This blog is part of the series ‘What the resilience of the Whitsundays can teach the world at the beginning of the hurricane season.’
A labyrinth of nature, both a scary looking forest of a fairy tale as a protective and genius looking dense network of flying roots and green branches. Flourishing at the boarder of ocean and land, mangroves are as a gift of heaven for nature, and for us.
The mangroves as well, suffered from the cyclone, yet survived and surrendered. Mangroves are after the coral reefs at the frontline of climate change; here they can protect the coastline against erosion, rising sea levels, storm wells and waves. Here they are the guardians of the coast, yet too underestimated.
Mangroves can help humanity in many different ways, I’ll give you their main functions on a row:
- Protection against waves
When wind and swell waves pass through mangroves (before reaching land), they loose energy by the rate of 13 to 66% over 100, of mangroves according to research of the Nature Conservancy and Wetlands International. In addition, mangroves reduce winds across the surface of the water, preventing the reformation of waves. As a result, when these waves hit land, they are reduced in height and energy, causing far less erosion and damage.
The denser the forest, especially the aerial roots of the mangroves, the more rapidly waves will be reduced. Rising sea levels due to tides or climate change, might cause the waves to pass over the roots, yet they can still be captured by the low branches of the mangroves. Hence, the complexer the aerial roots and low branches, the better weapened against waves. Mangrove trees do not necessary have to be old and solid to fulfill these tasks, therefore reforested areas can become an effective wave breaker in just a couple of years.
- Protection against storms
Besides tidal waves, tropical storms – including cyclones, hurricanes or typhoons – can cause large waves, combined with raised sea level (storm surge), strong winds and torrential rainfall, which all can result in widespread flooding.
Mangroves can be the preferred protective measure by (1) reducing the impact of waves, (2) reducing the storm surges by a ratio of 5 to 50cm per km width of mangroves, and (3) by trapping moving objects which were taken by the flood, (4) locally reducing high wind speeds, which might prevent further development of wind and swell waves in and immediately behind the mangroves. Moreover, the study mentions that mangroves might be damaged by the flood theirselves, yet breakage or uprooting of mangrove trees is apparently relatively rare.
- Protection against tsunamis
Take waves in the extreme and you end up with tsunamis. These guys are the ones that really challenge the wave reducing ability of mangroves. Mangrove belts of several hundred meters wide have been shown to reduce tsunami height by between 5 and 30%. The study of the Nature Conservancy and Wetlands International says that there is growing evidence, that mangroves reduced the impact of the tsunami in case of the tsunami in South East Asia of 2004, which killed 230,000 people. After this tsunami, a shared project initiated the Grean Coast project (2005 to 2008) to restore the mangroves, resulting in 2,000 hectares of restored mangroves in Indonesia among others.
The wider and the denser the forest vegetation to the better. Yet, the ability goes up to tsunamis of 4m, after which magnroves theirselves might be destroyed. Althought, this might be called a worse-case-scenario since the same remark can be made for man-made wave breakers, which seldonly exceed the height of mangroves.
In the event of a tsunami, mangroves can capture floating debris as well, reducing indirect storm damage.
- Protection against coastal erosion
Another protective ability of mangroves is the one against coastal erosion, a growing problem in a changing climate, with increasing storms and rising sea levels as a result. Waves and run-off water cause sediments to be removed from the shore and flow into the ocean, which result in a retreating shoreline. Yet, here is where the mangroves come in. With their network of dense roots, mangroves can bind and build soil. While the aerial roots slow down water flows above ground, they encourage deposition of sediments, hence reducing erosion. At the same time, the underground roots capture and bind soil and sediments together. As a result, typical mangrove soils are rich in organic matter produced by the mangroves. In addition, much of this organic matter accumulates because the mangroves are often waterlogged, meaning low oxygen content, which results in its turn in a thikening layer of peat.
This soil-forming and -binding ability is treathened when mangroves are lost due to deforestation. The stabel layers of peat and mud will start to erode, resulting in the disappearance of land into the sea. Especially in regions such as the Gulf ot ahiland, northern Java, and in Guyana, the conversion of mangroves into agriculture has resulted in the reatreating coasts at rates of several meters a year.
- Protection against rising sea level
The same ability to protect coasts from erosion, shows how mangroves eventually can protect against a rising sea level as well. Since mangroves actively build up soils over time – by capturing and binding sediments and forming own organic material – they can actually increase the height of the mangrove soil surface.
Knowing that recent sea level rise has averaged 3.2 mm per year, and this rate is accelerating and variable across the oceans, it might become urgent that rising coastal soils are more than welcome. Especially since rising sea levels will as well increase the impact of waves, storm surges and erosion.
The study of Nature Conservancy and Wetlands International mentions about mangrove soils grewing vertically at rates of up to 10mm a year in sites from Australia to Belize – hence faster than the rising sea level of 3.2. mm a year. Therefore, the researchers suggest that mangroves may be able to keep up with similar rates of sea level rise, where local conditions allow of course.
- Protection against climate change
Mangroves are not only a useful means in terms of climate change adaptation, but in terms of mitigation as well. Research published in the Journal Hydrobiologia shows how carbon captured in coastal ecosystems, in the soils beneath them – called blue carbon – increased by two-thirds over three years, hence serving as a meaningful carbon sink.
Yet, it is important to maintain the optimal growing circumstances for mangroves, which are
- Brackish water, a mix of salty sea water and fresh water
- A low oxigen containing underwater environment, hence processes such as drainage, deforestiation, extraction of groundwater and oil should be avoided, in order to avoid oxidation, which causes subsidence that counteracts the mangroves growth
By the way, do you know that
- contrary to mangroves, man-made tidal breakers prevent the capture of sediments of rainwater to the ocean, and the sediments brought in by the waves, which are crucial to build up soil.
Unfortunately mangroves are in big troubles globally, which you can read about in next post.