Mangroves migrating northward

Much of the Florida shoreline was once too cold for the tropical trees called mangroves, but the plants are now spreading northward at a rapid clip, scientists reported Monday. That finding is the latest indication that global warming, though still in its early stages, is already leading to ecological changes so large they can be seen from space.

Along a 50-mile stretch of the central Florida coast south of St. Augustine, the amount of mangrove forest doubled between 1984 and 2011, the scientists found after analyzing satellite images. They said the hard winter freezes that once kept mangroves in check had essentially disappeared in that region, allowing the plants to displace marsh grasses that are more tolerant of cold weather.

In one respect, the situation resembles the change in climate that has allowed beetles to ravage millions of acres of pine trees in the U.S. West and Canada, and more recently to gain a foothold in New Jersey.

In both the beetle and mangrove cases, scientists have found that it is not the small rise in average temperatures that matters, nor the increase in heat waves. Rather, it is the disappearance of bitter winter nights that once controlled the growth of cold-sensitive organisms.

"I think this idea of tipping points in the Earth's ecosystem is absolutely critical," said Kyle C. Cavanaugh, a researcher at the Smithsonian Environmental Research Center in Edgewater, Md., who led the research for the new paper, released on Monday by the journal Proceedings of the National Academy of Sciences. "The changes in temperature can be pretty small, but once you cross a threshold, you can get rather dramatic changes in the ecosystem."

Though scientists have long warned of the potential environmental consequences of unchecked global warming, the pace and scale of some recent developments have surprised them, given that the Earth has warmed by only about 1.5 degrees Fahrenheit since the 19th century. It is expected to warm substantially more than that over the coming century. Yet already, Cavanaugh said, "the changes are happening faster than we expected."

The northward spread of mangroves poses a more complicated set of ecological questions, however, than some other changes linked to global warming, such as the deaths of pine forests or coral reefs.

The mangrove forests that fringe shorelines in the tropics are among the earth's environmental treasures, serving as spawning grounds and nurseries for fish and as habitat for a wide Yet in many places, mangroves are critically endangered by shoreline development and other human activities.

So a climatic change that allows mangroves to thrive in new areas might well be seen as a happy development. Yet as they spread in Florida and elsewhere, the mangroves are displacing salt marshes, which are also ecologically valuable and also under threat from development.

Their ecology is markedly different from that of mangroves, raising new questions about what will be lost if marsh grasses are killed off by the invading trees.

"We can't put a price tag or a value on what is happening," said Daniel S. Gruner, a biologist at the University of Maryland who took part in the research. "We're not saying it's good or bad. It's just what the data show."

For years, scientists working in Florida had been noticing that mangroves seemed to be creeping northward along the coast. The new study is the first to offer a precise quantification of the change, using imagery from a satellite called Landsat, and to link it to shifts in the climate.

Historically, mangroves dominated the Florida shoreline south of Cocoa Beach, and salt marsh dominated north of St. Augustine. Along the 130-mile stretch between the two cities, mangroves and salt marsh competed for control of the narrow coastal strip where fresh water and salt water mix, with periodic cold snaps apparently tipping the balance in favor of the marsh grasses.

The study shows that lately, though, the mangroves have been winning. In a zone of 24,000 acres capable of hosting either type of plant, mangroves took over some 4,200 acres from 1984 to 2011, the researchers found, with the most dramatic gains at the northern end of the range.

The scientists pursued several explanations, including sea level rise and average temperature changes, and none of them panned out -- until they looked at the change in winter cold extremes.

The evidence suggested that cold snaps of 25 degrees Fahrenheit or below would kill off mangrove seedlings, a finding that was confirmed in laboratory research.

Records from weather stations in the coastal towns of Titusville, Vero Beach and Fort Pierce showed that it got that cold several times in the 1980s.

But the last bitter freeze in central Florida occurred in 1989, and cold sufficient to kill off mangrove seedlings has not occurred at all in recent years.