Safety in the trees?

A couple of weeks ago I touched on the fact that a common way of coping will sea level rise was by emulating or recreating natural barriers - both NASA and Tuvalu are on the case. So I wanted to use this post to investigate another natural barrier that has been used to mitigate the effects of sea level rise - mangroves.

Mangrove forests in Florida (NYTimes)

Mangroves are really great because they protect coastlines from erosion by dissipating energy from storms and waves, and because their complex root systems trap sediment, and so help the soil build up increasing elevation (Kumara et al, 2010). Mangroves can even use this accretion of sediments to survive small increases in sea level, protecting land further away from the coast from sea level rise at the same time (e.g.McKee et al, 2007). Because of this, mangroves have been lauded as a great solution to sea level rise, particularly in the tropics and developing world, where the expensive building of infrastructure techniques are just not an option.  


Kumara et al (2010) compared the sediment accumulation rates and increased elevation for four specifically planted mangrove sites in Sri Lanka with varying densities. They showed that higher density mangrove forests promoted sediment accumulation and elevation rise, and suggest that high density mangroves are a suitable coastal defence . However, this study was only carried out on one species over three years, so it is worth bearing in mind that the results may not be representative of other species or mangrove behaviour as the trees start to age.


The World Resources Institute recently released a report which included a case study on the use of mangroves to combat sea level rise in Vietnam. They point out that there has been varying success with the government enforced rehabilitation programme. In the north, where the aim was only to protect from sea level rise and reintroducing mangroves has put people out of work because they can no longer reach the sea so easily. Meanwhile in the south using mangroves to mitigate against sea level has been coupled with building of infrastructure including schools. This has been more successful, and provided a range of benefits. They suggest that as long as mangroves are incorporated into a more wide reaching development plan, they can be used successfully to protect from sea level rise (Powell et al, 2014).


However, it's not as simple as planting a mangrove forest by the coast, and leaving it to sort out the rising sea level. Research on the stratigraphic record of mangroves in the Bermuda suggests that they can cope with sea level rise at rates of up to 9 cm every 100 years (Ellison, 1993). If sea level rises faster than the rate of sediment accumulation, mangroves can't keep up and so retreat inland or die (Kumara et al, 2010).


So mangroves clearly do have an important role in protecting the coasts, however for areas with rapid sea level they cannot be the only form of protection, and should not be used as a cheap alternative. Regardless of the rate of sea level rise they are still very important for dissipating wave energy, preventing erosion,  and may have an important role in protecting coastal communities from extreme events such as tsunamis (Dahdouh-Guebas et al, 2005), so certainly planting mangroves is not going to do any harm on the coastal protection front.