Tropics
Online ISSN : 1882-5729
Print ISSN : 0917-415X
ISSN-L : 0917-415X
Regular papers
Mangrove habitat evolution related to Holocene sea-level changes on Pacific islands
Kiyoshi FUJIMOTO
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JOURNAL FREE ACCESS

1997 Volume 6 Issue 3 Pages 203-213

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Abstract

Geomorphological evolutions of mangrove habitats on the Pacific islands and prediction of the habitat dynamics were discussed using the examples of Kosrae Island in Micronesia, Bohole Island in the Philippines and Iriomote and Ishigaki Islands belonging to the Yaeyama Islands in Japan.
Most of the mangrove forests of Kosrae have been developed during the last 2000 years by accumulating mangrove peat with the gradual sea-level rise of 1 to 2 mm/yr. During the period of rapid sea-level rise of about 10 mm/yr between 4100 and 3700 yr B.P., the mangrove forests ceased peat accumulation and retreated landward. These evidences suggest that the critical rate of mangrove peat accretion with sea-level rise is between 2 mm/yr and 10 mm/yr. If the velocity of the anticipated sealevel rise exceeds the critical rate, all of the mangrove forests of Kosrae will retreat landward and reduce rapidly.
The mangrove habitats on Bohole Island have been formed by the same processes as on Kosrae Island except the expansion of clay layer overlying a mangrove peat layer during the last several hundred years. The clay deposit, which was inferred to have derived from the deforested slopes, supported Nypa fruticans thicket. Rapid sea-level rise over the critical rate of the mangrove peat accumulation will induce the retreat of mangrove forests from their seaward margin, however, the invasion of mangrove trees into the Nypa habitat will not occur immediately because the habitat conditions of the clay deposited area is no longer suitable for the growth of mangrove species except Nypa.
Mangrove habitats of Iriomote and Ishigaki Islands abruptly moved seaward by a relative sea-level fall that occurred about 1000 and 2000 yr B.P., apparently caused by seismic uplift. The thickness of the layers containing mangrove organic materials in present mangrove habitats were less than 1 m. The organic layers have deposited during the last several hundred years. During the last few decades, input of sediments caused by soil erosion has affected the seaward expansion of mangrove forests. However, the sediments supply will result the reduction of mangrove area because the seaward limit of mangrove expansion is restricted by the geomorphological framework.

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© 1997 The Japan Society of Tropical Ecology
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