Mangroves

Fort Matanzas National Monument is notable for many reasons, but few people probably realize that it once harbored the northernmost example of an entire plant species on the east coast! And yet, in 2006, an article appeared in the scientific journal “Castanea” detailing how researchers from the Universities of Georgia and Florida had documented the most northern specimen (at the time) of Rhizophora mangle, or red mangrove, within the boundaries of the park. It’s not known exactly how long Fort Matanzas held this illustrious record, but in time red mangroves expanded beyond the park to the north. But the entire reason the presence of one plant made such a stir is because the red mangrove is a very cold intolerant species of the tropics and sub-tropics. To find it as far north as Matanzas and beyond is thought to be yet another indicator of how a warming climate is causing shifts in the ranges of many plant and animal species.

There are three different species of mangrove found in Florida- the black, the red, and the white. The black mangrove is the most cold tolerant, while the white is the least tolerant of freezing weather. Because of this susceptibility to sustained cold weather, and because of their relatively rapid rate of expansion due to water-borne propagules, mangroves are like a natural barometer of long-term climate changes. So it was with a fair amount of excitement, but not total surprise, that park staff reacted to the news in September 2018 that white mangroves (Laguncularia racemose) had been documented in the park for the first time! The news was delivered by botanists from the Smithsonian Institution and Villanova University who were conducting research in the surrounding Guana-Tolomato-Matanzas NERR. Thus, another species was added to the park’s plant list (which now makes for 250 documented species, by the way).

Perhaps ironically, these tropical species that are expanding north with the warming climate may also be one of nature’s best defenses against some of the negative effects of global warming. For one thing, mangroves absorb and sequester vast amounts of carbon dioxide, one of the predominant greenhouse gases. Because mangroves grow in saturated soils that are low in oxygen content (anaerobic), bacteria and fungi that normally break down organic matter are not as efficient as they would be in well-aerated (aerobic) soils. Thus dead plant matter accumulates and eventually turns into peat, trapping the carbon from the plants’ tissues for upwards of millennia. Also, the complex intertwining of mangroves’ roots and branches forms a complex latticework that is very efficient at trapping sediment and absorbing the energy of wind and waves. There may be no better natural system for stabilizing and building a shoreline while protecting it and the uplands behind it from rising seas and associated storms.

Unfortunately, as with most coastal habitats, mangrove forests have been removed over the years to make way for human development. (At least 35% of global mangrove habitat has been lost since 1980.) Maybe this time we should play it smart and make way for the mangroves!