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added to the peat soils (soil made up almost entirely of decaying plant material) of
the forest decreases, which lowers the elevation of the forest floor, and leads to saltwater intrusion. Ultimately, the forests in these affected areas are converted to mudflats. When the next storm comes along, if it completely breaches the fringing
mangroves along the water’s edge, it can lead to a rapid shift to a marine environment. Mangrove biologists working along Cape Sable and the southwest coast of
Florida have documented these processes as a result of repeated strikes from the
Labor Day Storm, Donna, Andrew, Wilma, and now Irma. Each hurricane has
impacted the mangrove forests, and the subsequent storms set back any recovery
that had begun to take place.
The Future of Florida’s Coast – The Unanswered Questions
So, going back to our research question – does this process of overstepping caused
by hurricanes explain what happened to the shoreline of south Florida about
3500 years ago when coastal inundation occurred in less than 200 years? We think
it does. Within the last century, significant changes have occurred along the southwest mangrove coast that are similar to what we’ve interpreted from the cores. “The
present is the key to the past” tells us this is a likely and logical explanation for what
happened about 3500 years ago. And what about the future of the mangrove islands?
Only time will tell, but we have begun a program to monitor the changes on Jim
Foot Key over the next 5–10 years. When we returned to the island in April 2019,
almost 20 months after Hurricane Irma, we saw some positive signs of recovery
along the eastern berm with new mangroves taking root (Fig. 12.15). However, the
large trees along the berm were dead (Fig. 12.13c) and these berms are the only part
of the islands currently above sea level. The mature mangrove trees growing on the
berm before the storm contributed organic matter to the berm from their leaves and
roots. In addition, the roots trapped material washed in by high tides, which built up
the berm even more and provided additional space for new trees. The loss of these
mature mangroves following Irma means the island is less likely to be able to keep
pace with rising sea levels. The berm has been visibly flattened and a breach in the
berm has widened, allowing regular tidal exchange with water in the mudflat
(Fig. 12.16). The mudflat itself seems to have converted to a shallow lagoon, with
seagrass growing in the middle.
What do the results of our studies tell us about the future of the mangrove coast
of south Florida? Remember that the rate of relative sea-level rise 3500 years ago in
south Florida was only about 2.6 inches (6.6 cm) per century – today it is
9.8–14.5 inches (24.9–36.8 cm) per century, an increase of about 3–5 times. This
rate is expected to increase with continued melting of ice sheets on land throughout
the twenty-first century. In addition, many climate change scenarios indicate storms
will increase in frequency and intensity in the coming decades. The past tells us that
storms alone can cause mangrove coastlines to retreat landward. When high rates of
G. L. Wingard
added to the peat soils (soil made up almost entirely of decaying plant material) of
the forest decreases, which lowers the elevation of the forest floor, and leads to saltwater intrusion. Ultimately, the forests in these affected areas are converted to mudflats. When the next storm comes along, if it completely breaches the fringing
mangroves along the water’s edge, it can lead to a rapid shift to a marine environment. Mangrove biologists working along Cape Sable and the southwest coast of
Florida have documented these processes as a result of repeated strikes from the
Labor Day Storm, Donna, Andrew, Wilma, and now Irma. Each hurricane has
impacted the mangrove forests, and the subsequent storms set back any recovery
that had begun to take place.
The Future of Florida’s Coast – The Unanswered Questions
So, going back to our research question – does this process of overstepping caused
by hurricanes explain what happened to the shoreline of south Florida about
3500 years ago when coastal inundation occurred in less than 200 years? We think
it does. Within the last century, significant changes have occurred along the southwest mangrove coast that are similar to what we’ve interpreted from the cores. “The
present is the key to the past” tells us this is a likely and logical explanation for what
happened about 3500 years ago. And what about the future of the mangrove islands?
Only time will tell, but we have begun a program to monitor the changes on Jim
Foot Key over the next 5–10 years. When we returned to the island in April 2019,
almost 20 months after Hurricane Irma, we saw some positive signs of recovery
along the eastern berm with new mangroves taking root (Fig. 12.15). However, the
large trees along the berm were dead (Fig. 12.13c) and these berms are the only part
of the islands currently above sea level. The mature mangrove trees growing on the
berm before the storm contributed organic matter to the berm from their leaves and
roots. In addition, the roots trapped material washed in by high tides, which built up
the berm even more and provided additional space for new trees. The loss of these
mature mangroves following Irma means the island is less likely to be able to keep
pace with rising sea levels. The berm has been visibly flattened and a breach in the
berm has widened, allowing regular tidal exchange with water in the mudflat
(Fig. 12.16). The mudflat itself seems to have converted to a shallow lagoon, with
seagrass growing in the middle.
What do the results of our studies tell us about the future of the mangrove coast
of south Florida? Remember that the rate of relative sea-level rise 3500 years ago in
south Florida was only about 2.6 inches (6.6 cm) per century – today it is
9.8–14.5 inches (24.9–36.8 cm) per century, an increase of about 3–5 times. This
rate is expected to increase with continued melting of ice sheets on land throughout
the twenty-first century. In addition, many climate change scenarios indicate storms
will increase in frequency and intensity in the coming decades. The past tells us that
storms alone can cause mangrove coastlines to retreat landward. When high rates of
G. L. Wingard