Finding a level playing field for mangroves

Aerial view of a vast mangrove ecosystem in Bintuni Bay, West Papua showing colonization of Avicinea marina in the mudflat Photo credit: Daniel Murdiyarso
Aerial view of a vast mangrove ecosystem in Bintuni Bay, West Papua showing colonization of Avicinea marina in the mudflat. (Photo credit: Daniel Murdiyarso)

BOGOR, Indonesia – To prepare for a rise in sea level, you should surely first know where sea level is. The dense mangrove forests around many of Asia’s coasts appear flat, but there is an invisible gradient hidden in them.

As you move landward from the sea, the amount of salt dissolved in the water decreases. The waters become less saline and more brackish, as seawater increasingly mixes with fresh water from rivers and other inland sources.

Change the sea level by even centimetres, and that hidden gradient of salinity will be immensely altered.

Mangrove plants are rooted in this water. They are highly adapted to the salinity and frequency of inundation by sea water from tides. There are dozens of species with various coping mechanisms.

Many have something unique to help them. These are specialised aerial roots which extract oxygen from the air. These ‘pneumatophore pumps’ enable them to grow in ‘anoxic’ sediments, where oxygen is scarce or absent.

The plants position themselves on the fringes or in the interior, according to their adaptive capacity. Some cannot survive if the water is too salty, or not salty enough.

The forest plays a key ecological role. The root structure serves as a sediment trap and protects the interior from heavy waves, storm surges. It may also protect it from future sea level rises.

If sea levels rise, to do anything meaningful to tackle the results, we need to know in some detail where sea level actually is, or was. We also need to know how mangroves play their roles, and react to tides and moving sediments. It may not be at all obvious.

At the Center for International Forestry Research (CIFOR), we have initiated a project that could provide new data on this, initially for Southeast Asia. It is not expensive in research terms — around $75,000.

It shows that research into ‘adaptation’ for the effects of climate change — longer-term measures that will prevent damage repeatedly occurring — does not need to be costly. Furthermore, it can be carried out alongside and in tandem with research into ‘mitigation’ — fixing effects that are already seen. It has potentially huge long-term benefits.

So what has this $75,000 gone into? A very simple idea. Essentially, it is a network of buried posts.

They are being placed at key locations around the coasts of Southeast Asia, from Myanmar to Indonesia’s eastern province of Papua. In the end there should be around 40. So far 15 have been set up.

Aligned in a row of three, they are set up in the fringe, transition, and interior zones of the shore. On top of each is a piece of equipment called a Rod Surface Elevation Table, or ‘rSET’ with an array of pins to monitor surface elevation changes over time. A separate rod is needed to continuously record sea level, by which we can compare the rates of sedimentation and anticipated rise of sea level.

: Rod Surface Elevation Tables (rSETs) used to monitor surface elevation changes can help us to better understand the role that mangroves play against rising sea levels. Photo credit: Hanggar Prasetyo/Udayana University
Rod Surface Elevation Tables (rSETs) used to monitor surface elevation changes can help us to better understand the role that mangroves play against rising sea levels. Photo credit: Hanggar Prasetyo/Udayana University

It sounds deceptively simple. Actually placing one line of these posts is a full day’s work for a team of around 16 people. It needs four outrigger canoes to carry team and equipment to the site. To get there at the right time, depending on the tides, we might need to leave at 4am. Then, once there, we are at the mercy of the tides, so we need to know high and low tide times before we leave.

The post needs to go down a long way. The first few meters are not usually difficult – it is just passing through a soft layer of sediment. But to make sure its data is truly reliable, it needs to be drilled into the bedrock so it cannot move. An initial set of readings also needs to be taken once it is set up.

Lastly, it will need to be manually monitored every few months to check the levels. It is not the most exciting of work but crucially important.

In this way, we will know if soil accretion rates in mangrove forests are keeping pace with any sea level rise. We should get a much more complete picture of the role mangroves may play in protecting us when that sea level rise comes.

Written by: Dr. Daniel Murdiyarso
Affiliation: Center for International Forestry Research
Country: Indonesia


This post is entry #22 in the #IUFRO2014 Blog Competition. The most popular entry will receive a certificate and 500 USD. The second and third most popular entries will receive a certificate and copy of the new book, “Forests and Globalization: Challenges and Opportunities for Sustainable Development”.

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