The forests “both saw higher damages as wind speed increased, but the magnitudes of those damages between Florida and Puerto Rico differ greatly,” said Vivian Griffey, lead researcher and a master’s student at the University of Washington in Seattle. “The degree to which those damages occur within the given wind speed classes differs between the different areas.”
Not only did Puerto Rican mangroves lose more height on average, but there was also more variation in height loss in Puerto Rico than in Florida, she said. This variation got the researchers thinking that hurricane wind speed was not the only factor at play. Griffey will present this research on 13 December at AGU’s Fall Meeting 2019 in San Francisco, Calif.
From the Top
“Mangroves provide a number of different ecosystem services,” Griffey said. “One of the big ones that especially is important in Puerto Rico is their ability to buffer against hurricane storm surges.” Mangroves also store a lot of carbon per area and are a nursery habitat for tropical fish species. “They’re doing a lot of work for us,” she said.
In 2017, three major hurricanes struck the Caribbean and southeastern United States, causing damage from which residents are still recovering. The storms, particularly Maria and Irma, also damaged coastal ecosystems including saltwater mangrove forests.
The researchers wanted to map the damage done to the mangrove forests in each area: The two regions have the same species of mangroves and were struck by strong hurricanes around the same time. The team used a NASA satellite, Goddard’s Lidar, Hyperspectral and Thermal Imager (G-LiHT), to measure the heights of the mangrove trees in southern Florida and in several locations in Puerto Rico before and after the hurricanes.
“On average, there are far more damages in Puerto Rico than in Florida,” Griffey said. “In Florida we saw, on average, an 11% height loss. Whereas in Puerto Rico we saw, on average, a 33% height loss, which extended up to about 65% in some of the sites. Whereas in Florida, the largest percentage loss that we saw was only 17%.”
However, one thing that stood out as strange in the lidar data was that Irma and Maria had similar ranges of and maximum wind speeds, but the mangrove damage was very different between the two locations. The team also noticed a pattern to the damage in Puerto Rico.
“We saw the highest losses in Humacao, which is just north of where the storm made landfall,” Griffey said. “And then Jobos Bay saw the smallest amount of loss and the smallest range of loss as well.” Humacao is on the eastern coast of the island, and Jobos Bay is on the southwestern coast.
The researchers suspect that a combination of factors led to this pattern: Maria’s southeast-to-northwest trajectory, the counterclockwise rotation of northern hurricanes, and Puerto Rico’s mountainous central terrain.
“There’s very complex mountain topography in Puerto Rico,” Griffey said. “The way the winds interacted with that could potentially cause these patterns of damage we’re seeing on the eastern side.”
Mountains, Storm Surge, and Mangroves
Doug Morton, principal investigator on the project and chief of the Biospheric Sciences Laboratory at NASA’s Goddard Space Flight Center in Greenbelt, Md., said that storm history might be another factor that contributed to the different mangrove damages.
“Florida historically has seen more regular storm action, and Puerto Rico has had infrequent but very strong hurricanes that have impacted the island,” he said. “And so one of the curiosities—not finding the similar relationship between wind speed, tree height, and storm damage—may be that legacy effect or lack of recent storms making mangrove forests more vulnerable to damages from 2017 storms.”
There’s also the impact of storm surges to consider, Griffey said. Puerto Rico’s mountains mean that mangroves form a ring around the island, whereas Florida’s mangroves extend farther inland. Likewise, storm surges can’t propagate inland in Puerto Rico like they can in Florida. Although the storm surge buffer is beneficial for Puerto Ricans and inland infrastructure, it leaves the coastal mangroves exposed to damaging winds.“One of the patterns that pops right out in the poststorm data is this green ring around the smaller islands of mangroves in southwest Florida,” Morton said. “And those are shorter trees that were submerged during the storm. They were not only protected from damage, they actually retained their leaves, where many other parts of the coast of Florida and Puerto Rico had their leaves completely stripped.”
The team is continuing to explore the extent to which the regions’ geomorphologies, storm surge patterns, and hurricane histories may have factored in to mangrove damage. Those causes and the patterns of damage they left behind will be key to figuring out how the mangroves will recover.
“You see [some] mangroves recovering in 3-4 years,” Griffey said. “So it’s not all doom and gloom. They may be bad at resisting it, but they may be good at coming back from it. And that we don’t really know yet.”