Mangroves, salt-tolerant trees with intertwining branches, have been found to be expanding from tropical zones into more temperate areas. These trees line coastlines and prefer warmer temperatures, and so have traditionally been restricted to subtropical and tropical environments, however recently scientists are finding them at higher latitudes across the globe.
Mangroves can disperse via water, releasing propagules (similar to seeds) which are distributed by the ocean currents. The Kennedy Space Centre in Florida has found that mangroves have increased in abundance by 70% in the past 7 years over an area of 567 sq km (220 sq miles). A study from Florida also shows that northern mangrove populations may be releasing bigger propagules earlier than normal, helping them to take over salt marshes and avoid winter freeze events.
Mangrove expansion may have some benefits, including storm surge protection and increased carbon storage. A modeling effort found that a strip of mangroves 2m wide can reduce wave height by 90%, whereas it would take 20m of salt marsh habitat to reduce waves by the same amount. This protection is significant in reducing shoreline damage during storms and catastrophic events such as the 2004 Indian Ocean tsunami and Tropical Storm Wilma in Belize in 2005, which may become even more relevant as large storms become more frequent due to climate change. Mangrove expansion could also increase carbon storage along coastlines as they can store double the amount of carbon per-area as salt marshes. It is estimated that mangrove carbon storage at the Kennedy Space Centre increased by 25% in 7 years as mangrove forests spread.
One possibly negative impact of this shift is that as mangroves replace salt marshes, which provide important habitats for numerous bird and fish species, it is unclear as to how these animals will adjust in a different environment. Currently, there are ongoing projects to determine how the invasion of mangroves will alter the coastal protection capacity of wetland ecosystems in Florida. A recent study has also shown that many mangrove ecosystems are not building adequate new elevation to keep pace with rising sea levels and so their benefits may not even be realised.