Florida’s mangroves expand northward

Just north of St. Augustine, around a latitude of 30° N, lies the stretch of Florida’s east coast where the littoral vegetation switches from the mangrove forests of the south to the salt marshes of the north. Stubby, salt-tolerant mangrove trees thrive in warm climates. As Earth’s mean temperature rises, Florida’s mangroves should expand northward, says Kyle Cavanaugh of the Smithsonian Environmental Research Center in Edgewater, Maryland, and Brown University in Providence, Rhode Island. He and his collaborators analyzed 28 years of Landsat images of Florida’s east coast and found that mangroves are indeed pushing northward—but in a surprising way. South of 27° N, the area covered by mangroves remained the same during the study’s 1984–2011 span. North of that latitude, however, mangrove coverage grew—at a rate that increased with latitude. In the study’s northernmost zone, 29°–29.75° N, the coverage doubled. Locally, mean annual and mean winter temperatures have risen throughout Florida since 1984, but neither trend correlated with the expansion’s latitude dependence. Of all the environmental factors that Cavanaugh and his colleagues investigated, the only one to yield a strong correlation was a threshold: the annual change in the number of days with minimum temperatures below −4 °C. Such days are becoming rarer throughout Florida, but their frequency is falling fastest in the northern part of the state. Thanks to their deep roots, mangroves protect fragile coasts. Though benign, the dramatic expansion of Florida’s mangroves underlines the importance of identifying thresholds in other ecosystems that could trigger a rapid, possibly disastrous, response to climate change, the study’s authors say. (K. C. Cavanaugh et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.1315800111 .)