Mangrove Forests Pushed North By Climate Change
December 31, 2013

Changing Climate Pushes Mangrove Forests North Along Atlantic Coast

redOrbit Staff & Wire Reports - Your Universe Online

Cold-sensitive mangrove forests have been expanding northward along Florida’s Atlantic coast for the last few decades due to extreme weather changes, ecologists reported this week in the Proceedings of the National Academy of Sciences.

The findings, which are based on 28 years of satellite data, offer a new and unique view of the speed and scale on which changes in climate extremes have affected vital ecosystems, the researchers say.

Florida’s Atlantic coast from the Miami area northward gained more than 3,000 acres (1,240 hectares) of mangroves from 1984 to 2011 – all of which occurred north of Palm Beach County. Meanwhile, mangroves doubled in area between Cape Canaveral National Seashore and Saint Augustine during that time.

The researchers attribute the change to a decline in the frequency of killing frosts in northern Florida. Indeed, between the study's first five years and its last five years, nearby Daytona Beach recorded 1.4 fewer days per year when temperatures fell below 28.4 degrees Fahrenheit (-4 degrees Celsius), while the number of killing frosts in southern Florida went unchanged during the study period.

The mangroves' movement up the Florida coast is a striking example of climate change's impact on nature, the study’s authors said. Rising temperatures lead to new patterns of extreme weather, which in turn cause major changes in plant communities, they noted.

Unlike many studies that focus on changes in average temperatures, the current study shows that changes in the frequency of rare, severe events can determine whether landscapes hold their ground or are transformed by climate change.

Study co-author Daniel S. Gruner, professor of entomology at the University of Maryland, said the mangrove forests are now edging out salt marshes.

"This is what we would expect to see happening with climate change, one ecosystem replacing another," he said. "But at this point we don't have enough information to predict what the long term consequences will be,” said Gruner, who co-leads an interdisciplinary research project on mangrove ecosystems.

Kyle Cavanaugh, a Smithsonian postdoctoral research fellow and the study’s lead author, noted the mixed impact of the mangroves’ migration.

"Some people may say this is a good thing, because of the tremendous threats that mangroves face," he said. "But this is not taking place in a vacuum. The mangroves are replacing salt marshes, which have important ecosystem functions and food webs of their own."

Mangrove forests grow in calm, shallow coastal waters throughout the tropics. Salt marshes fill that niche in temperate zones. Both provide vital habitat for wildlife, including endangered species and commercially valuable fish and shellfish.

Some animals use both types of habitat, while others, like marsh-nesting seaside sparrows or the honeybees that produce mangrove honey, rely on one or the other.

Both provide valuable ecosystem services, buffering floods, storing atmospheric carbon and building soils, and both are in decline nationally and globally. Mangrove forests are cut down for charcoal production, aquaculture and urbanization or lose habitat to drainage projects, while salt marshes are threatened by drainage, polluted runoff and rising sea levels, the researchers said.

Florida naturalists noticed that mangroves now grow in places that once were too cold for such tropical trees. "We knew this was happening, but no one knew if it was a local or a regional phenomenon," Cavanaugh said.

The researchers used photographs of Florida's Atlantic coast taken by Landsat 5, a satellite launched by NASA and the US Geological Survey in 1984 that kept working until 2011, to track changes in the Earth's land cover.

"It very quickly became a gold standard to examine the effects of climate change, because it lets you look back in time," said Cavanaugh, an expert in remote sensing.

The satellite images revealed the mangroves' expansion into terrain formerly inhabited by salt marsh plants.

Although the study only looked at the Atlantic Coast, the same trend is taking place on Florida's Gulf Coast, with mean winter temperatures have risen at seven of eight coastal weather stations in the study area, the researchers said.

However, if overall warming benefited mangroves, the mangrove cover should have increased all over Florida, not merely in the north. Furthermore, average winter temperature, rainfall, and urban or agricultural land use did not explain the mangroves' expansion.

The researchers said that only fewer freezing days at the northern end of their range matched the trend.

"There's an enormous amount of uncertainty as to what these changes mean," Cavanaugh said.

For now, it is apparent that changes that are well underway in Florida's climate have seemingly led to significant changes along hundreds of miles of coastline.

The researchers are now studying the effects of weather changes on coastal insects and birds, and whether coastal ecosystems' ability to store carbon will be affected. They are also looking at whether juvenile fish and commercially valuable shellfish will remain abundant in the changing plant communities.

Cavanaugh is also analyzing Landsat 5 imagery for Mexico, Peru, Brazil, Australia and New Zealand to see if mangroves are expanding elsewhere as they are in Florida.