Vegetatio 106: 137-146, 1993. M. M. Grandtner and T. Kikuchi (eds). Vegetational Patterns in Relation to Landforms. © 1993 Kluwer Academic Publishers. Printed in Belgium.

137

Development process of tidal-flat type mangrove habitats and their zonation in the Pacific Ocean A geomorphological study

K. Fujimoto ~ & T. Miyagi 2

1Kyushu Research Center, Forestry and Forest Products Research Institute, 4-11-16, Kurokami, Kumamoto, 860, Japan; 2Department of Geography, Tohoku-Gakuin University, Sendai, 980, Japan Accepted 18.6.1992

Keywords: Late Holocene, Mangrove habitat development, Philippines, Pohnpei Island, Sea-level change, Zonation

Abstract

Using the sites of Pagbilao, the Philippines and Pohnpei Island, the Federated States of Micronesia, zonation and development process of mangrove habitats on tidal flats situated in the geomorphic environment excluding estuary, delta, and lagoon or backmarsh behind barrier or beach ridge were discussed from the viewpoint of geomorphology. Zonations of the mangrove forests were observed from seaward to landward in both areas. Most of the zones correspond with the variations of the ground level or deposit. Mangrove peat which has a thickness of about 2 meters was deposited in the main part of the mangrove habitats in both areas. On the other hand, some large Sonneratia alba were observed in the Rhizophora apicuIatahabitat on Pohnpei Island. The authors presumed that some of the large S. alba have survived by regeneration from fallen stems since the mangrove forest developed on the present site. The maximum depth of the mangrove peat layer reaches 1.7 meter below the present sea level in Pagbilao and over 2.5 meters at Pohnpei Island. The bottom of the mangrove peat was dated at about 2,000 y.B.P, in both areas by the radiocarbon method. The mangrove peat depositional areas have not been moved during the last 2,000 years. Therefore, the mangrove forests seem to have grown in the present sites since 2,000 y.B.P, and accumulated peat in connection with the subsequent sea-level rise.

Nomenclature: Scientific names for mangroves follow Tomlinson (1986).

Introduction

Mangrove habitats are divided into three types based on their geomorphological situations, which Papers presented at the Vth INTECOL Congress at Yokohama 1990.

are the type located in an estuary or delta (estuary-delta type), the type located on and around a back marsh or lagoon behind a barrier or a beach ridge (backmarsh-lagoon type), and the type located on a tidal flat situated in the geomorphic environments excluding an estuary, delta, backmarsh and lagoon (tidal-flat type). The

138 development of the mangrove habitat of the estuary-delta type is mainly controlled by fluvial processes (Thorn et aI. 1974; Kikuchi et al. 1978, 1980, etc.). The framework of the backmarshlagoon type one has developed in connection with the bar or beach ridge development (Miyagi et al. 1989). These two types of mangrove habitats are generally stable, except when the geomorphic processes creating these geomorphological situations are interrupted. On the contrary, though the tidal-flat type one seems to have scarcely been affected by these geomorphic processes, it has been developed during late Holocene time. The main purpose of this paper is to clarify the development processes of the tidal-flat type mangrove habitat to understand the mechanism maintaining its habitat from the view point ofgeomorphology, using examples both in Pagbilao, the Philippines and at Pohnpei Island, the Federated States of Micronesia. In addition, the relationship between

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the substructural conditions and the mangrove zones is discussed.

Mangrove habitat of Pagbilao, the Philippines Pagbilao is located in the southern part of the Luzon Island (Fig. 1). It is observed that mean tidal range is 91 cm, and diurnal tidal range reaches 146 cm, at Tayabas River entrance situated at 14 km west of Pagbilao. Profiles located on Fig. 2 are shown on Fig. 3a-b-c. The profiles a and b are situated on the tidal flat, and the profile c is situated along a river. They show the topography, geology and vegetation. Along the profiles a and b, Avicennia marina is found on the seaward fringe, followed landward by Rhizophora apiculata. On profile a, Avicennia officinalis forest with Ceriops decandra and Heritiera littoralis forms the innermost zone. Osbonia

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octadonta grows between the A. officinalis forest and the R. apiculata one. Along the profile b, Bruguiera gymnorrhiza forms the innermost zone. A. marina stands mainly on the depositional area of sand. R. apiculata and B. gymnorrhiza stand on the

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A. officinalis grows on the depositional area of flood deposits behind the mangrove peat area. Along these profiles, mangrove peat overlies the sand layer which contains shell fragments. The maximum depth of the boundary between the mangrove peat layer and the sand layer reaches about 1.7 m below the present sea level. Radiocarbon ages obtained from the bottom of the mangrove peat indicate about 2,000 y.B.P. From these facts, the development process of this mangrove habitat seems to have been as follows. The mangrove forest formed at about 2,000 y.B.P, in the present site, when the relative sea level stood at 1.7 m below the present one. The subsequent relative sea-level rise resulted in the accumulation of mangrove peat, which has enabled the mangrove forest to maintain its position and habitat.

The profile c along the river shows a mixed forest constructed by A vicennia officinalis, Ceriops decandra, Ceriops tagal and so on. The geological structure is rather complicated. Radiocarbon ages obtained from this profile are younger than those from the profile a on the tidal flat. These facts indicate that an active erosion and sedimentation occurred along the river.

Mangrove habitats on Pohnpei Island Pohnpei Island, East Caroline Islands, is located at about 2,000 km to the east of the trenches between the Pacific plate and the Philippine Sea plate (Fig. 1). Hence, the crustal movement characteristic of the mobile belt is not recognized in this island, but a subsidence caused by hydroisostatic adjustment is recorded (Bloom 1970; Matsumoto et al. 1986). It is observed that mean tidal range is 70 cm, and mean spring tidal range is 104 cm at Pohnpei Harbor. Fig. 4 shows Pohnpei Island and the distribu-

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tion of mangrove forests around it. They occurs especially in the south and the west, i.e. on the leeward of the trade winds. We made topographical, geological and botanical surveys in the southwestern part of the island. Profiles located on Fig. 5 are shown on Fig. 6. Along these profiles, we found the following zonation. Rhizophora stylosa is found on the seaward fringe. Rhizophora apiculata and Bruguiera

gymnorrhiza constitute the main part of the mangrove forest. Xylocarpus granatum forms the innermost zone. All of these forests develop on the mangrove peat depositional area. However, the habitat of X. granatum is situated near the coluvial footslope, where the ground level is slightly higher than in the other habitats. On the other hand, along the profile b, some large S. alba which has a diameter of more than 1 meter stand in the

tagal, C: Cepifora sp., Ea: Excoecaria agallocha, HI: Heritiera littoralis, LI: Lumnitzera littorea, Nf: Nypa fluticans, Oo: Osbonia octadonta, Ra: Rhizophora apiculata, R: Rhizophora sp., Sa: Sonneratia alba, Sc: Sonneratia caseoralis, Xg: Xylocarpus granatum, X: Xylocarpus sp., 1. mangrove peat, 2. peaty clay, 3. clay, 4. silt, 5. fine sand, 6. medium sand, 7. coarse sand, 8. granule or pebble, 9. wood fragment, 10. shell fragment. (after Fujimoto et al. 1989, revised).

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R. apiculata forest. These large S. alba seem to have survived by vegetative regeneration in spite of environmental changes. The mangrove peat overlies the sand layer or coral bench. The mangrove peat layer shows its maximum depth at more than 2.5 meter below the present sea level at the seaward margin of the mangrove forest along profile a. Radiocarbon ages obtained from the bottom of the mangrove peat indicate about 1,200 to 1,900 y.B.P. These facts indicate that, by accumulation of peat as the sea level rise, the mangrove forests in these areas have continued to live since 1,900 y.B.P. It is possible that the radiocarbon age of mangrove peat doesn't always offer the correct data of the paleo-sealevel, because the penetration of roots from above, the reworking of peat by the burrowing crabs and so on may occur. However,

it is worth noticing that the almost same age as Pagbilao was obtained from the bottom of the mangrove peat in Phonpei Island. It seems that the coral fragments dated as about 5,900 y.B.P, shown in Fig. 6.(1), which were obtained from about 1.7 m below the present sea level, are reworked ones, because Bloom (1970) reported four radiocarbon dates indicating that the relative sea level of this island at that time stood at about 4 m below the present one.

Discussion

Features of the zonation observed in these mangrove forests fall into the types found in Chapman (1976). Most of the zones correspond with the variations of the ground level or deposit as men-

143

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( 1 ) T o p o g r a p h y , geology and vegetation along profile a on Pohnpei Island. A: height of mangrove, B: vertical scale of topographical and geological profiles, Bg: Bruguiera gymnorrhiza, Ra: Rhizophora apiculata, Rs: Rhizophora stylosa, Sa: Sonneratia alba, Xg: Xylocarpus granatum, 1. mangrove peat, 2. c l a y e y p e a t - peaty clay, 3. silt, 4. medium sand, 5. coarse sand, 6. granule or pebble, 7. humus, 8. shell fragment, 9. coral fragment, 10. coral block and coral reef, 11. horizon of 14C dating (After Miyagi & F u j i m o t o 1989, revised.)

tioned above. The authors were especially interested in the existence of large Sonneratia alba in the Rhizophora apieulata habitat on Pohnpei Island, and presumed that some of the large S. alba have survived by regenerating from the fallen stems since the mangrove forest developed on its present site. On the other hand, it is clear that the mangrove habitats in these two areas have developed by accumulating the mangrove peat during the last 2,000 years of the sea-level rise as mentioned above. The mangrove peat overlies the marine sand layer or coral bench, which formed in the

lower-tidal or sub-tidal environment, at a few meters below the present sea level in both areas. This fact indicates that the sea level was a few meters below the present one at about 2,000 y.B.P., so that the upper part of the marine sand layer or coral bench became an environment which enabled the mangrove forest to develop. Fig. 7 shows the curve proposed for the relative sea-level changes and a schematic representation of the mangrove habitat development on Pohnpei Island. The relative sea-level changes curve was shown in Fujimoto and Miyagi (1990), using the radiocarbon dates of the mangrove peats reported

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See the key of Fig. 6.(1). (After Miyagi & Fujimoto 1989).

by Miyagi and Fujimoto (1989), Matsumoto et at. (1986) and Bloom (1970). The development process of a tidal-flat type mangrove habitat on Pohnpei Island may be summarized as follows. Stage 1: Around 3,000y.B.P., the sea level continued to rise. The mangrove habitat developed on the inner part of the tidal flat depending on the rate of the sea-level rise and the landform structure. The coral bench or platform developed widely on the outer part of the habitat. Stage 2: In accordance with the relative sealevel rise until 2,500 y.B.P., the mangrove habitat extended landward and mangrove peat continued to be accumulated. The sea level rose to about 1 meter below the present one.

Stage 3: Around 2,000 y.B.P,, the sea level fell to about 2.5 meters below the present one. The mangrove habitat shifted consequently seaward. The former mangrove habitat seems to have been replaced by a salt marsh or a back swamp forest. Stage 4: The sea level gradually began to rise again. It is during this period that the mangrove habitat extended not only landward but also upward as the peat accumulated with the sea-level rise. In Pagbilao, the base of the mangrove peat layer accumulated in Stage 3 reaches 1.7 m below the present sea level, but in Pohnpei Island, it reaches 2.5 m below the same level. The difference in the maximum depth of the mangrove peat indicates the difference of the relative subsidence

145

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rate between Pagbilao and Pohnpei Island, caused by different trends of the hydro-isostatic adjustment. But the basic development process of the mangrove habitat in Pagbilao seems to be the same as in Pohnpei Island. Namely, the fall of the sea level around 2,000 y.B.P, provided an initial condition for the present mangrove habitat formation, and the mangrove peat accumulation during the sea-level rise after that has enabled the mangrove forests to maintain their location and habitats in both areas. The sea-level fall around 2,000 y.B.P, was observed not only in these two areas but also in Japan Islands (Fujimoto, 1990). These areas seem to be respectively situated in the different trend area of the hydro-isostatic adjust-

merit. Hence, the sea-level fall about 2,000 y.B.P. is possibly an eustatic change. Therefore, most of the mangrove habitats of a tidal-flat type seem to have generally developed in connection with the process indicated in this study. Mangrove habitats of a tidal-flat type as they are now are geomorphically unstable, because the initial condition for the habitat development has already disappeared. The habitat has been maintained by the mangrove peat accumulation only during the last 2,000 years. If a mangrove forest of this type is cut down, therefore, the coastal erosion will begin immediately. Moreover, if the rate of the sea-level rise caused by greenhouse effect exceeds that of the mangrove peat accumu-

146 lation, most of the mangrove habitats of this type will be lost. The mangrove habitat of a tidal-flat type is the most fragil of all types of mangrove habitats.

Acknowledgements The authors are grateful to Professors T. Tamura and T. Kikuchi of Tohoku University for their unfailing guidance, Mr. T. Ludwig of Department of Conservation & Resource Surveillance of Pohnpei State, F. S. M. and Mr. M. Gawel of Department of R & D of F. S. M. for their kind support for the field research work in Pohnpei Island, and to Miss E. Melana of Department of Environment and Natural Resources, Philippines, for cooperation for the research in the Philippines.

References Bloom, A. L. 1970. Paludal stratigraphy ofTruk, Ponape and Kusaie, Eastern Caroline Islands. Geol. Soc. Am. Bull. 81: 1895-1904. Chapman, V.J. 1976. Mangrove vegetation. Cramer. Germany. 447 pp. Fujimoto, K. 1990. Successional reconstruction of late Holoeene sea-level fluctuations in Matsushima Bay, Northeastern Japan. Geographical Review of Japan, Ser. A 63: 629-652. Fujimoto, K. & Miyagi, T. 1990. Late Holocene sea-level fluctuations and mangrove forest formation on Ponape Island, Micronesia. J. Geogr. (Tokyo) 99: 507-514.

Fujimoto, K., Miyagi, T. & Melana, E. 1989. Fundamental study to predict effects to mangrove habitat by rapid sea level rise due to greenhouse effect, in the suburbs of Pagbilao, Philippines. In: Miyagi, T. & Maximino, G. eds), The processes of mangrove habitat development and their destruction by human impacts in Luzon Island, Phillipines. pp. 31-43, APIC, Japan. Kikuchi, T., Tamura, T., Makita, H. & Miyagi, T. 1978. Arrangement of swamp forests and landforms of the alluvial plain in the lower Nakama River, Iriomote Island, Ryukyu Archipelago, Japan: I. Mangrove swamp. Ann. Tohoku Geog. Assoc. (Sendai) 30: 71-81. Kikuchi, T., Tamura, T., Makita, H. & Miyagi, T. 1980. Arrangement of swamp forests and landforms of the alluvial plain in the lower Nakama River, Iriomote Island, Ryukyu Archipelago, Japan: II. Barringtonia racemosa forests and Pandanus tectorius thickets. Ann. Tohoku Geogr. Assoc. (Sendai) 32: 71-81. Matsumoto, E., Matsushima, Y. & Miyata, T. 1986. Holocene sea-level studies by swampy coastal plains in Truk and Ponape, Micronesia. In: Sugimura, A. (ed), Sea-level changes and tectonics in the middle Pacific. pp. 95110, Report of HIPAC project in 1984 and 1985 (Second Research). Miyagi, T. & Fujimoto, K. 1989. Geomorphological situation and stability of mangrove habitat at Truk Atoll and Ponape Island in the Federated States of Micronesia. Sci. Repts. Tohoku Univ., Ser. 7 (Geogr.) 39: 25-52. Miyagi, T., Melana, E., Hirano, S., Fujimoto, K. & Ajiki, K. 1989. The development of mangrove habitat and their landform condition in the Luzon Island, Philippines. In: Miyagi, T. & Maximino, G. (eds), The processes of mangrove habitat development and their destruction by human impacts in Luzon Island, Philippines, pp. 1-15, APIC, Japan. Thorn, B. G., Wright, L. D. & Coleman, J. M. 1975. Mangrove ecology and deltaic-estuarine geomorphology: Cambridge Gulf-Ord River, Western Australia. J. Ecol. 63: 203232. Tomlinson, P. B. 1986. The botany of mangroves. Cambridge University Press. Cambridge, England. 413 pp.