The mangroves of Florida are on the move. Mangrove forests in the north of the state have doubled in area in the last 28 years, thanks not to global warming as such, but because the number of sharply frosty days has dropped.
The discovery is in itself not a surprise – mangrove growth is limited by temperature – but once again it confirms a pattern of climate change and species migration in response to man-made global warming.
Kyle Cavanaugh and colleagues report in the Proceedings of the National Academy of Sciences that they examined 28 years of satellite imagery over one particular stretch of an important ecosystem on the coast of northern Florida between 1984 and 2011. All the increase occurred north of Palm Beach County, and growth in the area between Cape Canaveral National Seashore and St Augustine doubled.
“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”, said Dr Cavanaugh, a researcher at the Smithsonian Institution in the US.
Simple pattern
“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 the expansion is a regional phenomenon. It’s a very large-scale change.”
The other important aspect of the research was that it eliminated some of the other possible causes behind the expansion of mangroves into the salt marshes a little to the north.
The pattern of change over the 28-year period, in which the area colonised by mangroves grew by another 1,240 hectares, was not a consequence of a shift in average temperatures, or of the change in management patterns, or an altered pattern of precipitation, or changes in the tidal regimes, or even altered rates of nutrient flow and sedimentation.
What emerged from the painstaking match of seasonal weather and mangrove advance was quite simple. With a decline in the number of days in the year in which a sharp frost occurred, and in which the temperatures fell to minus 4°C or below, the mangrove reach began to extend.
Salt marsh loses
It was not the overall warmth, but the change in the frequency of severe events that mattered most. And this change in frequency was surprisingly slight: just 1.4 fewer frosty days a year at Daytona Beach seemed to be enough to account for the expansion.
In the region overall, the scientists write, minimum temperatures have been rising faster than either the daily mean temperature, or the daily maximum. The implication is that stands of the mangrove genus Avicenniawill go on colonising new habitat.
Mangroves are important ecosystems for both wildlife and for coastal citizens, and worldwide, mangrove forests have been under threat. But even a welcome change may not be entirely for the good.
“The expansion isn’t happening in a vacuum”, said Dr Cavanaugh. “The mangroves are expanding into and invading salt marsh, which also provides an important habitat for a variety of species.”
