The belowground carbon storage and burial rate were examined for the mangrove forests on Pohnpei Island in Micronesia, the southwestern coast of Thailand, Iriomote Island in Japan and the Mekong Delta in Vietnam. Stored carbon in a coral reef-type mangrove habitat of Pohnpei Island consisting of a 2-m thick mangrove peat layer, which is a type of mangrove habitat found in tropical Pacific islands, was estimated to be 130 kg C m^-2 (=1300 t C m^-2). In the southwestern coast of Thailand, stored carbon up to 1 m in depth was estimated to be 50 kg C m^-2 for R. apiculata forest, 39 kg C m^-2 for R. apiculata - Bruguiera spp. or Xylocarpus spp. forest and 27 kg C m^-2 for Ceriops tagal forest. In Iriomote Island, it was estimated to be 50 kg C m^-2 for Rhizophora stylosa forest. In the Mekong Delta, it was estimated to be 33 kg C m^-2 for R. apiculata forest on a flood deposit of ca. 100 years ago and 27 kg C m^-2 for a 20-year-old R. apiculata plantation on the former Sonneratia - Avicennia habitat. Carbon burial rates of these young forests were estimated to be 298 g C m^-2 yr^-1 during the last 100 years and 580 g C m^-2 yr^-1 during the last 20 years, respectively, which were faster than those estimated using the long-term Rhizophora deposits formed during several hundred or several thousand years, which were 25 to 93 g C m^-2 yr^-1. Carbon burial rate of Rhizophora forest was faster than those of other types of mangrove forest. These results suggest that Rhizophora forest has higher belowground carbon sequestration ability than other types of mangrove forest and displays its greatest ability during the early stage of the forest.

Mangrove forests usually create thick, organically rich sediments as their substrata. Most of the substrata in the tropics except under deltaic environments consist of mangrove peat which mainly derives from mangrove roots (Scholl, 1964ab; Woodroffe, 1981; Fujimoto & Miyagi, 1993; Fujimoto et al., 1995b; Fujimoto et al., 1996). This shows that mangrove forests have great belowground productivity and play a significant role in carbon sequestration not only above ground but also below ground.

There have been many studies on aboveground biomass and productivity of mangrove forests (e.g., Golley et al., 1962; Lugo & Snedaker, 1974; Briggs, 1977; Christensen, 1978; Suzuki & Tagawa, 1983; Tamai et al., 1986; Day et al., 1987; Lee, 1990; Kusmana et al., 1992; Saintilan, 1997ab), but few on the belowground biomass (Golley et al., 1962; Lugo & Snedaker, 1974; Briggs, 1977; Komiyama et al., 1987; Saintilan, 1997ab). Twilley et al. (1992) estimated the belowground carbon storage based on restricted data about the belowground biomass, excluding the sediments. This paper discusses carbon storage in the sediments of various types of mangrove habitat in the Asia-Pacific region and the burial rates using the data obtained from Pohnpei Island in Micronesia (Fujimoto et al., 1999a), the southwestern coast of the Malay Peninsula in Thailand, Iriomote Island in southwestern Japan and the Mekong Delta in Vietnam.

Pohnpei Island is situated in the humid tropics without a clear dry season (lat. 6oo 45' to 7oo 00'N and long. 158oo 05' to 20' E) and mangrove forests grow on the coral reef fringing the island though some of them are situated in estuaries. The former is referred to as coral reef type and the latter as estuary type. The typical mangrove habitat of the coral reef type consists of a 2-m thick mangrove peat layer (Fujimoto & Miyagi, 1993) and is usually covered by Rhizophora apiculata dominant forest with Bruguiera gymnorrhiza, Sonneratia alba and sometimes Xylocarpus granatum (Fujimoto et al., 1995b, Kikuchi et al., 1999).

Field research in Thailand was conducted in the Khlong Thom lowland (lat. around 7oo 50' N and long. around 99oo 05' E) and the Satun lowland (lat. around 6oo 35' N and long. around 100oo 05' E), which are mainly situated in estuarine environments in the humid tropics with a weak dry season. The mangrove forests in these areas were divided into six communities, i.e. Sonneratia alba - Avicennia alba community, Rhizophora apiculata community, R. apiculata - Bruguiera spp. community, Ceriops tagal - Xylocarpus spp. community, Lumnitzera littorea community and Excoecaria agallocha community. These six communities distributed from the middle part to the upper part of the tidal zone and corresponded to the sediments (Mochida et al., 1999).

Iriomote Island is situated in the subtropics (lat. 24oo 15' to 26' N and long. 123oo 40' to 56' E) and the mangrove forests are valuable as the most northerly in the world. Data for this study were collected from the Rhizophora stylosa community and the B. gymnorrhiza community on the small deltas at the mouth of the Nakama River and the Shiira River, respectively.

Field research in Vietnam was conducted at Tam Giang III in the Ca Mau Peninsula, southern tip of the Mekong Delta (lat. around 8oo 50' N and long. around 105oo 15' E) and the Can Gio Forest Park in eastern Ho Chi Minh City (lat. around 10oo 25' N and long. around 106oo 52' E) in the humid tropics with a clear dry season. Data for this study were collected from the R. apiculata pure forest converted from the mixed forest with R. apiculata that developed about 50 years ago on the flood deposit of ca. 100 years ago at Tam Giang III and the 20-year-old R. apiculata plantation on the former Sonneratia ovata - Avicenia spp. habitat in Can Gio that was destroyed by the herbicides and defoliants used by the United States during the Vietnam war.

A hand-operated, piston-type, thin-wall sampler 7.5 cm in diameter was used for sampling non-disturbed cores, which cannot penetrate coarse live roots and undecomposed large organic debris. Two-centimeter thick samples for estimating carbon content were collected at intervals of 10 to 30 cm. The remains were used for analyzing coarse organic matter content. The coarse organic matter including live roots was isolated by wet sieving using 0.5 mm meshed sieves. Total carbon and nitrogen contents were analyzed using a CN corder (YANACO MT-600). Samples collected in Vietnam were analyzed by the oxidation-reduction titration method for carbon content in the laboratory of the Ho Chi Minh City Agriculture & Rural Development Service.

Table 1 shows the stored carbon in the four areas researched in the Asia-Pacific region. The stored carbon in a coral reef-type mangrove habitat covered by R. apiculata dominant forest consisting of a 2-m thick mangrove peat layer in Pohnpei Island, which is a type of mangrove habitat found in tropical Pacific islands, was estimated at 130 kg C m^-2 (=1300 t C ha^-1) and that in an estuary-type habitat reached around 200 kg C m^-2 up to 3.5m in depth (Fujimoto et al., 1999a). In southwestern Thailand, the stored carbon up to 1 m in depth in the R. apiculata, R. apiculata - Bruguiera spp. or Xylocarpus spp., C. tagal forests were estimated to be about 50 kg C m^-2, 39 kg C m^-2 and 27 kg C m^-2, respectively. In Iriomote Island, it was estimated to be 50 kg C m^-2 for the R. stylosa forest at the mouth of the Nakama River and 22 kg C m^-2 for the B. gymnorrhiza forest at the mouth of the Shiira River. In the Mekong Delta, it was estimated to be 33 kg C m^-2 for the R. apiculata forest on the flood deposit of ca. 100 years ago in the Ca Mau Peninsula and 27 kg C m^-2 for the 20-year-old R. apiculata plantation on the former Sonneratia - Avicennia habitat in Can Gio.

Belowground carbon storage of mangrove habitats in the Asia-Pacific region

Ra: Rhizophora apiculata, Rs: Rhizophora stylosa, Bg: Bruguiera gymnorrhiza, B: Bruguiera spp., X: Xylocarpus spp., Ct: Ceriops tagal, So: Sonneratia ovata, A: Avicennia spp.

Table 2 shows the carbon burial rates of mangrove habitats in the Asia-Pacific region. Fujimoto et al. (1999a) reported that the carbon burial rates of the mangrove peat layer formed under the R. apiculata dominant forest in Pohnpei Island were 55 g C m^-2 yr^-1 between 2620 and 1800 years BP in the falling sea-level phase, 93 g C m^-2 yr^-1 between 1800 and 1380 years BP in the rising sea-level phase and 53 g C m^-2 yr^-1 during the last 1380 radiocarbon years in the stable sea-level phase. These results suggest that Rhizophora forests display their greatest belowground carbon sequestration ability during the rising sea-level phase within the possible peat accumulation rate.

In the southwestern coast of Thailand, relatively high carbon burial rates were obtained from the R. apiculata - Bruguiera spp. or Xylocarpus spp. communities, i.e. 70 g C m^-2 yr^-1 during the last 610 radiocarbon years and 105 g C m^-2 yr^-1 during the last 710 radiocarbon years, where sedimentation rates, about 2 mm yr^-1, were also relatively high (Fujimoto et al. 1999b).

Carbon burial rates of mangrove habitats in the Asia-Pacific region

The high sedimentation rate and high carbon burial rate seemed to have been caused by the input of both inorganic and organic matters from the upper stream. In the R. apiculata community, the carbon burial rates were calculated at 25 g C m^-2 yr^-1 during the last 2920 radiocarbon years and 32 g C m^-2 yr^-1 during the last 1310 radiocarbon years. The lowest value was obtained from the C. tagal community, which was 14 g C m^-2 yr^-1 during the last 1670 radiocarbon years.

The mangrove habitat on Iriomote Island usually has a 1-m thick mangrove organic layer formed during the last 1000 years (Fujimoto & Ohnuki, 1995). The carbon burial rates estimated using the general period of formation and thickness of the mangrove organic layer were about 50 g C m^-2 yr^-1 for the R. stylosa forest at the mouth of the Nakama River and about 22 g C m^-2 yr^-1 for the B. gymnorrhiza forest at the mouth of the Shiira River.

The carbon burial rate of the R. apiculata forest in the Ca Mau Peninsula developed on the flood deposit was estimated to be about 298 g C m^-2 yr^-1 during the last 100 years, assuming that the carbon content of the lower part of the flood deposit with few mangrove roots was the initial value of the flood deposit. At Can Gio, the carbon burial rate during the last 20 years after planting the R. apiculata was estimated to be 580 g C m^-2 yr^-1, assuming that the carbon content of the lower layer with few mangrove roots was the initial value at the time of plantation.

These results suggest that Rhizophora forest has higher carbon sequestration ability than other types of mangrove forest and displays its greatest belowground carbon sequestration ability during the early stage of the forest.