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and other plants, and the area just behind the mangroves had lost 2.2 inches (5.6 cm)
in elevation. Large piles of seagrass were all around the edges of the island. On the
southwestern corner of the island where the storm surge was most likely highest, we
found thick (2–3 inches = 5.1–7.6 cm) deposits of bay mud, with seagrass mixed
into the mud.
This is the yin and yang of hurricanes. They tear things down and erode, but they
also build up islands and shorelines by depositing sediments. Mangrove shorelines
have been shaped by hurricanes for millennia because both occur in the tropical to
sub-tropical regions of the world. The question is which hurricane impacts – the
constructive or destructive – dominate over time and what is the net effect of these
opposing processes? Were the mangroves dead or had they just lost their leaves?
And if the mangroves were dead, did this loss of the trees outweigh the beneficial
effects of sediment deposition? Finally, most important to our research, could these
processes explain the coastal inundation of south Florida 3500 years ago?
Just as the present can be the key to the past, the past can also be the key to the
present. To understand what the longer-term effects of Hurricane Irma might be on
the islands, we looked at records of hurricane impacts to the mangrove coasts of
south Florida over the last century. Hurricanes strike south Florida every 7–8 years
on average, and a major storm occurs every 14–19 years (based on NOAA data
gathered since 1886). Within a 90-mile (145 km) radius of Florida Bay, 21 major
hurricanes (category 3 and above) have made landfall in the last 130 years. Records
from earlier storms (Labor Day Storm in 1935, Donna in 1960, Betsy in 1965)
describe similar impacts to what we saw on the islands following Hurricane Irma.
However, few quantitative studies were done, and satellite imagery was not available until after 1972, so we cannot make any direct comparisons between these
older storms and Irma. Extensive research, however, was done by a number of people following Hurricanes Andrew (1992) and Wilma (2005) on impacts to the southwest Florida coastal mangroves. Andrew destroyed tens of thousands of acres of
mangrove forest in Everglades National Park and deposited an average of 3 inches
(7.6 cm) of mud across 40 square miles (104 sq. km) of forest. Wilma destroyed
3000 acres (1214 hectares) of mangrove forest, defoliated nearly 100% of the upper
canopy of the trees, and added another several inches of sediment to the forest floor,
doing significant damage to forests that had begun to recover from Andrew. For
many of the mangroves, this sediment supply smothered the above ground root
structures (prop roots and pneumatophores), so they could not absorb oxygen,
which killed the trees.
A picture began to emerge as these researchers combined field monitoring and
aerial imagery to understand how a succession of hurricane strikes to the same area
could impact the mangrove forests and coastline. The repeated strikes were causing
the shoreline to retreat inland in some areas by a process of “overstepping” the
coastal berms. When winds knock over the mangrove trees the environment of the
forest floor changes significantly. The loss of the protective cover of the tree canopy
changes the soil temperatures and the fallen trees create depressions where the roots
had been. These changes to the forest floor affect the ability of mangrove seedlings
to take root. In addition, the absence of trees means the amount of organic matter
12 Climate, Sea Level, and People – Changing South Florida’s Mangrove Coast
and other plants, and the area just behind the mangroves had lost 2.2 inches (5.6 cm)
in elevation. Large piles of seagrass were all around the edges of the island. On the
southwestern corner of the island where the storm surge was most likely highest, we
found thick (2–3 inches = 5.1–7.6 cm) deposits of bay mud, with seagrass mixed
into the mud.
This is the yin and yang of hurricanes. They tear things down and erode, but they
also build up islands and shorelines by depositing sediments. Mangrove shorelines
have been shaped by hurricanes for millennia because both occur in the tropical to
sub-tropical regions of the world. The question is which hurricane impacts – the
constructive or destructive – dominate over time and what is the net effect of these
opposing processes? Were the mangroves dead or had they just lost their leaves?
And if the mangroves were dead, did this loss of the trees outweigh the beneficial
effects of sediment deposition? Finally, most important to our research, could these
processes explain the coastal inundation of south Florida 3500 years ago?
Just as the present can be the key to the past, the past can also be the key to the
present. To understand what the longer-term effects of Hurricane Irma might be on
the islands, we looked at records of hurricane impacts to the mangrove coasts of
south Florida over the last century. Hurricanes strike south Florida every 7–8 years
on average, and a major storm occurs every 14–19 years (based on NOAA data
gathered since 1886). Within a 90-mile (145 km) radius of Florida Bay, 21 major
hurricanes (category 3 and above) have made landfall in the last 130 years. Records
from earlier storms (Labor Day Storm in 1935, Donna in 1960, Betsy in 1965)
describe similar impacts to what we saw on the islands following Hurricane Irma.
However, few quantitative studies were done, and satellite imagery was not available until after 1972, so we cannot make any direct comparisons between these
older storms and Irma. Extensive research, however, was done by a number of people following Hurricanes Andrew (1992) and Wilma (2005) on impacts to the southwest Florida coastal mangroves. Andrew destroyed tens of thousands of acres of
mangrove forest in Everglades National Park and deposited an average of 3 inches
(7.6 cm) of mud across 40 square miles (104 sq. km) of forest. Wilma destroyed
3000 acres (1214 hectares) of mangrove forest, defoliated nearly 100% of the upper
canopy of the trees, and added another several inches of sediment to the forest floor,
doing significant damage to forests that had begun to recover from Andrew. For
many of the mangroves, this sediment supply smothered the above ground root
structures (prop roots and pneumatophores), so they could not absorb oxygen,
which killed the trees.
A picture began to emerge as these researchers combined field monitoring and
aerial imagery to understand how a succession of hurricane strikes to the same area
could impact the mangrove forests and coastline. The repeated strikes were causing
the shoreline to retreat inland in some areas by a process of “overstepping” the
coastal berms. When winds knock over the mangrove trees the environment of the
forest floor changes significantly. The loss of the protective cover of the tree canopy
changes the soil temperatures and the fallen trees create depressions where the roots
had been. These changes to the forest floor affect the ability of mangrove seedlings
to take root. In addition, the absence of trees means the amount of organic matter
12 Climate, Sea Level, and People – Changing South Florida’s Mangrove Coast