The mangrove forests that line much of Florida’s eastern coast are moving north because of climate change, scientists led by the Smithsonian Environmental Research Center report December 30 in the Proceedings of the National Academy of Sciences. The migration isn’t because of rising average temperatures, however. Instead, it’s driven by a reduction in the number of extremely cold days.

Mangroves are a group of about 80 different trees and shrubs that grow in tropical regions around the world. They’re usually characterized by their roots, which grow partially above the ground, giving the plants the appearance that they’re held up by a tangle of stilts. That nest of roots holds the trees above the water, which rises and falls with the tides.

Mangrove ecosystems are complex, providing homes and food for a diverse array of species both on land and in the water. And they’re valuable economically--mangrove forests provide an array of services worth some $1.6 trillion per year, including serving as nurseries for commercially important fish, protecting coasts from destructive storm waves and providing places for recreation.

From a human standpoint, though, coastal land is often valuable for other reasons that don’t mesh well with a tangled forest (such as for beachfront homes), and mangroves have been disappearing faster than even tropical rainforests. At least 35 percent of the world’s mangrove forests have been destroyed in the last decade. But destruction hasn’t happened everywhere, and there’s been some evidence of mangroves in Florida beginning to encroach upon another valuable ecosystem—salt marshes.

“The expansion isn't happening in a vacuum,” the PNAS study’s lead author Kyle Cavanaugh, a Smithsonian postdoctoral researcher, said in a statement. “The mangroves are expanding into and invading salt marsh, which also provides an important habitat for a variety of species.”

To figure out what’s been driving that expansion, Cavanaugh and the other researchers began with a set of Landsat satellite images spanning a 28-year period, from 1984 to 2011. Historically, the northern limit at which the tropical mangrove species found in Florida—mostly red mangrove (Rhizophora mangle), black mangrove (Avicennia germinans) and white mangrove (Laguncularia racemosa)—can grow is approximately the 30°N line of latitude, just north of St. Augustine. Scientists have thought that this limit is set by temperature.

But between 1984 and 2011, the extent of mangroves near that northern limit grew, and they doubled in space on the coast between 29°N and 29.75°N. The researchers looked at a variety of factors that might explain the range expansion, comparing changes in mangrove area to several temperature measures as well as changes in precipitation and land use. Average annual temperatures and average winter temperatures rose at most of the weather stations in that area, but analysis of satellite data showed that the mangrove movement didn't correlate with either factor. Nor was it associated with changes in precipitation, agricultural or urban runoff or patterns in the tides. The important factor was the number of days in which the temperature was lower than -4° Celsius, the team discovered.

“We found a strong relationship between increases in mangrove area and decreases in frequency of extreme cold events,” the researchers write

It seems that for mangroves, -4°C is a threshold. If a winter gets colder than that, then a mangrove plant can’t survive until spring. But climate change is altering the frequency of freezing events, pushing regions at the northern limit of the mangroves’ range from being unfriendly to the plants’ winter survival into being mangrove friendly. And that’s allowing mangrove seeds to take root in salt marshes, make it through the winter and replace marsh with forest.

“This is what we would expect to see happening with climate change, one ecosystem replacing another,” study co-author Daniel S. Gruner, of the University of Maryland College Park, said in a statement. “But at this point we don't have enough information to predict what the long-term consequences will be.”

Climate change may be allowing mangroves to extend their range north, but it isn’t all good news for the plants. Rising sea levels are expected to threaten some 10 to 20 percent of the world’s mangroves by 2100, and precipitation decreases and temperature increases may make survival difficult for mangroves in arid regions. And even if temperature conditions are ripe for mangroves to expand their range, they could be hemmed in by coastal development.