Researchers have observed an increase in mangrove trees in southern Amelia Island, Florida, in recent years. This is well north of the plant’s typical cold-sensitive habitat.
Historically, mangroves have been limited to southern Florida. However, they are now increasing along the temperate zones farther north, from St. Johns to Nassau Counties.
This migration pattern is related to warmer winters and hurricanes. Warmer winters allow mangroves to survive further north, while hurricanes help to disperse their seeds.
A survey conducted by Dr. Candy Feller of the Smithsonian Institute in 2004 found no mangroves along the southern tip of Amelia Island. However, a return visit to the same site in 2017 found mangroves over six feet tall. In May 2023, the same trees had grown to nine feet tall and spread 20 feet wide.
Feller links this expansion to Hurricanes Frances and Jeanie, which hit Florida in 2004. These hurricanes sent wave energy right up the east coast, just when mangrove plant propagation was at peak season. This helped to disperse mangrove seeds throughout the region.
Hurricanes are an efficient mechanism for dispersing mangrove propagules. Thousands of propagules were washed ashore in Northeast Florida after Hurricane Ian in 2022.
Hurricanes are nothing new so why didn’t the plants establish locally decades ago?
The answer is that freezes have kept the species in check. In the past, freezes would periodically kill off mangroves that had migrated too far north. However, climate change is making winters warmer, which is allowing mangroves to survive farther north.
Feller’s research has shown that the relationship between mangrove tree sightings with hurricane and freeze records over the past 300 years is cyclical. Freezes (marked by blue circles) temporarily reduce mangrove numbers, while red dots represent more tree reports after hurricanes.
In addition to warmer winters and hurricanes, mangroves are also becoming bigger and stronger. This is due to a process called self-fertilization. When mangrove leaves die back from a freeze, the leaf kills release nitrogen that stimulates the growth into a hardier tree. These trees can spread quicker than in the past and are often more resistant to cold.
This self-fertilization process has helped mangroves to survive past freezes and to become more cold-tolerant. As a result, they are now able to survive farther north than ever before.
Climate change is impacting our local winters with a decrease in freeze frequency to the point where the mangrove growth pattern has gotten a head start from past freeze recovery cycles.
Case in point, this year only had four freezes in Jacksonville after six harsh frozen nights during Christmas week in 2024. After that, only three nights in January 2023 froze. How did the trees fare at the survey site? Feller discovered all red and black mangroves survived while the white mangroves suffered leaf damage.
As climate change continues, we can expect to see even more mangroves expanding north in Florida. These trees provide important ecosystem services, such as shoreline protection and water filtration. Their northward migration is a sign of the changing climate, but it also offers hope that these valuable trees will be able to adapt and thrive in the future.