Thailand's mangroves: a forest between land and sea, from collapse to recovery

Neither wholly terrestrial nor wholly marine, the mangrove fringes the estuaries and muddy shores of Thailand, from the upper reaches of the Gulf to the coasts of Ranong. It lost more than half of its area between 1961 and the mid-1990s, cleared for shrimp farming, agriculture, mining and charcoal. Today it is regaining ground, driven by replanting, public policy and, more recently, the carbon market. To understand this forest is to read together its ecology, the history of its retreat, the debates over the causes and the uncertainties surrounding its recovery.

An amphibious forest, governed by the tides

At high tide, salt water rises between the trunks until only the foliage shows above the surface. A few hours later it withdraws, revealing a dark mud bristling with roots. The mangrove, pa chai len (ป่าชายเลน) in Thai, where pa means forest, is one of the few forests that live in this way with their feet in the sea. In Thailand, mangrove trees establish themselves on sheltered, muddy shores and in the lower reaches of estuaries, within the band between low-water and high-water levels.

The country offers this habitat a considerable coastline. According to the marine protected areas database of the Department of Marine and Coastal Resources (DMCR), the Thai coast measures 3,148 km. It breaks down as follows: 2,055 km on the Gulf of Thailand, shared among 17 provinces, and 1,093 km on the Andaman Sea, shared among 6 provinces. Other scientific studies give 2,614 km. The length of a coastline depends on the method of measurement, and each figure must be tied to its source. The Ministry of Natural Resources and Environment, for its part, refers to 24 coastal provinces monitored by satellite. The sources consulted do not explain the discrepancy with the DMCR count.

The two coasts have not shared the same history. The Gulf, particularly its central section near Bangkok, has been profoundly transformed. The country's last mature natural forests are found on the Andaman coast, in the provinces of Ranong, Phang Nga and Trang.

An architecture of layers and bands

A mature mangrove is organised in two layers. The upper layer can reach 20 m. It is dominated by the two Rhizophora species found in the country, R. apiculata and R. mucronata, which Thai groups together under the name kongkang. Alongside them grow Heritiera littoralis (ngon kai) and species of Xylocarpus. A taxonomic revision published in 2024 confirmed that Thailand has only these two species of Rhizophora.

On newly formed mudflats, Avicennia (A. alba, A. officinalis) are the first to settle. They are followed by the Rhizophoraceae and Sonneratia caseolaris. Popular accounts translate this succession into space as a zonation, that is, a pattern of bands running parallel to the shore:

• on the seaward side, a fringe of Avicennia, no more than 10 m tall;
• then a belt of Rhizophora, around 8 m on average;
• finally, further inland, a mixed zone.

This pattern helps in reading the landscape. It is, however, a general model that each site qualifies, and the source describing it is of secondary standing.

A living, mobile and productive habitat

How many species?

The question seems simple, but the answers vary according to what is being counted: only the "true" mangrove trees, or all the plants associated with them. UNESCO records more than 50 mangrove species in the Ranong Biosphere Reserve alone. An inventory carried out in a national park in Phang Nga (Giesen et al., 2006) counts at least 28. A secondary source puts forward 87 species of mangrove trees and associated plants, in 41 families, across the country. What these figures offer is an order of magnitude rather than a definitive total.

The fauna is just as rich. The Ranong reserve is home to more than 300 animal species, among them the dugong (Dugong dugon), a marine mammal that feeds on seagrass beds. These seagrass meadows extend the mangrove beneath the sea to a depth of around ten metres. Forest, seagrass and coastal waters thus form an ecological continuum: protecting one without the others makes little sense.

A forest on the move

The mangrove is not a fixed backdrop. In Pak Phanang Bay, on the Gulf coast, it advanced seawards by an average of 53 m a year over thirty years, according to a study by Panapitukkul and colleagues published in 1998. This advance follows the build-up of sediment.

Conversely, rising sea levels should in principle push it to migrate inland. In Surat Thani province, the sea rose by 5.28 mm a year between 2008 and 2021. Yet researchers observe that the mangrove there has moved little inland, because urban development blocks its way. Caught between rising water and occupied land, the forest risks seeing its space shrink.

Genetic diversity worth preserving

A study published in 2023 revealed, in several species, a clear genetic differentiation between populations on the eastern coast of the peninsula (the Gulf) and those on the western coast (the Andaman). The same authors point out that reforestation has focused mainly on the two Rhizophora species. The challenge is therefore twofold: replanting areas is not enough; the restored forests must also remain diverse and suited to their coast of origin.

This knowledge owes much to the Phuket Marine Biological Center (PMBC), founded in 1968. Its research on the Bangrong mangrove in Phuket benefited from long-standing cooperation with Denmark, which continued until 2000.

1961–1996: the great retreat

The first national inventory, carried out in 1961, put Thailand's mangrove at around 368,000 ha. The Royal Forest Department series, cited by Huitric, Folke and Kautsky (2002), gives 364,000 ha. The figures available for the following years are:

• 312,700 ha in 1975;
• 168,683 ha in 1993;
• around 167,500 to 168,500 ha in 1996, the low point of the curve. A 2024 scientific review even gives 160,000 ha.

The volume edited by Edward Barbier and Suthawan Sathirathai (2004) estimates the loss at around 56.5% of the original area. Other studies speak of 50 to 60%. In thirty-five years, then, the country lost more than half of its mangroves.

This retreat did not strike everywhere with equal force. According to a DMCR series, the central region fell from around 66,900 ha to under 11,000 ha between the first and last surveys, which probably date from 1961 and 2014. Over the same period, the Andaman coast fell only from around 194,000 to 176,000 ha. The upper Gulf, on Bangkok's doorstep, lost 85% of its mangroves between 1975 and 1993. At provincial level, Nakhon Si Thammarat is reported to have lost nearly 88% of its own between 1961 and 1996 (Charuppat and Charuppat, 1997).

The causes of the retreat

The literature identifies shrimp farming as the main cause of the decline. The upper Gulf, close to markets and infrastructure, was the first to be developed. Clearance also served agriculture and tin mining. To this was added charcoal, produced under forest concessions that the state brought to an end in 1998, with the last concession in Ranong expiring in 2000. The sources consulted do not allow the scale of this exploitation to be measured or its beginnings to be dated.

The effects of shrimp farming go beyond mere clearance. Studies record soil salinisation, eutrophication of the water (an excess of nutrients that starves it of oxygen), antibiotic residues, and abandoned ponds where vegetation no longer grows back. In Pak Phanang, sediment discharged from the ponds slows the growth of neighbouring mangrove trees and increases their mortality.

Are shrimp solely to blame?

This dominant narrative is nonetheless contested. According to Giri and colleagues (2008), in the area they studied, 50% of converted mangroves went to agriculture, 41% to aquaculture and 2% to urban development. In Krabi, a remote-sensing study covering the years 1989 to 2007 observed only moderate exploitation, without the boom-and-bust cycle of shrimp farming often described elsewhere. An econometric analysis goes further: it concludes that intensive farming, developed in the 1990s, reduced losses compared with extensive and semi-intensive forms, which require more space.

These studies do not exonerate shrimp farming. They do, however, remind us that its weight varies by region, period and farming method, and that agriculture has often played an underestimated role.

The recovery: real progress, disputed figures

From the late 1990s, the trend reversed. The end of charcoal concessions in 1998 marked a turning point. In Ranong, replanting trials began as early as 1994 on a former concession. They drew on the mangrove research centre established in 1982 by the Royal Forest Department at Ngao. In 2002, this centre and the Ranong reserve came under the authority of the DMCR. Attached to the Ministry of Natural Resources and Environment, this department manages, rehabilitates and protects mangroves under the Marine and Coastal Resources Management Act. The country's forests in fact fall under three administrations: the Royal Forest Department, the Department of National Parks (DNP) and the DMCR.

The recovery figures show an increase, but they do not form a consistent series:

• around 1.5 million rai (240,000 ha) in 2004 according to one source, 265,800 ha according to another. The rai is the Thai unit of land area, equal to 0.16 ha;
• around 245,300 ha in 2014;
• 1.737 million rai, or around 277,900 ha, in 2020, according to the satellite inventory by the DMCR and GISTDA, the Thai space agency.

This last figure represents an increase of around 200,000 rai compared with 2014. The minister Varawut Silpa-archa attributed it to raised awareness among the public and businesses. The director-general of the DMCR added the effect of the lines of bamboo stakes planted since 2007 in 13 coastal provinces to curb erosion and trap sediment.

A study published in 2024, based on Landsat images processed with Google Earth Engine, nonetheless reached a very different result: 601,642 ha of mangrove in 2020, compared with 339,613 ha in 1989. That is more than double the official figure. The sources consulted do not explain this discrepancy, which probably stems from the methods used to classify the images. The reference figure remains that of the DMCR, but this divergence calls for caution. The DMCR also states that it has 1.1 million rai of mangrove under its responsibility, much of which has been converted into aquaculture ponds: the area administered does not correspond to the area under forest.

2004: did the mangrove protect the coast from the tsunami?

The tsunami of 26 December 2004 struck the Andaman coast and revived interest in the mangrove as a natural defence. Several studies support this idea.

• In Phang Nga province, remote-sensing analyses measured around 27% land-cover change in sparsely forested areas, compared with less than 3% in areas with dense mangrove cover. Their authors recommend a forest buffer strip of 1,000 to 1,500 m.
• A survey conducted in twenty villages on the Andaman coast concluded that healthy mangroves provided "substantial protection". It also stressed that their condition depended on the local social fabric and on the way outside aid had been distributed.

Other studies strongly qualify this finding. At Cape Pakarang, in Phang Nga, a study combining field surveys and modelling found that around 70% of the mangrove had been destroyed. The survival of Rhizophora depended on trunk diameter: 72% of trunks measuring 25 to 30 cm withstood the wave, compared with 19% of those measuring 15 to 20 cm. The review by Cochard and colleagues (2008), covering Aceh and southern Thailand, judged the buffering role of coastal vegetation to be "probably rather minor". It even noted cases where vegetation may have increased the danger, by providing debris or channelling the flow. Finally, it expressed concern that funding for replanting might come at the expense of tsunami preparedness.

The current state of knowledge therefore suggests speaking of partial protection. This depends on the width, density and maturity of the forest. The mangrove is not a barrier, and it replaces neither warning systems nor evacuation.

Mangrove landscapes

Ranong, the great Andaman stronghold

Lying around 650 km south of Bangkok, the Ranong Biosphere Reserve was recognised by UNESCO's Man and the Biosphere (MAB) Programme in 1997. It is described as the largest continuous expanse of mangrove in the country. Its area varies according to the source: 31,007 ha, around 40% of it marine, according to UNESCO; 30,309 ha, including 10,148 ha of mangrove forest, according to a scientific study. It has a little over 18,000 inhabitants.

The principle of a biosphere reserve is to combine conservation with human activity. In the core zones, only small-scale fishing is practised. In the transition zone, shrimp ponds sit alongside rubber plantations and oil palms. In 2024, a UNESCO educational mission included a visit to a community of Moken, sea nomads, on the island of Koh Lao.

The Ranong Ramsar site, designated in 2002 under the international convention on wetlands, covers 1,084 km². The IUCN presents it as the only place in the region where intact primary mangrove survives. The press also mentions a 200-year-old kongkang bai lek and a 300-year-old taboon dam. These ages are not based on any known dating method and should be read as estimates. Since December 2021, the Ranong mangroves have formed part of the "Andaman Sea Nature Reserves", included on the World Heritage Tentative List.

Krabi and Phang Nga Bay

The Krabi River estuary has been on the Ramsar list since 2001 (213 km²), and Phang Nga Bay since 2002. The Khao Thong mangrove in Krabi covers around 3,700 rai and serves as a testing ground for supervised ecotourism. Pressures remain visible there: in 2014, oil palms and shrimp ponds were advancing towards the estuary, south of the town.

Pak Phanang and the Gulf

On the Gulf coast, Pak Phanang Bay, in Nakhon Si Thammarat province, held around 35,000 ha of mangrove in 2016, an area almost unchanged in 2021 according to a recent study. Its Ramsar status remains uncertain: one study says it was announced in 2000, while other sources mention only a proposed listing.

Further north, at Bang Pu in Samut Prakan province, survives one of the last fragments of mangrove in the upper Gulf. An agreement concluded in December 2002 between the Royal Thai Army and WWF made it the country's first urban nature education centre. It stands as a witness to what the mangrove once was on the capital's doorstep.

Carbon, taxation, symbols: the mangrove at the heart of public policy

The mangrove has also become a national symbol. In 2020, Thailand celebrated its first National Mangrove Forest Day, set for 10 May. The government announcement presents it as a tribute to the royal attention paid to mangrove conservation, although the extract consulted does not specify which sovereign is meant.

The blue carbon forest

The climate has given the mangrove new weight. The government has set itself the goal of adding 300,000 rai (48,000 ha) of mangrove between 2022 and 2031. In 2022, the DMCR published a guide for bringing plantations into T-VER, the Thailand Voluntary Emission Reduction Program, which issues carbon credits. For 2023, the target was 2,382 ha of new plantations in 16 provinces. The former director-general of the DMCR, Sopon Thongdee, asserts that mangroves store "five times more" CO₂ per unit of area than terrestrial forests. This is an institutional statement, not a figure established in the sources consulted.

Companies have committed to these programmes. Dow, with the IUCN, has announced a project covering 5,000 rai across five provinces, with a budget of at least 30 million baht. PTT Global Chemical runs a programme at Klaeng, in Rayong province. This information comes from the companies' own press releases. Mongabay reported in 2024 that these plans arouse both hope and concern in coastal communities, who want to know who will control the replanted land and who will benefit from the carbon revenue.

Pressures that persist

The threats have not gone away. According to the DMCR, some stretches of the Gulf lose up to 5 m of land a year to erosion. A 2019 land tax law had an unexpected effect: to avoid the tax on land deemed "vacant", some owners converted their mangroves into farmland. The government has approved in principle a tax exemption for mangroves.

The story of Thailand's mangroves is therefore neither one of irreversible loss nor one of completed restoration. The retreat from 1961 to 1996 is well documented, even if the exact role of shrimp farming remains disputed. The recovery is real, but its scale depends on the methods of measurement. Its future is now being decided on several fronts at once: the diversity of plantations, the room left to the forest in the face of the rising sea and the spreading city, and the sharing of benefits with those who make their living from these shores.

Sources and further reading

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3. DMCR, Marine Protected Area Database of Thailand: Executive Summary, Department of Marine and Coastal Resources, n.d..
4. non relevés, Mangrove Forest Conservation in Thailand (tableau de données DMCR 2005, 2013, 2014), FFTC Agricultural Policy Platform, c. 2019.
5. Bangkok Post, Thailand's mangrove forests to be land-tax exempt, Bangkok Post, c. 2024-2025.
6. Gouvernement royal thaïlandais, Government supports mangrove reforestation to earn carbon credits, thailand.go.th, c. 2022.
7. IUCN Thailand, Thailand celebrates Thailand National Mangrove Day, IUCN, 2021.
8. IUCN Thailand, Thailand celebrates its first National Mangrove Forest Day, IUCN, 2020.
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10. Gouvernement thaïlandais (relayé par ACTMANG), Press release by Thailand Government: Designating May 10 as National Mangrove Day, mangrove.or.jp, n.d..
11. Mongabay, Thailand's budding mangrove restoration plans spark both hope and concern, Mongabay, 2024.
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