Mangrove Ecosystems – A Sustainable Solution for Climate Change!

Mangroves are very unique and advantageous ecosystems in the world. Mangrove forests globally found only on coastlines in tropical and subtropical areas. Mangroves are trees and shrubs, and herbs, which are equipped with unique adaptations, enabling them to live in saline and inundated environments. They are the only species of trees on the planet that can tolerate saltwater. Mangrove forests can reach up to 30m in height under their ideal conditions of high rainfall or where there is a good flow of groundwater to reduce the salinity in the environment. More can be read here Archived.

Mangroves are extremely important to the coastal ecosystems they inhabit. Physically, they serve as a buffer between marine and terrestrial communities and protect shorelines from damaging winds, waves, and floods. Mangrove thickets improve water quality by filtering pollutants and trapping sediments from the land, and they reduce coastal erosion.

Ecologically, they provide a habitat for a diverse array of terrestrial organisms. Mangroves are often among the first species to colonize mud and sandbanks flooded by seawater, but an increase in coastal development and altered land use, and many other factors led to a decline in global populations. More can be read from here  Archived.

When we looked into this, we came to know about a piece of news as follows, “The best solution to reduce climate change is to restore the mangrove environment”. We discussed with Pioneer mangrove researcher, Professor Mala Damayanthi Amarasinghe in the department of plant and molecular biology, faculty of science at Kelaniya University to find out the truth or falsity of this news.

Claim: Restoring the mangrove environment is the easiest and most suitable solution to mitigate climate change.

How Mangroves contribute towards mitigating climate change!

Pioneer mangrove researcher, Prof. Mala  Amarasinghe remarked that the amount of carbon stored by mangrove plants and soils is at least four times greater than that stored by other tropical forests, making these coastal forests extremely valuable in the fight against climate change.

She further explained that the carbon is stored in the biomass or in muddy soil where mangroves live in. Carbon storing capacity differs with plant growth. Seedlings (small plants) store less carbon compared to saplings (plants are too young to bear flowers and fruits) and more carbon is absorbed and stored by larger trees normally mangrove tree has an average lifetime of 100 years. Therefore, a good part of the carbon dioxide absorbed by the trees will not be released back into the atmosphere for a long time unless there is a disturbance.

Mangroves play an important role in carbon sequestration by storing carbon largely in their soils as organic matter produced by dead roots and other plant parts. The current extent of mangrove forests is responsible for absorbing an amount of CO₂ equivalent to 14% of that absorbed (for photosynthesis) by oceans and 3% of the amount absorbed by tropical forests.  

Mangroves in Sri Lanka

Pioneer mangrove researcher, Prof. Mala Damayanthi Amarasinghe explains that during her research on mangroves she has encountered 23 different true mangrove species in Sri Lanka. Among these one species is recorded only once in the past 40 years. They include rare, endangered and threaten species as mentioned below.     

Species Diversity of Mangrove Plants in Sri Lanka

1	Acanthus ilicifolius L.	13	Heritiera littoralis
2	Aegiceras corniculatum (L) Blanco	14	Lumnitzera littorea (Jack) Voigt
3	Avicennia alba blume	15	Lumnitzera racemosa Willd.
4	Avicennia officinalis L.	16	Nypa fruticans (Thunb.) Wurb
5	Bruguiera cylindrica (L) Blume	17	Rhizophora apiculata BL.
6	Bruguiera gymnorriza (L) Lamk.	18	Rhizophora mucronata Lamk.
7	Bruguiera hainessi C.G.Rogers*	19	Scyphiphora hydrophyllaceae gaertn.*
8	Bruguiera parviflora Wight & Arnold ex.Griffith	20	Sonneratia alba J.Smith*
9	Bruguiera sexangula  (Lour.) Poir	21	Sonneratia alba J.smith*
10	Ceriops decandra (Griff.) Ding Hou*	22	Sonneratia caseolaris (L) Engler
11	Ceriops tagal (Perr.) C.B. Robinson	23	Xylocarpus granatum Koing
12	Cynometra iripa Kostel*

Distribution of mangroves in Sri Lanka

Inter tidal areas found in sheltered coastlines of lagoons and estuaries located in coastal areas of Sri Lanka are referred to as fringing mangroves. Basin mangroves occur in depressions, commonly found in the Jaffna peninsula. Mangroves found along estuaries of major rivers are referred to as riverine mangroves, located on the southern and south-western coasts of Sri Lanka. Scrub mangroves occur in dry zones, where soil salinities are high and these mangroves are often in bad conditions. Overwash mangroves occur in patches where the species are inundated during high tide. More can be read from here  Archived.

Mangrove reforestation is a valuable offset credit in carbon trading and a solution for climate change.

Carbon trading is the process of buying and selling permits and credits that allow the permit holder to emit carbon dioxide. The model used in all current carbon trading schemes is called ‘cap and trade’. In a ‘cap and trade’ scheme, a government or intergovernmental body sets an overall legal limit on emissions (the cap) over a specific period of time, and grants a fixed number of permits to those releasing the emissions and polluters should pay for it. More details from here  Archived.

There is an offset process also! Mala Damayanthi Amarasinghe, professor in the department of plant and molecular biology at Kelaniya University says, companies that release CO2 try to grow mangroves, as this would in turn mean that their emissions (the cap) could get the necessary approvals easily. There is an offset process also! Mala Amarasinghe, a professor in botany at the department of plant and molecular biology at the university of Kelaniya says, companies that release CO2 tend to invest in planting mangroves as a measure to reduce their emissions below the legal limits (the cap) cut down carbon tax. However, every country doesn’t have the capability of growing mangroves since countries that grow mangroves well are given a guarantee and they can earn money through this. The value of this depends on the amount of carbon stored by mangroves. This process is operated through carbon credits.  Carbon emitters increasingly look towards tropical countries where mangroves grow to buy carbon credits. However, if these plants are not monitored and sufficient care is given, the desired results cannot be achieved says Prof. Mala Amarasinghe.   

Mangrove reforestation or afforestation is not an easy task.

There are about 80 different species of mangrove trees. All of these trees grow in areas with low-oxygen soil, where slow-moving waters allow fine sediments to accumulate. More can be read here. Archived.  Professor Mala Amarasinghe stated that people should have a clear idea about soil salinity when growing mangroves because the salinity tolerance of mangroves is different from species to species. For instance, Avicennia marina  (Manda in Sinhala) has high salt capacity but not the species like Rhizophora aepiculata  (Kadol) and Sonneratia caseolaris (Kirala). Mangroves cannot withstand freezing temperatures or very high temperatures since they can be found only at tropical and subtropical latitudes where temperatures are relatively high than in temperate areas since the amount of sunlight received during the daytime is also high. This affects especially the survival of seedlings that are planted along open shorelines without any shelter/ shade from large trees.

Nursery-reared seedlings tend to have a higher survival rate in plantations. The tidal small tidal range prevailing in seas around Sri Lanka together with coastal, and topographical features limit the width of the intertidal zone thus restricting the coastal areas that can support mangroves. Therefore, it is very important to protect the existing mangroves and to rehabilitate degraded mangrove areas rather than to search for new inter-tidal areas for mangrove planting.   

Prof. Mala Amarasinghe explained that NARA has taken some initiative to propagate rare mangrove species at kadol kale (Negambo) while traditional fishermen especially in the Negombo estuary play a considerable role in propagating and conserving mangroves. She said that indigenous knowledge of fisherman are very important and taken into consideration in planning and implementing mangrove plantation/ restoration programs because many such efforts have proven to be a failure due to inadequate knowledge and experience of planners and implementers of mangrove planting projects and poor after-care given on plantations.  

Artisanal fishermen of Negombo estuary/ lagoon use methods based on indigenous knowledge to cultivate mangroves in the right places and to protect them as an integral part of brush park fishery says, prof. Amarasinghe. They are equipped with the knowledge on how to extract mature branches of mangrove trees/shrubs selectively and to use them in constructing brush parks which they use to fish. 

Fishermen plant mangroves and take good care of the plants. She further explains these bush parks were situated in munnakkare, thalahena and wedikanda areas and this fishing method along with mangrove cultivation as a source of twigs and branches required to make brush parks are being practised for many generations in Negombo lagoon.  They have a good understanding of the conditions required for different mangrove species and where should the seedlings be planted for better survival and growth.  Therefore, this traditional method has been very successful in re-establishing mangroves on mud flats. Says, Professor Mala Amarasinghe.

Sustainable management of mangroves

Under sustainable management, mangroves provide many livelihood opportunities for local communities including shellfish gathering, fishing, and beekeeping. Communities living alongside mangroves could reap many such benefits from a thriving mangrove forest.

Mangroves also generate indirect income for communities. A place where freshwater, marine and terrestrial habitats meet, mangroves are instruments of biodiversity conservation. They are home to thousands of different species, including globally threatened species. Below are some animal types found in mangroves.

Molluscs – such as Mud Whelks (Terebralia), Periwinkles (Littoria), ship worms,

Crustaceans – such as crabs, prawns (most dominant crustaceans)

Insects – mostly found in non-inundated areas

Fish – approximately 600 species of fish have been recorded in mangrove forests of the Indo West Pacific Realm

Reptiles and Amphibians – including snakes, lizards, crocodiles and sea turtles

Birds – birds are the most abundant terrestrial vertebrates found amongst mangroves, who turn to these habitats in search of shelter, food, or rest.

Mammals – monkeys, deer, otters, racoons, small cats and tigers. Mangrove habitats of Sundarbans and Sumatra are home to the largest tiger populations in the world.

Marine mammals such as manatees, dugongs, and dolphins have also been observed, due to the important habitats often found adjacent to mangroves. This lifecycle supports both biodiversity and community livelihoods through sustainably conducted reef tourism and offshore fishing. More information can be taken from here   Archived.

Mangroves are highly productive ecosystems with an array of benefits to both animals and humans. Mangrove forests provide surrounding communities with timber for the construction of houses, boats, fishing gear and for fuel wood. Some mangrove species are also used in leather production, as they are rich in tannins, a chemical commonly used in leather production.

Mangroves also provide food to local communities. Some fruits, such as those from Sonneratia in Sri Lanka, are used to make drinks and ice creams. Nypa’sflower stalks are used in herbal teas and its fruits are consumed.

Mangroves around the world play a crucial role in supporting coastal fishing communities. In South-East Asia Apart from the immense economic benefits for the fisheries industries, mangroves can also be used for tourism, due to the biodiversity found within.

Mangrove forests also support filtration actions, where they help trap sediments, including pollutants and nutrients. Being primary producers, they also absorb these nutrients, playing a role in pollution reduction.

Mangroves, therefore, deserve conservation for all the above ecological functions in general and their carbon sink function in particular that contributes to reducing global warming.  

Read the Sinhala version of this article here