Mangroves on the Move?

Red mangroves growing along the shoreline. Photo by Jimmy Jacobs

Red mangroves growing along the shoreline. Photo by Jimmy Jacobs

Three species of mangrove trees are prevalent on the coasts of South Florida and two of them figure prominently in producing habitat for saltwater game fish. The red mangrove is found along shorelines and features a web of roots that form impenetrable tangles beneath the trees.     Black mangroves are also found near the shore and often are flooded by rising tides. They, however, don’t have the root tangle, but rather a series of root stubs that protrude up from the ground around the trees.

The white mangrove is found on higher, drier ground and has very little influence on fish species.

Mangrove forests often form on shallow flats and can collect debris and sand to eventually become islands. But, while still flooded the bases of the trees offer protection to a host of forage species, and hunting grounds for predators. Mangroves also protect shorelines from erosion, especially during gale and hurricane conditions.

In recent decades the Sunshine State’s mangroves have been disappearing to shoreline development in most areas of the state.  However, a new study published in the magazine Hydobiologia authored by Villanova Biology Professor Samantha Chapman, coupled with a report from Smithsonian Environmental Research Center scientists, points to some good news on the mangrove coasts.

Prof. Chapman’s research shows that mangroves provide as much as eight times more protection for coastlines than do salt marshes, and the mangrove stands offer a much more cost effect form of protection than do man-made breakwaters.

As for the Smithsonian work, it used aerial photographs from 1984, overlaid by satellite imagery from 2011, to show the northern limit of mangroves at the 30-degree north latitude line, which is just north of St. Augustine. Surprisingly, it also showed that the mangrove trees were encroaching northward, with the greatest build up found in the area between 29 and 30 degrees of latitude. Through there the mangroves are replacing salt marshes.

The main reason for this expansion seems to spring from less extreme cold weather in this part of Florida in recent decades. The number of days when the air temperature fell below 25 degrees had declined during that period, allowing the tropical trees to move north.