- Mangroves are hugely valuable, both ecologically and economically, but many are highly threatened by clearing and rising sea levels.
- This new study finds some mangroves may be able to keep pace with rising seas because of influxes of sediment – but many more may not, and may become submerged in the coming decades.
- The the authors say it’s not too late to turn things around in some areas.
Living at the interface of land and sea, many of the world’s mangroves are under heavy threat as sea levels rise from global warming. Now, new research published in Nature finds that a big part of that threat is due to inland activities like dam construction and deforestation. It warns that such activities may submerge many Indo-Pacific mangroves by 2070.
Mangroves are coastal forests comprised of trees that can survive living in and near brackish water. They are hugely valuable, both ecologically and economically, serving as nurseries for many fish species, protection of shoreline from storms, and carbon storehouses.
“Intertidal mangrove forests occur on tropical and subtropical shorelines, and provide a wide range of ecosystem services – to fisheries, in coastal protection and in carbon sequestration – with an estimated value of $USD194,000 per hectare per year,” said Catherine Lovelock, a University of Queensland ecologist and coauthor of the new study.
But many mangroves are also hugely threatened and are being cleared at three to five times the rate of terrestrial forests, according to a report from the United Nations Environment Program. In total, the report pegged the economic loss from this clearing at $42 billion per year.
For this new analysis, researchers with institutions around the world looked at recent trends in the mangroves of the Indo-Pacific – a vast area comprising the seas off eastern Africa, Southeast Asia, Australia, and Pacific islands from Hawaii to French Polynesia. They examined the amount of sediment that is replenished in mangroves, which effectively changes the elevation of the mangrove as it accumulates. This sediment helps buoy up mangroves in the face of rising seas, and its nutrients are vital for continued mangrove growth.
The analysis resulted in good news for mangroves in some parts of the study area.
“Our modelling shows mangroves are likely to persist in east Africa, the Bay of Bengal, eastern Borneo and north-western Australia – areas where there are relatively large tidal ranges and/or higher sediment supply,” Lovelock said.
Lovelock said the sediment supply was so high in these areas that it could help mangroves there “keep pace with sea-level rise.”
But for other areas, the prognosis is grimmer. The researchers found that of all the mangroves they studied in the Indo-Pacific, 69 percent had rates of sediment accumulation that were not enough to keep them above expected sea-level rises.
One of the main reasons for this, according to the authors, is that anthropogenic activity inland from mangroves is stymying the natural flow of sediment down to the coast. So not only is human-caused global warming making the ocean rise, human activity is also starving mangroves of the soil and nutrients they need to keep up with it.
“The Indo-Pacific region holds most of the world’s mangrove forests, but sediment delivery in this region is declining, due to activities such as dam construction,” she said. “This is of particular concern as this region is expected to have variable but high rates of future sea-level rise.”
The study also includes degradation of inland forests as a major driver of reduced sediment quality, resulting in less organic matter for mangroves.
The authors point to the Chao Phraya River delta in Thailand, where an 80 percent reduction in the sediment supply has, together with groundwater pumping, “resulted in kilometers of mangrove shoreline retreat.” They also warn of impending damage to the mangroves of the Mekong delta in Vietnam due to upstream development.
But Lovelock and her colleagues offer some hope, saying that it’s not too late to turn things around in many of the areas they studied. They urge the maintenance of sediment supplies in areas most likely to be affected by dams and forest degradation, and planning for inland migration of mangroves as they try to retreat from rising seas.
“Appreciation of the financial contribution of mangroves has been slowing the trend of decline,” Lovelock and coauthors Neil Saintilan and Kerrylee Rogers write in an accompanying article in The Conversation. “However, long-term survival will require planning that includes both the continued provision of sediment supply, and in many cases the provision of retreat pathways, to allow mangroves to respond to sea level in ways they always have.”
- Lovelock, C. E., Cahoon, D. R., Friess, D. A., Guntenspergen, G. R., Krauss, K. W., Reef, R., … & Triet, T. (2015). The vulnerability of Indo-Pacific mangrove forests to sea-level rise. Nature.