Japan Geoscience Union Meeting 2018

Presentation information

[JJ] Poster

H (Human Geosciences) » H-CG Complex & General

[H-CG28] Coastal wetlands: geomorphologic, biologic and anthropogenic processes

Sun. May 20, 2018 3:30 PM - 5:00 PM Poster Hall (International Exhibition Hall7, Makuhari Messe)

convener:Kiyoshi Fujimoto(Nanzan University)

[HCG28-P01] Forest structure and site environments of a mangrove community in the surface erosion area possibly caused by rapid sea-level rise on Pohnpei Island, Micronesia.

*Kiyoshi Fujimoto1, Kenji Ono2, Shin Watanabe3, Shingo Taniguchi3, Keita Furukawa4, Yasumasa Hirata2, Kodai Hasada5, Saimon Lihpai6 (1.Nanzan Univ., 2.Forestry and Forest Products Research Institute, 3.Univ. of the Ryukyus, 4.Ocean Policy Research Institute, 5.Hosei Univ., 6.Pohnpei State Government, FSM)

Keywords:Mangrove community, Surface erosion, Sea-level rise, Mangrove peat

In the western Pacific low-latitude region between the Philippines and Micronesia, the sea level is rising at over 10 mm/yr (IPCC 2013). At Pohnpei Island in Micronesia, rapid sea-level rise at 16.9 mm/yr had been observed between 2002 and 2010 (Australian Bureau of Meteorology 2010). It is decided by the relative relation between possible accumulation rate and sea-level rise rate there whether the mangrove forest survive or die. Possible accumulation rate at the site situated in the sediment inflow area is calculated by the sum of sedimentation rate by inflow and mangrove peat accumulation rate, while the site situated in the no inflow area is decided by the mangrove peat accumulation rate only. In the no sediment inflow area, Rhizophora pure forest is formed as a pioneer community and mangrove peat is created. The mangrove peat accumulation rate is considered to decrease with the vegetation succession because of the decrease of tree density of Rhizophora sp. In Pohnpei, the surface erosion is not recognized in the Rhizophora stylosa and apiculata communities, but it is recognized in the communities dominated by other species. We set up a permanent plot with 20 m wide and 130 m depth in the significant surface erosion area in the south part of the island in September 2017 to observe the forest and habitat dynamics. This presentation will report the present status of forest structure and surface erosion in the plot.
The plot is situated behind a R. stylosa community with about 30 m depth. Four species, Bruguiera gymnorrhiza, R. apiculata, Xylocarpus granatum and Sonneratia alba, were identified in the plot. Number of each species were 116, 40, 17 and 13, respectively. Tree density was calculated at 715 trees/ha. 90 % of R. apiculata trees were shrub with smaller than 5 cm in diameter. Diameter of B. gymnorrhiza and X. granatum show unimodal distribution whose mode is 20.0 to 29.9 cm and 30.0 to 39.9 cm, respectively, while S. alba shows inhomogeneous distribution between 20 and 90 cm in diameter. Aboveground biomasses were calculated at 256 t/ha for B. gymnorrhiza, 173 t/ha for S. alba, 94 t/ha for X. granatum and 25 t/ha for R. apiculata. Substrata consists of mangrove peat, which shows that former dominant species was Rhizophora sp. Ground elevation in the plot was between -57 and +19 cm. Area above mean sea level sparsely distributed around 100 m line, which indicates that surface erosion is surely progressing. At the base of B. gymnorrhiza trees, the buttress roots and the like of prop roots under the buttress roots were exposed by the surface erosion and the gaps between ground surface and its buttress root appeared. The average gap height reached 43 cm between 0 and 20 m line, 31 cm between 110 and 130 m line, which suggest that the amount of erosion possibly decrease with distance from seaward forest edge.