Hurricanes may be limiting the height of mangroves along the U.S. Gulf Coast.
This knowledge could affect the ways that land managers plan for storm mitigation measures because the tropical plants—which have adapted to survive in loose soil and salty water—can buffer coastal infrastructure and shorelines from some of the worst effects of wind and wave surges.
“The coastline is stable when you have mangroves.”
“The coastline is stable when you have mangroves,” said Marcelo Cohen, a paleoclimatologist and paleoecologist at the Federal University of Pará in Brazil and at Louisiana State University.
Cohen has studied mangrove growth around the Americas for years, paying particular attention to how these plants are migrating north and south from their tropical strongholds into more temperate zones.
Researchers had long believed that colder weather was the controlling factor in mangrove height at the northern end of the plants’ range in the Gulf of Mexico. But more recent research in the Everglades and elsewhere in the tropics has shown that hurricanes can also control the height of the mangrove canopy.
Cohen wanted to see whether a similar effect was happening on the Gulf Coast. In a study published recently in Science of the Total Environment, he and his colleagues used data from a combination of lidar sensing and drone-captured photogrammetry imagery of mangrove forests in the northern Gulf Coast. These imaging techniques tracked the height and characteristics of mangroves in Port Fourchon, La., and Cedar Key, Fla., which sit at similar latitudes and have similar winter temperatures, from 2002 to 2023. On average, Cedar Key has been hit by a hurricane every 17 years in the past century. Port Fourchon gets hit more frequently: on average every 5.9 years, including by Katrina in 2005, Zeta in 2020, and Ida in 2021.
Storm Pruning
Cedar Key’s maximum mangrove canopy height was about 12 meters, whereas Port Fourchon’s was around 2.5 meters. Both are much shorter than the 25 meters mangroves can reach in the warmer Everglades to the south.
Analysis of the drone photogrammetry from 2018 to 2023 revealed that hurricanes effectively pruned the mangrove trees at Port Fourchon by breaking branches, stripping leaves, and knocking down the tallest trees.
Mangrove height in Port Fourchon, La., is still recovering from Hurricane Zeta in 2020 and Hurricane Ida in 2021. Shown here are an aerial photo taken in March 2018 (left) and another taken in November 2021 (right). Credit: Marcelo Cohen
“The tops of the trees were broken—[hurricanes] are cleaning the surface,” Cohen said.
The trees that weren’t uprooted grew more slowly after the storms, likely using energy to recover from the damage. In fact, the rate of increase in mean canopy height has still not recovered 3 years after Ida and nearly 4 years after Zeta, Cohen’s team found. Winter freezes in 2014 and 2018 also defoliated mangroves at Port Fourchon, but the trees recovered in less than a year. Meanwhile, the mangroves at Cedar Key grew more steadily.
This shows that although temperature likely provides the upper limit for mangrove heights at Cedar Key, it’s hurricanes that are putting a cap on their growth at Port Fourchon, Cohen said.
“Hurricanes absolutely do structure canopy height.”
“Hurricanes absolutely do structure canopy height,” said Ken Krauss, a research ecologist with the U.S. Geological Survey who was not involved in Cohen’s study. At the same time, he wondered whether other factors might be causing the differences observed between Port Fourchon and Cedar Key.
Although Cohen and his team considered the disparate winter temperatures of the two locations, Krauss noted several other differences. Cedar Key has a greater subsidence rate, and mangroves can colonize the salt marsh more actively than in Port Fourchon, for example. And Cohen and his team also didn’t examine factors such as water temperature, which varies between these two regions.
Still, Krauss has published work showing that cyclones indeed influence mangrove height in tropical regions, and he said it’s possible that this is also occurring in the northern Gulf Coast. In fact, he said the analysis that Cohen and his team did is great, but he’d like to see follow-up that examines other factors to test what he sees still as a “new hypothesis.”
Cohen said this information has implications for authorities who work on storm mitigation measures. Planting a buffer of mangroves is one of various measures that can protect infrastructure farther inland from storm surge and wind damage. But in areas such as Port Fourchon that see more frequent storms, the protective quality of mangroves may be limited.
—Joshua Rapp Learn (@JoshuaLearn1), Science Writer
Citation: Learn, J. R. (2024), Hurricanes may prune Gulf mangroves, Eos, 105, https://doi.org/10.1029/2024EO240284. Published on 8 July 2024.
Text © 2024. The authors. CC BY-NC-ND 3.0Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
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