots send up “pneumatophores” (breathing roots) which stick vertically out of the sediment. They look like a bunch of small sticks. They help the black mangrove get oxygen to its roots, which are often buried in anoxic mud. The white mangrove is generally the furthest from the water.
Both red and black mangroves form distinctive embryos called propagules. Those of the red mangrove are elongated and somewhat cigar-shaped. The propagules of the black mangrove look a bit like lima beans. Both can often be found on local beaches—some have traveled from areas to our south, while others may come from local mangrove trees. When the propagules fall off the parent plants, they will often start to grow roots as they float in the water. If they wash into a suitable area, the propagules’ roots will quickly grow into the sediment and the leaves will soon emerge.
While mangroves are quite salt-tolerant, and have adaptations to help them deal with growing in or near sea water, they do not require salt water. However, because most trees cannot tolerate salty conditions, mangroves will thrive along the coastline, without having to outcompete other trees. They do especially well in low energy, muddy habitats.
Mangroves are protected
Mangroves are protected in Florida by the Department of Environmental Protection, which has established rules that govern trimming of these trees. Property owners who are wishing to reduce the height of mangroves may be required to obtain a permit. They should contact an authorized professional mangrove trimmer to ensure that no laws are violated (see http://www.dep.state.fl.us/water/wetlands/mangroves/).