Mangroves – a weapon against climate change

Mangroves – a weapon against climate change

What are Mangrove forests

Occurring in 123 countries mangrove forests are the only woody salt-tolerant plant that lives in salt water along the world’s subtropical and tropical coastlines. Mangroves are typical regions of transition between two biological communities, having some components of both marine and terrestrial biomes, but have also developed several unique structural and functional adaptations, such as physiological mechanisms to tolerate salt and aerial roots that enable the plants to respire in anoxic, waterlogged soils. They support many species of plants, invertebrates, fish, and birds. Mangrove forests have commonly been shown to provide multiple provisioning services that increase the economic or food security of local communities such as timber, fuelwood, medicinal, and food resources. Regulating services such as wave attenuation, and more recently, carbon sequestration and storage. How are mangrove forests different to wetlands? Click on this link to find out more on wetlands

Mangroves - a weapon against climate change
A functional mangrove forest provides many resources to animals and humans.

Mangroves are architecturally simple compared with terrestrial forests, usually harbouring few tree species and lacking an understory of ferns and scrubs. However, the standing biomass of some mangrove forests in equatorial regions can be immense, rivalling the height and weight of many tropical rainforests. The development of mangrove forests occurs where near-horizontal topography coincides with sea level; a relatively stable period of sea level is, thus, a prerequisite for the development of old-growth forests.

These mangroves are naturally disturbed by tsunamis, floods, cyclones, lightning, pests and disease, and become more susceptible when human stressors such as pollutants are introduced. However, mangroves often exhibit considerable resilience to disturbance, undergoing a perpetual change in ecosystem development commensurate with the evolution of the environmental settings they inhabit. I have been to one of the very few mangrove forests in South Africa, along with them looking quite peculiar having their roots protruding into the sky instead of into the ground. They hold a remarkable amount of biodiversity, from Nile crocodiles, hippos. Crabs, fish and a whole host of birdlife.  

Importance of Mangrove forests

Coincidentally, poverty and dense human populations flourish along these low-latitude coasts, partly explaining the high (1–3%) annual deforestation rates of these tidal forests. Mangrove forests are a valuable ecological and economic resource, providing essential services such as food and fuel resources; nursery grounds for fish, mammals and other semi-terrestrial and aquatic fauna; depocenters for sediment, carbon and other elements; and, in some instances, offering some protection from coastal erosion due to tsunamis and intense tropical storms.

The average monetary value of mangroves has been estimated as second only to the value of estuaries and seagrass meadows, and higher than the economic value of coral reefs, continental shelves and the open sea. Of higher eventual importance is the role of mangroves in storing carbon to help improve the impact of climate change. Like other forests, mangroves vary in size and age and, therefore, differ in rates of production and the balance between carbon production and respiration.

Destruction of Mangrove forests

Despite their uses to humans, approximately 50% of the world’s mangrove forests have disappeared over the past 50 years, ironically reflecting their importance as a valuable economic resource. Major causes for this destruction have been urban development, aquaculture, mining, and overexploitation of timber, fish, crustaceans and shellfish. Despite the socio-economic and ecological importance of mangrove forests, these ecosystems continue to decline or be degraded due to anthropogenic impacts, natural causes, or the additive effects of both. Future anthropogenic impact on coastal ecosystems and mangroves will only increase as human populations in coastal regions are steadily increasing.

Mangroves - a weapon against climate change
Large areas of mangrove forests have been destroyed to make room for aquaculture.

Mangrove forests are also threatened by climate change impacts, especially increased rates of sea-level rise and reduction in availability of freshwater due to reduced flows in rivers that sustain mangroves. India has the second-largest mangrove forested area (3400 km2) in South Asia, after Bangladesh. However, India has experienced the most significant mangrove losses (~580 km²) between the years 2000 to 2012 to deforestation due to agriculture, aquaculture, embankments and coastal development, and diversion of freshwater away from the estuaries. Bhitarkanika mangroves experienced immense deforestation pressure during 1951–1961 due to a surge in population growth following the resettlement of refugees from Bangladesh. The population influx resulted in mangrove deforestation to have land for settlement, agriculture, and aquaculture. Globally, deforestation and conversion of mangroves have been shown to contribute 10% of the total global emissions from tropical deforestation, even though mangroves account for only about 0.7% of the world’s tropical forest area². Carbon losses from mangrove conversion can be high not only because of losses from above ground carbon pools but also below ground pools. Over just 20 years mangrove forest cover had declined about 26%, to an estimated 3.1Mha. In 2005, mangrove forest cover had further decreased to 2.9 Mha. Indonesia has lost 30% of its mangrove forests between 1980 and 2005; this is equivalent to an annual deforestation rate of 1.24%.

 Recent estimates of Indonesia’s mangrove cover suggest a total loss of 40% in the past three decades. Aquaculture development was the leading cause after it expanded rapidly in 1997–2005 and resulted in an officially recorded active pond area of about 0.65Mha. It was also reported that the revenue from shrimp export approached US$ 1.5 billion in 2013; almost 40% of the total revenues arising from the Indonesian fishery sector.

Mangroves - a weapon against climate change
Aquaculture farms stand where mangroves use to florish.

Although mangrove tree species can tolerate inundation by tides, they can die, and their former habitat can convert to open water or tidal flats when sea level rise causes the frequency and duration of inundation to exceed specific thresholds. In Vietnam, the area of mangrove forests has declined dramatically during the last century. In the early 1940s, Vietnam had more than 400,000ha (ha) of mangrove forest. In 2014, the mangrove forest area was reduced to 85,000ha, with much lower biodiversity and biomass, and a tiny percentage of that is a natural forest. Of the mangrove forest area in the Mekong River Delta, 161,277ha had been converted for shrimp farming as well as other uses.

Rehabilitation and Conservation

While efforts have recently been made to restore and conserve mangrove forests in this region, it is not entirely clear how restored/ planted mangroves might differ from relatively intact and pristine mangroves with regards to C storage function. The present global extent of mangrove forests is 152 000 km2 of which ~8% (11 870 km2) occurs along the coast of South Asia. India has a total of 4628 km2 of mangrove forests (0.14% of the global area), of which 60% are found along the eastern coast, 27% on the western coast, and 13% on the Andaman and Nicobar Islands. India’s east coast mangroves are much more biodiverse and cover a larger area than those found in the west coast due to differences in geomorphological settings.

The mangroves found along the Odisha coast have significant conservation value due to their rich and unique biodiversity. Floral and faunal diversity around Bhitarkanika area includes more than 300 species of vegetation, 31 species of mammals, 29 species of reptiles, and 174 species of birds. This region is also a critical habitat for the endangered Crocodilus porosus (saltwater crocodile). Approximately 70 species of true mangroves and mangrove associates are found here.  A study done in Sri Lanka found that 54% of planting attempts resulted in complete failure, and roughly 40% of the sites chosen for planting had no success. Of the14 sites that had some recruitment, 50% had survival rates of less than 10%.

Here is an example of an area where mangrove restoration is currently underway.

These figures are of grave concern given that 13 million USD was invested in such planting efforts and the trend of investing in mangrove planting is still growing. Only three project sites had high survival rates. The failure which we observed can be attributed to several causes. Firstly, the degree of damage caused by the tsunami of 2004 was a major factor in selecting restoration project sites. Conversion of mangroves and potential planting sites to shrimp farming has been traced as the major socioeconomic issue in Sri Lanka. As far as we observed, no political support exists for mangrove planting.

Nevertheless, shrimp farming extends under political patronage. Mangrove restoration projects in Sri Lanka have generally not been successful in restoring mangroves despite the good intentions which fueled them in the first place. The findings of this study demonstrate a frequent mismatch between the purported aims of restoration initiatives and the realities on the ground. The need to conserve and restore mangroves is critical, and our results should not be used as a motivation to stop investing in mangrove conservation. Well prepared and well-managed mangrove planting with post-care can lead to successful restoration, as has been amply shown, e.g. in Kenya, East Africa, to the benefit of multiple stakeholders.

In Demak, Indonesia, mangrove forests were converted to lands for unsustainable aquaculture. As a result, the coastal communities started facing severe consequences like land erosion, floods and no protection against storms and massive waves.  The restoration of a mangrove green belt in muddy coastal areas suffering from erosion and abrasion can be done through, among others, construction of permeable structures. The Building with Nature Indonesia Consortium transferred the ownership of permeable structures built so far from local contractors to the communities. Placement of new structures and maintenance are now entirely carried out by the community.

Mangroves - a weapon against climate change
Including local communities in restoration improves mangrove forest conservation.

The function of those permeable structures is to capture sediment that will function as a substrate for mangroves to grow naturally. Since 2015, the Building with Nature (BwN) program has been building permeable structures, together with locally-formed community groups, in three villages in Demak District. To date, cumulatively 4.7 km of permeable structures have been constructed and maintained. Some structures were abandoned as the site conditions turned out to be unsuitable, other structures had to be rebuilt due to problems with durability. Hence, in total ~1.8 km of the structure remains in place, behind which we expect ~20 ha of mangrove to be restored. In the first three years (2015-2017) the construction and maintenance of permeable structures were done through local contractors that were asked to engage with Building with Nature community group members.

This was done to ensure that group members understand the construction and maintenance process, and to foster a sense of belonging. In July 2018, the structures and continuation of their maintenance were handed over to the community groups, which was further handed over to the local village government. Since then, maintenance of existing structures and the construction of new structures has become the duty of community groups.

Take home message

Mangrove forests, like wetlands, have significantly been undervalued in the past. Consequently, they have been cut down to make room for human purposes such as aquaculture and housing. Only recently, perhaps in the last three decades have scientists realised the critical role these ecosystems have. It is not too late to restore these dynamic ecosystems to their former glory with projects similar to the story above. Mangrove forests will play a significant role in fight against climate change, and so restoration and conservation efforts will grow in importance in years to come.

For more information about mangroves and their conservation, click on this link

Join me next week as I take a look at climate change denialism.


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