Pertanika 7(2),"91-99 (1984)

Structure of Mangrove Vegetation at Siar Beach, Lundu, Sarawak.

SABERI BIN OTHMANUniversiti Pertanian MalaysiaCawangan Sarawak, Kuching,

Sarawak

RINGKASAN1

Suatu penelitian tentang tumbuhan 'Mangrove' telah di lakukan di Muara Sungat Star, Lundu,Sarawak. Hasil penelitian ini menerangkan penzonan tumbuhan 'Mangrove' yang didapati berdasarkanordinasi plot. Penelitian ini juga membincangkan hubungan diantara penzonan tumbuhan di hutan 'Man-grove' Muara Sungai Siar ini, dcv.gan 'Mangrove' lainnya di Sarawak khasnya dan Malaysia umumnya.

SUMMARY

A study of mangrove vegetation was conducted at Siar River estuary, Lundu, Sarawak. The zona-tion of plants is described based upon the ordination of stands. The relation between the zonation ofplants at this mangrove forest with other mangrove vegetation is also discussed.

INTRODUCTION

Mangrove and estuarine vegetation are amongthe types of natural vegetation which are highlysubjected to destruction. Because of their increas-ing commercial value, the exploitation of man-grove forest is acclerating with time. This activitywould disturb the coastal environmental whichwould affect the aquatic productivity in particularand the ecological conditions in general. It mayalso create problems of ensuring adequate re-generation of the mangrove forests.

In Sarawak, the mangrove forests cover anarea of approximately 173,614 ha of which40,874 ha have been constituted forest reservesand protected forests (Chai, 1974). These forestsare concentrated along the coastlines and estuariesof the Sarawak River (1st Division), the RejangDelta (6th Division), and the Trusan River (5thDivision) (Chai, 1974). Those that have notbeen constituted reserved or protected are sub-ject to continuous destruction. These forestsusually are situated close to the fishing village ora recreation centre. The mangrove forest at SiarBeach is in this category.

This study attempts to determine the distri-bution of plants in Siar River estuary, near Lundu,1st Division, Sarawak.

STUDY AREA

The study area is located at the Siar RiverEstuary, between latitudes 109°52E-109° 53'Eand longtitudes 1° 44'N - 1° 45*N, (Wolfendenand Haile, 1963). This estuary is about 12 km.north-east of Lundu (Fig. 1). The river which isabout 1.28 km. long, and flows into the SouthChina Sea is shallow — about 2m. at low tide andabout 4m. wide at high tide.

The Siar River Estuary area mainly consistsof mangrove vegetation in which the prominentspecies are Rhizophora mucronata, R. apiculata,Bruguiera cylindrica, Avicennia alba and Sonne-atia alba.

The soils of this area are derived from biotiteadamellite rocks (Wolfenden and Haile, 1963).These include hybridised rocks ranging in compo-sition from gabbro to adamellite.

The climate of the area is of the wet type.The rainfall curve is located above the 100mmmark, with an annual rainfall of 3868.5mm (Mayanand Ngiam, 1981). The climatic diagram for theMeteorological Station at Kuching is presentedin Fig. 2.

Key to author's name: O. Saberi

91

SABERI BIN OTHMAN

South China Sea

Sematan

Kalimantan

Seratok

Siar River

i

South China Sea

1.6 km.

Fig. 1. Star beach of Lundu, Sarawak.

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STRUCTURE OF MANGROVE VEGETATION AT SIAR BEACH, LUNDU, SARAWAK.

oJ

0J F M A M J J A S O N D J

Fig. 2, The climate diagram for Kuching.

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SABERI BIN OTHMAN

MATERIALS AND METHODS

Twenty plots, each measuring SxlOm, wereestablished randomly along the Siar River (Fig. 3).Within each plot, all trees having a diametergreater than 10 cm. were identified and recorded.

Detection of Natural Groupings of Species:

The relative density of each species was usedfor the analysis of plant association by standordination method. Stand ordination is flexible,and an efficient technique, which objectivelyutilises continuous measures of abundance of allspecies in the construction of the ordinationdiagram (Bray and Curties, 1957; Mueller-Domboisand Ellenberg, 1974; Kershaw, 1975; Othman,1980), is necessary.

Fig. 3. The distribution within the study-site ofthe 5 x 10 m plots used for the measure-menat of density and ordination analysisof vegetation

RESULTS

Species Composition:

Within the plots and their surrounding, 12species of typical mangrove trees and shrubs wererecorded (Table 1).

Rhizophora mucronata was the dominantspecies, whilst R. apiculata was the co-dominantspecies. Table 2 shows the density of species foundin the study area.

TABLE 1List of species occuring within the plots.

The Nomenclature follows Backer and VanSteenis (1951), Ding Hou (1958), Kochumen

and Wyatt-Smith (1964), and Chai (1975)

SpeciesARECACEAE

1. Nypa fruticans Wurmb.

2. Oncosperma sp.

POLYPODIACEAE

1. Acrostichum aurewm L.

RHIZOPHORACEAE1. Bruguiera cylindrica Bl.2. B. parviflora (Roxb.) w. & A. ex. Griff.3. Rhizophora apiculata Bl.

4. R. mucronata Lamk.

SONNERATIA£EAE

1. Sonneratia alba J.E. Smith.

AVICENNIACEAE

Avtcennia alba Bl.

TABLE 2Density (Number of trees per ha.)

found in the study area

Species

Rhizophora mucronataR, apiculataAvtcennia alba

Bruguiera cylindricaB. parviflora

Sonneratia alba

Nypa fruticansOncosperma sp.

Density

360/ha

240

112

50

46

80

25

20

Natural Grouping of Species

Had the vegetation of stands integeratedcontinually and evenly, that is there were nosharp zonation of vegetation, and even distribu-tion of stands could have been expected in the

94

STRUCTURE OF MANGROVE VEGETATION AT SIAR BEACH, LUNDU, SARAWAK.

2-diamensionaI stand ordination diagram. How-ever, had there been a sharp zonation, clumps ofstands spatially seperated could be expected in thestand ordination diagram. Figure 4 appears to showsomething midway between these two expecta-tions. There appear to be two groups of looselyassociated stands seperated by a gap of about 20%of x-axis, and 80% of Y axis respectively. Theseare tentatively termed 'grouping' 1 and 2. Incomparing the two groupsings it can be seen thatRhizophora mucronata was present in both butdominant in "grouping 1' (Fig. 5). In contrastto R. mucronata, Bruguiera cylindrica wasalso present in both but dominant in 'grouping2' (Fig. 6). Avicennia alba, Sonneratia albaand B. parviflora was only found in 'groupings1 * (Fig. 5). Species of Rhizophora apiculatawas almost evenly distributed in both groupings(Fig. 6). Small patches of Nypa fruticans andOncosperma spp. can be seen in the study area,and largely confined to 'grouping 2* (Fig. 6.).When the density of the species of the two groupsof stands suggested by ordination is examinedseperately, marked differences can be seen. 'GroupT is clearly dominated by R, mucronata andto a much lesser extent by Avicenni alba, Bru-guiera parviflora and Sonneratia alba, whilstin the 'Group 2' R. apculata and Bruguieracylindrica have almost similar density and A.alba, and B. parviflora are virtually absent.

• 6

' Group 2 f

•18 # 1 2

2 «

Jg

Fig. 4. Ordination of X / Z values of the 20stand sin Fig. 3. According to their X andZ values, the stands have been subjective-ly placed into two groups:—

DISCUSSION

Stand ordination of the study site at SiarBeach, Lundu, revealed two groupings of vege-tation, that is 'grouping 1' and *grouping 2 \The stands of *grouping 1* are confined to theseaward side, whilst the stands of 'grouping 2'are confined to the landward side (Fig. 3). R.mucronata was the most abundant plant inthe study site on a density basis. If the densityin the two stands are examined seperately, thedensity of R. mucronata, B. parviflora, S. albaand A, alba are highest in 'grouping 1' (Fig. 5).'Grouping 2' was characterised by the presenceof R. mucronata, R. apiculata, B. cylindrica,Nypa fruticans and Oncosperma sp. (Fig. 6).

Fig. 5M.

Fig. 5.b.

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SABERI BIN OTHMAN

The 'groupings' suggested from ordinationof stands may explain the zonation of mangroveplants at Siar Beach, Lundu, The distributionof A. alba, B. parviflora and 5. alba are re-stricted to the area frequently inundated bysea water. Whilst, the species of N. fruticansand Oncosperma sp. are confined to the area

seasonally inundated by sea water, that is duringequinoctial high tide. Thus, there is a clear zona-tion of plants in Siar Beach, Lundu that is theA. alba — B. parviflora — S. alba zone and N.fruticans — Oncosperma sp. zone. Similar zona-tion have been reported for mangrove in Penin-sular Malaysia (Watson, 1928; Chapman, 1976),Banyuasin at Musi River estuary, South Sumatra

Sonneratia

O •*8.

••

Fig.

alba

/

5.c.

* •

Fig. 5.d.

Fig. 5. The relative density presentation ofplants belonging to 'Grouping V in thestand ordination.

a. R.b. A.c. B.d. S.

mucronataalbaparvifloraalba

6OoO

1 - 20%

21 - 40%

41 - 60%> - 61%

Brufcuiera

o" o

ri

Fig. 6.a.

cylindrica

\

O7 ° Y~\7oo O

Fig. 6.b.

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STRUCTURE OF MANGROVE VEGETATION AT SIAR BEACH, LUNDU, SARAWAK.

Nypa fruticans

OFig. 6.c.

Oncosperma spp.

•

*

/

o

# o0

Fig. 6.d.

Fig. 6. The relative density presentation ofplants belonging to 'Grouping 2' in thestand ordination.

a. R. apiculatab. B. cylindricac. N. fruticansd. Oncosperma spp.

Representation of relative density valuesare as in Fig. 5,

(Seokerdjo and Kartawinata, 1979), and in otherparts of Sarawak (Chai, 1975). The species ofR. apiculata and B. cylindrica are present inboth distributional areas. However, their densities

are highest in 'grouping 2'. Whilst, R. mucronatais present in both groupings, its density ishigher in 'grouping 1', that is at the seawaidside.

The Possible Factors of Influnce for Groupings

Mangroves are naturally stressed ecosystems,and natural stressors include salinity. There isa possible gradient in salinities from theseaward to the landward side. The speciesin 'grouping 1' are subjected to tidal inunda-tion more frequently compared to the speciesin 'grouping 2', that is Oncosperma sp. and N.fruticans. Thus, it is possible that the species in'grouping V tolerate higher soil salinities com-pared to the species in 'grouping 2\ Differences insoil salinities have been reported to be correlatedto the distribution of mangrove species (e.g.Walter and Steiner, 1936; Mac Nae, 1968; DeHan, 1931, Davis, 1940; Chapman, 1944 etc.).Avicennia sp. has been proven to secrete saltsfrom leaf (Me Millan, 1974). Other species, R.mucronata, Bruguiera sp. and Sonneratia sp.have a low xylem sap tension (Scholander, 1968).The ability of these species to regulate salt balancein their tissues may explain why these speciescan tolerate high soil salinities. Thus, this explana-tion may justify the confinement of these speciesat the seaward margin in Siar Beach, Lundu.Avicennia marina showed its optimum growthin solution half the concentration of seawater(Conner, 1969).

Other possible factors that influence thegrouping of mangrove plants in Siar Beach, Lundu,is tidal water. Tidal control on the spread of theplants from the seaward margin to landwardmargin is related to the degree of submergence,or exposure, or the salinity encountered at therespective levels. Tidal inundation classes of DeHaan (1931) and Watson (1928) showed a closerelationship with the distribution of mangrovespecies (Chapman, 1975). The confinement ofparticular mangrove species to the particularinundation classes in Sarawak has been describedby Chai (1975). The occurence of species atrespective inundations class in Siar Beach, Lundu,is in agreement with that of Chai (1975).

Relation between Plant Zonation and SuccessionalProcess.

Does the plant's zonation in Siar Beach,Lundu, recapitulate successional process? Toanswer this question, let us have a look into the dis-tribution of plants in this area. A. alba occupiedthe clayey area of the study area, whilst, 5. albaoccupied the area with mixed sand and clay.

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SABERI BIN OTHMAN

Both species have been reported to be pioneerspecies in both areas respectively (Chapman,1975). R. mucranata is distributed in bothzones, although its densities is highest in zonewhere A. alba and S. alba grow. One of thespecies in 'grouping 1\ B. parviflora, is not apioneer species. Usually, this species is establishedbehind the Avicennia stand (Chai, 1975). Theestablishment of B. Parviflora in mixed stand ofR. mucronata and A. alba could be explaineddue to the character of its seed and the condi-tion of the ground. It is quick to establish,especially on the soft clayey ground, perhapsbecause of the small size of the seedling (Chap-man, 1975).

When the ground is more firm and dry, thestand is characterized by the establishment ofa mixed zone of Oncosperma sp. and Nypafruticans. Here, Oncosperma sp. is establishedat the landward margin of the mangrove. Oncos-perma sp. and Nypa fruticans are included inthe inundation class 6 of de Haan (1931) andinundation class 5 of Watson (1928), i.e. floodedonly by abnormal or equinoctial tides. Theseareas are situated at the landward edge of themangrove bordering with dry land forest. Chai(1975) reported that Oncosperma sp. is esta-blished at the landward margin of the mangrove,bordering with dry land forest.

Although the species of R. apiculata andB. cylindrica are distributed in both groupings,their densities are higher in 'grouping 2\i.e. where the ground is sticky clay. These specieshave been reported in Watson's inundationclass 3 (Chai, 1975). Since the occur-ence of these species is in-between the twogroupings, they can be regarded as a transitioncommunity between pioneer community R.mucronata-A. alba-S. alba and Oncosperma sp.-N.fruticans community. Thus, there is a possiblesuccessional process operating in the mangroveecosystem at Siar Beach.

ACKNOWLEDGEMENTS

I wish to thank the Universiti PertanianMalaysia, for financial support, and En. AhmadHusni Mohd. Haniff for helpful comments duringthe preparation of the manuscript.

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STRUCTURE OF MANGROVE VEGETATION AT SIAR BEACH, LUNDU, SARAWAK.

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(Received 4 August 1983)

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