By Lisa D. Mickey
It’s possible you have seen the arrival and proliferation of mangrove trees where you live and have wondered about the origin of these trees.
Are they native with a history in Florida? Are they invasive plant migrants from the tropics?
And why are they important to where you live, especially now, when tropical storms have displayed increased frequency and intensity on the shorelines throughout Florida?
Short answer: mangrove trees are, indeed, native to Florida and have been here for many years. They are not invasive species.
According to Dr. Marty Main, a professor of wildlife ecology and conservation at the University of Florida, for a species to be categorized as invasive, it: has to be nonnative; has to cause ecological damage; and it has to cause concern for human health issues.
“Mangroves have many very good ecological properties,” said Dr. Main, founder of the university’s Florida Master Naturalist Program. “They stabilize the shorelines and provide habitat for juvenile fish and many other species.”
In fact, there are three different species of mangroves in Florida. The red mangrove (Rhizophora mangle) is known for its downward-extended reddish-colored prop roots and typically grows closest to the water. It is followed by the black mangrove (Avicennia germinans), which features vertical pencil-shaped roots protruding from the water called pneumatophores that help this tree breathe. The most upland of the three species is the white mangrove (Laguncularia racemosa), the smallest and least cold tolerant of the trees.
All three species produce propagules, which are seeds that grow on the trees throughout the year and drop off the limbs and into the water each autumn. The red mangrove propagules resemble long green beans, while the black mangrove’s propagules look similar to fat lima beans. The white mangrove’s small seeds are about the size and shape of almonds (Pictured below respectively).
All of these propagules can remain viable for around a year, floating and eventually rooting on shorelines, sandbars and oyster reefs miles away from their parent tree, potentially becoming new trees.
Mangrove Range Expansion
Mangrove forests have traditionally grown on tropical and subtropical saltwater coastlines, but in recent decades, these trees have transitioned into more northern and western regions of Florida as water and air temperatures have climbed.
Scientists have been able to document the presence of mangrove trees for many years, especially in the south Florida region, but following several extended days of freezing temperatures in 1989, with more cold weather recorded in the early 1990s, mangrove trees have responded to climate-related temperatures by bouncing back.
Aerial photography, as well as satellite imagery, has documented the northern range expansion of mangrove trees since the 1940s. It’s believed that Florida’s Atlantic coast has grown by nearly 3,000 trees since the early 2000s.
Aerial photo of mangrove barrier islands in Florida.
In a 2022 academic paper co-authored by Dr. Jason Evans of Stetson University, analysis of satellite imagery has identified “substantial increases in mangrove area” over the past three decades.
The study added “decreases in the occurrence of killing frosts, as associated with anthropogenic climate warming, are favoring mangrove expansion in Florida.”
Evans’ research spelled out that “the actual mechanism for mangrove expansion could be the decrease in the frequency of extreme cold events, which allows freeze-intolerant mangroves to expand.”
“Ponce Inlet was often considered to be a break line for extensive mangrove swamps in the past,” said Dr. Evans, associate professor of environmental science and studies at Stetson. “South of Ponce Inlet was observed to have widespread mangrove ecosystems, whereas north of Ponce Inlet, mangroves were patchier and likely dominated by the more cold-tolerant black mangroves interspersed within Spartina [cord grass] marshes.”
Riverside Conservancy’s headquarters, based in Edgewater, is what Evans describes as well within the traditional native range for all three of the Florida mangrove species, even though they were heavily impacted by the 1989 freeze.
“That pattern of mangrove expansion and freeze-driven contraction probably would have been the pattern for many hundreds, if not thousands, of years in what is now the northern Mosquito Lagoon,” added Evans, who also serves as executive director of Stetson’s Institute for Water and Environmental Resilience.
And that ebb and flow of a changing planet is nothing new, said Dr. Main.
“Florida looks much different now than it did when European settlers arrived here or before that, back to the Pleistocene [known as the Ice Age, from circa 2.58 million to 11,700 years ago] when Florida was largely a savannah, three times the size that it is today,” he added
When Florida residents observe tidal or coastal saltmarshes transitioning from grass-based vegetation into mangrove-dominated forest, Dr. Main warns this change is not an invasion by another species.
“This is a native species that is expanding its range according to changing environmental conditions,” he said. “You might end up with a different ecosystem, but everything changes. Nothing in nature remains static.”
Value of Mangrove Trees
Brown pelicans nesting on top of mangroves. The downy chicks (grey) are just starting to grow their first set of feathers.
It is estimated that estuarine mangrove forests serve as the nursery for 75-80 percent of oceanic species. Mangroves also provide habitat for up to 80 percent of commercially and recreationally important fish species. Some 700 species of fish have been documented in the Indian River Lagoon, many of which began their lives around the mangroves.
In addition, more than 300 species of birds have been recorded in the lagoon region, using mangrove trees for nesting and foraging or as a stopover point for migratory birds.
The sheer density of the trees with their tangled and pointing roots serve as a safe habitat for juvenile fish and crabs attempting to avoid predation. Tiny blooms on the trees early each summer also prove inviting for honeybees, which collect the mangrove pollen to render tasty mangrove honey.
A blossom of a black mangrove, which begin to bloom in the fall.
And to sit alongside mangrove trees in the lagoon for quiet observation typically showcases a plethora of fish, crabs, shrimp, conchs, whelks, oysters, clams, birds and even juvenile sharks and sub-adult sea turtles. The habitat is vibrant and viable with the mangrove trees providing a backdrop for the varied life that lives in its roots and on its limbs.
Mangrove forests also store atmospheric carbon. It is estimated that mangroves absorb and sequester as much as 10 times the carbon of a similar size area of terrestrial trees.
“Mangroves are one of the plants that fall into the ‘blue carbon’ category, meaning that it is one of the organisms on the coastline and in the estuary that are helping to capture, absorb and store carbon,” said Carolyn Kovacs, Florida Sea Grant agent for the University of Florida’s IFAS Extension offices in Flagler and Volusia counties.
“Mangrove trees are protected by law and it’s important to protect them because of how they benefit us environmentally,” added Kovacs.
Perhaps the value of mangrove trees is most evident to residents of waterfront homes who have intact mangrove forests along their shorelines. These densely entangled trees with their expansive root systems collect sand and stabilize shorelines. During tropical storms, mangrove trees help absorb the energy of high winds and tidal surges.
Such a mangrove fortress could prove even more valuable in the coming decades. According to the National Oceanic and Atmospheric Administration (NOAA), sea level is projected to rise 10-12 inches in the next 30 years along America’s eastern coastline, while global tropical storm intensities are projected to increase by up to 10 percent.
“These trees are important in protecting our land and private properties,” added Kovacs. “They really do absorb storm surge and help prevent water from reaching land and entering houses on those shorelines.”
And yet, it is estimated that up to 50 percent of the world’s mangrove forests have been lost due to coastal development, urbanization and illegal cutting. Even with the long list of environmental benefits mangrove trees provide, not everyone appreciates the uniqueness of these salt-tolerant trees.
A 1979 photo of a mangrove being completely cut down. In 1996, Florida passed the Mangrove Trimming and Preservation Act to protect mangroves from being clear-cut.
Some home owners want an unobstructed view of the water adjacent to their homes. Some complain of a sulfuric odor at low tide.
“It’s not the mangroves, it’s the sediment that sometimes creates a smell,” said Kovacs. “There are a lot of little microbes living in the sediment among the root systems and they produce gases. It just means there’s a lot of good bacteria and organisms living and working in a healthy ecosystem. It might smell a little funkier at low tide, but that’s just the smell of life.”
Mangroves In the Coming Years
Kovacs believes sometimes there is public confusion about mangrove trees because of changes in central Florida over the last 35 years. Some residents have lived in the area long enough to remember the presence of mangroves, the loss of mangroves to freezing temperatures, the transition of mangroves back to salt marshes and marsh grasses, and then the transition of salt marshes and grasses back to mangrove trees.
Newer residents to the area may not have witnessed climate-related changes over the decades and may only be familiar with mangrove expansion and vegetative dominance, viewing mangrove trees as invaders intent on usurping saltmarsh habitats.
“If you have only been here for the past 10 years, it will probably appear that there were marshes that are now mangrove forests,” said Kovacs.
“That may be true, but it’s part of something that has been happening naturally,” she added. “With a changing climate, there are a lot of extremes, so it’s possible that we could have future cold temperatures and it could shift again in either direction.”
And as the planet experiences changes of many kinds, mangrove trees will serve as an indicator of global transitions, sometimes exemplified as the sentinels in our own back yards.
“People have to recognize there will be many changes and for them to assume some species are invasive because they are responding to the climate change that is occurring is simply inaccurate,” added Dr. Main.
“As the planet warms, it’s going to become unsuitable for some species, while benefitting others,” he added. “I think we should expect to see changes of all kinds and to understand that when it comes to our planet, changes will continue.”