Mangrove Habitat Dynamics and Holocene Sea-level changes in the Southwestern Coast of Thailand.

Abstract

Evidence of mangrove habitat dynamics in relation to the Holocene sea-level changes offer significant bases for predicting the influence of an anticipated sea-level rise. We clarified the mangrove habitat dynamics during the mid to late Holocene in the southwestern coast of Thailand based on the relationships between ground level, sediments and present vegetation, spatial distribution of mangrove forest-floor deposits and their formative periods, The mangrove forest-floor deposits wereidentified by detailed observation of the sedimentary facies and vertical distribution of root density using non-disturbed core samples. Three cycles of sea-level fluctuations were found during the last 8000 years. The first sea-level fall occurred just before 7200 yr BP and mangrove forest moved seaward. The sea level fell up to about 3 m below present mean sea level. The rate of the subsequent sea-level rise, which was estimated at 4.7 mm/yr, exceeded the maximum rate of mangrove peat accumulation because the mangrove forests retreated landward. The maximum sea level was estimated to have reached more than 1 m above present mean sea level around 6150 yr BP. The sea-level fell slightly again before 4000 yr BP, after that it rose gradually up to about 1 m above present mean sea level around 3500 yr BP with the mangrove peat accumulation in Rhizophora apiculata forest, The sea-level fell again up to about 1 m below present mean sea level around 2200 yr BP and the mangrove habitat expanded seawaward. The R, apiculata habitat consisting of mangrove peat formed during the former high stand of sea level changed to the Lumnitzera littorea habitat with the sea-level fall. During the last 2000 years, the sea level rose gradually again at about O.6 mm/yr and the Rhizophora apiculata forests maintained their habitats by accumulating mangrove peat with the sea-level rise. In the depositional areas of inorganic sediments, whose sedimentation Tate was calculated at about 2 mm/yr, dominant species changed from R. apicutata to Bruguiera, Ceriops or Xylocarpus spp. with the increment of the ground level and the habitats have expanded laterally during the last several hundred years.