Florida's climate has seemingly led to significant changes along hundreds of miles of coastline. "The expansion isn't happening in a vacuum," says Kyle Cavanaugh. "The mangroves are expanding into and invading salt marsh, which also provides an important habitat for a variety of species." (Credit: "mangrove" via Shutterstock, font by Tyler Finck/FontSquirrel)

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As cold snaps wane, mangroves head north

Prompted by one less hard freeze a year, protected mangrove trees risk becoming an invasive species as they expand northward along the Atlantic coast of Florida.

That surprising finding, reported by a team of ecologists in the Proceedings of the National Academy of Sciences, illustrates the speed and scale on which alterations in climate extremes have affected crucial ecosystems.

Going back 28 years, a resident of Palm Coast could have encountered some mangrove forests. Those same forests are now about 100 percent greater according to researchers’ measurements, which are based on analysis of satellite imagery of the coastal area mangroves.

A very harsh winter in 1989 caused mortality in mangroves and damaged citrus crops. Satellite images show how mangroves rebounded when they were no longer threatened by cold snaps. (Credit: Kyle Cavanaugh/James Kellner)
A very harsh winter in 1989 caused mortality in mangroves and damaged citrus crops. Satellite images show how mangroves rebounded when they were no longer threatened by cold snaps. (Credit: Kyle Cavanaugh/James Kellner)
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Changes along the Florida coast are already well underway, but the effect on the ecosystem—and what it might mean for food webs—is not certain. (Credit: Kyle Cavanaugh/James Kellner)

“Before this work there had been some scattered anecdotal accounts and observations of mangroves appearing in areas where people had not seen them, but they were very local,” says study lead author Kyle Cavanaugh, a postdoctoral researcher at Brown University and at the Smithsonian Institution. “One unique aspect of this work is that we were able to use this incredible time series of large-scale satellite imagery to show that this expansion is a regional phenomenon. It’s a very large-scale change.”

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James Kellner, assistant professor of ecology and evolutionary biology and a senior co-author, adds that the paper goes beyond merely documenting the northward push of mangrove forests by accounting for the most likely cause. Cavanaugh and his colleagues tested various hypotheses by correlating the satellite observations with reams of other data. What emerged from their tests of statistical significance was the area’s decline in the frequency of days where temperature dips below minus 4 C or 25 F. That, not coincidentally, is a physiological temperature limit of mangrove survival.

One fewer freeze

“The most intuitive explanation is not the explanation that actually explains this pattern,” says Kellner, who is part of Brown’s Environmental Change Initiative. “The one people would most probably point to is an increase in mean temperature.”

But in the analysis, the team had to rule out increases in mean annual or winter temperatures as well as changes in precipitation and changes in nearby urban and agricultural landcover. They also ruled out sea level rise.

Instead seemingly subtle differences from 1984 through 2011 of just 1.4 fewer days a year below 25 F in Daytona Beach or 1.2 days a year in Titusville appear to explain as much as a doubling of mangrove habitat in those areas.

Expanding and invading

In a state where mangroves enjoy environmental protections and where the plants are the namesake of streets, parks, businesses, and at least one golf course, it might appear at first blush that more mangrove habitat could be a good thing. Florida orange growers might be happy to kiss hard freezes goodbye. But Cavanagh and Kellner caution against any celebration of this apparent consequence of climate change.

“The expansion isn’t happening in a vacuum,” Cavanaugh says. “The mangroves are expanding into and invading salt marsh, which also provides an important habitat for a variety of species.”

The next question is to understand how these changes affect the lives and interactions of the species in each ecosystem.

“There’s an enormous amount of uncertainty as to what these changes mean for the food webs,” Cavanaugh says. For now, what’s apparent is that changes well underway in Florida’s climate have seemingly led to significant changes along hundreds of miles of coastline.

In addition to Cavanaugh and Kellner, the paper’s other authors are Alexander Forde and Daniel Gruner of the University of Maryland, and John Parker, Wilfrid Rodriguez, and Ilka Feller of the Smithsonian.

The NASA and the National Science Foundation supported the research.

Source: Brown University

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