Mangrove forests cover only a small percentage of the planet in comparison to other forest types – roughly 1.9% of coastline in the world – but they contain the largest source of carbon sequestration per hectare of land and are a major player in the carbon cycle of the oceans.
This means they have high economic value as “blue carbon” – carbon captured in oceans and coastal ecosystems – but around the world they have often been destroyed in the course of coastal development, agricultural and mining activities.
Increasingly, however, they are being acknowledged as formidable carbon sinks. But has this acknowledgement come too late? Does the ongoing loss mean we’ve squandered our best last chance to put the brake on global warming?
Carbon is stored as biomass in the sediment captured through the growth of mangroves. The carbon produced by its decomposing roots alone is a major contributor to this complex sink. The removal of mangroves adds 10% to the total carbon lost from global tropical deforestation through greenhouse gas emissions.
Sebastian Thomas, a lecturer in Sustainability Governance and Leadership at the University of Melbourne, believes that when it comes to calculating its greenhouse emissions target of 26% to 28% below 2005 levels, Australia has “cherry picked” the nicest number in the lot. Despite the shift toward the renewable energy sector, he maintains the target nominated by Australia is “unambitious” and we’re not going to meet it. Australia’s actual trajectory in his view, based on a minority report from the Climate Change Authority, looks more like an increase of 11%.
In his paper 2014 paper published in Ecological Economics, “Blue carbon: Knowledge gaps, critical issues, and novel approaches”, he discussed the way blue carbon is being fast-tracked into the economic psyche of the world, and how mangroves, once considered “just swamps full of mosquitos”, are now acknowledged as an important ecological resource.
“It’s not about biomass – it’s about soils,” he says.
“The reason why we are losing all these mangrove forests and sea grass meadows and so on is not because they’re being harvested for timber resources, as is the case for terrestrial forests. The reason why we’re losing those forest ecosystems is that they’re being converted for other uses.
“For example in Asia, mangroves are a nutrient rich source for shrimp farmers to harness. However, after five to seven years this biological resource is drained and the farmer is left without shrimp, profit or a livelihood.”
Coral bleaching on the Great Barrier Reef is an acknowledged issue, but little attention has been paid to the concurrent dieback of mangrove forests
Mangroves connect the land to coral reefs through the cyclical flow of currents, which carry the carbon and nutrients needed for survival by coral reefs, known as virtual living oases in the open oceans. Land-based agriculture, farming, cattle grazing and mining collectively contribute to poor water quality and greenhouse gas emissions by transporting sediment, pesticides, coal residue and fertiliser that result from these activities.
The increased turbidity from these pollutants in the water also affects the light conditions required for photosynthesis, as well as the filtering of water by marine organisms for feeding and respiration. This in turn can dramatically affect marine plant and animal numbers.
Climate change is also a factor. Coral bleaching on the Great Barrier Reef is an acknowledged issue, but little attention has been paid to the concurrent dieback of mangrove forests along north Queensland’s coastline.
“Everybody knew about the reef and we know how much damage the reef has sustained, particularly in the northern third, but the fact that we lost a 100,000 hectares of mangroves was not noticed by anybody,” states Thomas.
Mangroves are an opportunistic species and take advantage of any new spaces created by inundations, storms and sediment accretion to establish growth. To access fresh water, they morph the physical structure of roots, stands and stems. The prop roots, or pneumatophores, that emerge from the sediments are like a bed of nails to walk on. They possess a cell membrane that is permeable to water but excludes most salt from entering the xylem vessels, which conduct water and minerals from the roots to the canopy.
The sediment is also a hive of activity, even when it doesn’t appear to be. Millions of bacteria feed on organic matter, using the salt water and iron as sources of energy to produce sulfide as a by-product – which is why mangroves can smell obnoxious when the sediments are disturbed.
The sediment layer is anoxic – that is, it contains almost no oxygen – and it is in here that carbon is stored. Mature, undisturbed mangroves trap a higher percentage of carbon, so they are a more cost-effective than young or restored areas.
Thomas believes there are unrecognised associated services that come with mangrove ecosystems.
“My scientific view is that they are obviously valuable resources from a carbon point of view,” he says.
“But secondly the carbon is just a proxy – just a button you could push to achieve all these other outcomes.
“If you can recognise the values of mangrove forests, sea grass meadows and salt marsh estuaries, if you can recognise how important they are and how valuable they are from an economic sense, in terms of providing fisheries stock, mitigating disaster costs and providing livelihoods for all kinds of people, they become a good thing to preserve.”