Mangrove dynamics and environmental changes on Koh Chang, Thailand during the last millennium

Abstract

A 1500-year record of mangrove dynamics has been established from palaeoecological analyses on three cores from Salak Phet Bay, Koh Chang island in the eastern Gulf of Thailand. The occurrence of Rhizophora, accompanied by other mangrove species, suggested that Salak Phet Bay supported a mangrove community from at least 1500 cal yr BP. From 1500 cal yr BP the mangrove extent decreased indicating less inundation frequency, possibly in response to a sea-level fall until 1300 cal yr BP. Following this regression, sea-level rise resulted in an increased presence of mangrove taxa until 500 cal yr BP. The study documents that Salak Phet Bay was characterised by relatively low saline conditions based on the occurrence of the moist-loving species (Oncosperma) around 1500-500 cal yr BP. After 500 cal yr BP mangrove taxa gradually decreased and terrestrial herbaceous taxa, mainly grasses, increased suggesting that the frequency of marine inundation was reduced as sea level fell. Drier conditions were also recorded by an increase in terrestrial grasses and a decrease in Oncosperma after 500 cal yr BP. In the uppermost sediments the increased presence of Rhizophora is probably associated with recent global sea-level rise although changes in mangrove composition are possibly related to human activities within Koh Chang. The sedimentation rate and the mangrove migration at Koh Chang have kept pace over the past 1500 years but this may be challenged under predicted future rapid sea-level rise as accommodation space for mangroves to migrate inland is required to maintain viable mangrove forests. Keywords pollen analysis, palaeoecological analysis, the eastern Gulf of Thailand, sealevel change

ENSO may have contributed to sea level changes in the Gulf of Thailand during the Late-Holocene

“…The timing of the RSL fall is consistent with the age of the rock shelter paintings in the Sam Roi Yot Mountain (The Fine Arts Department, 2004). These RSL fluctuations overlapped with those obtained from the pollen records from a mangrove forest in Koh Chang (#1) that demonstrated the RSL fall at c. 1500-1300 cal y BP and a rise at c. 1300-500 cal y BP (Englong et al, 2019) (Figure 1b and Supplementary 5, available online). The different environmental conditions between Ko Chang and the Sam Roi Yot wetland could explain the inconsistency in the timing of the sea-level changes.…”

“…The subsamples were sieved with a mesh size of 0.2 mm, and the sieved remains were treated by the acetolysis method (Ellison, 2008). Pollen grains were extracted using the heavy liquid approach with sodium polytungstate (3Na 2 WO 4 •9WO 3 •H 2 O) (Englong et al, 2019). Afterward, an aliquot of each treated subsample was dried onto a coverslip and then mounted onto a glass slide.…”

Late Holocene coastal dynamics south of the Chanthaburi estuary, eastern Gulf of Thailand

“…While the information about the development of RSL change in the Gulf of Thailand and the neighbouring Sunda Shelf during this time is controversial (cf. Englong et al, 2019; Wan et al, 2020), some studies suggest it may have dropped by almost 1 m during this time (Tjia, 1996; Oliver and Terry, 2019). If these reconstructions are correct, a strong regression tendency may likely have caused the seaward migration of the beach ridges (Fig.…”

Assessing mangrove deforestation using pixel-based image: a machine learning approach

“…Recently, in Thailand, Palaeoecological analysis was conducted to assess mangrove species evolution and changes over a period of one thousand five hundred years [11]. The study area covered the Salak Phet Bay of Koh Chang Island in the Eastern Gulf of Thailand.…”

“…We calculate the classifiers performance concerning various execution measurements such as recall, precision, F-measure, accuracy region under ROC curve, and gamma measurement. The mean absolute error 𝐸 𝑛 of a particular a variable 𝑛 is calculated by (11):…”