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Litterfall production in a tropical mangrove ofSarawak, MalaysiaMohammed Muzammel Hoquea, Abu Hena Mustafa Kamala, Mohd Hanafi Idrisa, OsumanuHaruna Ahmedb, A.T.M. Rafiqul Hoquec & Md. Masum Billaha

a Faculty of Agriculture and Food Sciences, Department of Animal Science and Fishery,University Putra Malaysia, Bintulu Sarawak Campus, 97008 Bintulu, Sarawak, Malaysiab Faculty of Agriculture and Food Sciences, Department of Crop Science, University PutraMalaysia, Campus Bintulu, 97008 Bintulu, Sarawak, Malaysiac Department of Mechanical and Environmental Informatics, Tokyo Institute ofTechnology, Ookayama, Meguro-ku, Tokyo, JapanPublished online: 09 Mar 2015.

To cite this article: Mohammed Muzammel Hoque, Abu Hena Mustafa Kamal, Mohd Hanafi Idris, Osumanu Haruna Ahmed,A.T.M. Rafiqul Hoque & Md. Masum Billah (2015): Litterfall production in a tropical mangrove of Sarawak, Malaysia,Zoology and Ecology, DOI: 10.1080/21658005.2015.1016758

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Litterfall production in a tropical mangrove of Sarawak, Malaysia

Mohammed Muzammel Hoquea*, Abu Hena Mustafa Kamala, Mohd Hanafi Idrisa, Osumanu Haruna Ahmedb,A.T.M. Rafiqul Hoquec and Md. Masum Billaha

aFaculty of Agriculture and Food Sciences, Department of Animal Science and Fishery, University Putra Malaysia, Bintulu SarawakCampus, 97008 Bintulu, Sarawak, Malaysia; bFaculty of Agriculture and Food Sciences, Department of Crop Science, UniversityPutra Malaysia, Campus Bintulu, 97008 Bintulu, Sarawak, Malaysia; cDepartment of Mechanical and Environmental Informatics,Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan

(Received 11 November 2014; accepted 5 February 2015)

Assessment of litterfall production is essential to ascertain the status of nutrient cycling and forest health. Sarawak,Malaysia, is endowed with numerous pristine and estuarine mangroves; however, research on primary productivity ofthese forests is scanty. Therefore, litterfall production in a pristine Sibuti mangrove of Sarawak was estimated using littertraps for one year (January–December 2013), and forest structure was studied through tree census. The forest was domi-nated by Rhizophora apiculata (relative density [RD] was 77.11%), followed by Xylocarpus granatum (RD was16.92%) and other species (RD was only 5.97%). The annual dry weight of litterfalls was estimated to be1640.82 g m−2, of which R. apiculata contributed 92.94%, followed by X. granatum (4.01%) and other species (3.05%).Leaves were the most abundant (57.21%) contributory component of litters, followed by propagules (11.89%), flowers(10.85%), twigs (8.56%), and stipules (8.45%). No significant correlations were found for total litterfall and its majorcomponents with climatic variables and forest structure. Total litterfall of R. apiculata and X. granatum did not showany seasonal variations. However, leaf litter of R. apiculata showed a significant seasonal variation between intermediate(January–April) and wet (September–December) seasons. The litterfall production of Sibuti mangrove forest is higher incomparison to other tropical mangroves of the world. The higher productivity of the forest could be due to nutrientavailability, pristine nature, and stand maturity of the forest rather than climatic influences. The findings of the studysuggest that litterfall production of the pristine Sibuti mangrove is not influenced by the variability of climatic factors.

Maisto medžiagų apykaitos miške ir jo būklės įvertinimui labai svarbūs nuokritų susidarymo tyrimai. Daug žmogausveiklos nepaliestų ir nepakankamai ištirtų estuarinių mangrovių yra Malaizijoje, Saravako valstijoje. Saravake esančiojeSibuti mangrovėje nuo 2013 m. sausio iki gruodžio miško nuokritų susidarymas buvo tiriamas nuokritų gaudyklėmis.Įvertinta medžių rūšinė sudėtis: dominavo Rhizophora apiculata – 77,11% ir Xylocarpus granatum – 16,92%, kitų rūšiųmedžiai sudarė 5,97%. Sausas nuokritų svoris per metus buvo 1640,82 g/m2, iš jų R. apiculata – 92,94%; X. granatum– 4,01%, kitų rūšių – 3,05%. 57,21% nuokritų sudarė lapai, 11,89% – vegetatyvinio dauginimosi dalys, 10,85% – žiedai,8,56% – šakelės, 8,45% – prielapiai. Nuokritų ir jų dalių kiekis nebuvo patikimai susijęs su klimato sąlygų kaita armiško struktūra. Nustatyta, kad bendras R. apiculata ir X. granatum nuokritų gausumas nepriklauso nuo sezono, tačiauR. apiculata lapų nuokritų gausumas ženkliai skyrėsi pereinamojo (sausio–balandžio) ir drėgnojo (rugsėjo–gruodžio)sezonų metu. Sibuti mangrovėje nuokritų susidarymo produktyvumas yra didesnis nei kitose pasaulio tropinėsemangrovėse. Didesnis šio miško produktyvumas daugiau priklauso nuo maisto medžiagų kiekio, gamtos natūralumo irmiško brandos negu nuo klimatinių sąlygų. Tyrimo rezultatai rodo, kad klimato sąlygų kaita neturi įtakos Sibutimangrovės nuokritų susidarymui.

Keywords: litter production; Rhizophora apiculata; seasonal variation; Sibuti mangrove; Sarawak

Introduction

Mangroves are the world’s most productive and uniquewetland ecosystems in intertidal regions of the tropicaland subtropical coasts (Nagarajan et al. 2008; Estradaet al. 2014). These unique ecosystems are regularly inun-dated by tidal action, while numerous materials areexchanged within mangroves and adjacent aquatic eco-systems (Ye et al. 2011). Litter production is the mainsource of nutrient return; it enhances soil organic matterand energy (Triadiati et al. 2011). It is an indicator of theprimary productivity of mangroves in view of nutrientcycling, and also subsequently contributing to the nearby

estuarine ecosystems (Tam, Wong, and Lan 1998;Metcalfe, Franklin, and McGuinness 2011). Nutrientcycling and soil fertility in a forest ecosystem depends onthe rate and frequency of litter productivity and decompo-sition (Triadiati et al. 2011). In general, forest productiv-ity is directly related to nutrient availability and uptakeby the plants (Boto and Wellington 1983). Adequate sup-ply of nutrients is vital for maintaining the balance of amangrove ecosystem (Reef, Feller, and Lovelock 2010).Moreover, primary productivity usually varies from placeto place, species to species, and is also due to speciescomposition (Saberi 1989; Hossain and Hoque 2008).

*Corresponding author. Emails: hoque_doe@yahoo.com, gs35249@mutiara.upm.edu.my

© 2015 Nature Research Centre

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Stand age (Gong et al. 1984), forest type (Day et al.1996; Hossain and Hoque 2008), and stand density(Hossain and Othman 2005) are also responsible for thevariability of litter production in a forest ecosystem. Inaddition, litter production may be influenced by some ofthe climatic variables, especially wind speed, temperature,rainfall, and evaporation (Hossain and Hoque 2008;Farooqui et al. 2012; Sharma et al. 2012; Abu Hena et al.Forthcoming). However, very few studies of litter produc-tion in tropical forests are available in relation to monthlyclimatic variables (Sasekumar and Loi 1983).

In Malaysia, there are about 577,500 ha of mangroveforest reserves, and Sarawak is the second largest estatethat covers 23% of the total area of mangrove forests(Jusoff 2013). Most of the studies related to mangroveshave been conducted in Matang, (Gong et al. 1984; Putzand Chan 1986) Pulau Langkawi, (Latiff 2012) and afew in Selangor (Hossain et al. 2008), Malaysia. How-ever, very few research works have been conducted inSarawak mangroves, especially on ecological and silvi-cultural aspects (Chai 1974; Billah et al. 2014). In mostcases, the managed young mangrove stands with averagetree heights of 8–13 m were preferred for research on lit-ter productivity in Malaysia. For example, Matang man-groves (Gong et al. 1984), South Banjar Forest Reserve,Selangor (Sasekumar and Loi 1983), Selangor NaturePark (Hossain 2004), Siar Beach and Lundu, Sarawak(Saberi 1989) are remarkable. Studies on the ecologicalroles, especially on the primary productivity of an undis-turbed and matured forest of Sarawak, are scarce, whichleads to incomplete understanding of mangrove functionsin this region. Therefore, this study was carried out toexplore the species and temporal patterns of litterfall pro-duction, in relation to some climatic variables and foreststructure of the Sibuti mangrove, Sarawak, Malaysia.The outcome of this study would be helpful to under-stand the productive roles of an undisturbed mangrove inSarawak that could be useful for sustainable conservationand management in future.

Materials and methods

Study area

The study area, Sibuti mangrove (3°59′25.76′′N and113°43′51.6′′E), is located at the edge of the SouthChina Sea, approximately 45 km to the west of Miritown, Sarawak, Malaysia (Figure 1). The forest isbounded by Bungai farmlands in the north and SibutiRiver in the south and east. This site is an undisturbedtropical mangrove forest, (Saifullah et al. 2014) domi-nated by Rhizophora apiculata, followed by Xylocarpusgranatum and Nypa fruticans (Gandaseca et al. 2011).On average, the forest is flooded 34 times per month(DOMSM 2013). The forest is under class 3, as it isinundated more than 28 times every month (Watson1928). Topographically, the forest is almost flat from theriver bank to the inner side with a very little increase inelevation at the edge of the forest. The monthly climatic

parameter (maximum, mean, minimum temperature, andtotal rainfall) variation is shown in Figure 2. The aver-age monthly rainfall and temperature in this area were316.17 mm and 28.41 °C, respectively, in 2013 (Figure 2;Meteorological Department, Niah Forest ResearchStation, Miri, Sarawak, Malaysia). The fluctuations andoverall speed of wind were negligible and variedbetween 1.8 and 2.52 kph throughout the year with anaverage of 2.16 kph (DOMSM 2013).

Forest structure study

Three sampling plots, each of 100 m × 100 m (rivermouth, middle, and end part of the forest), were purpo-sively selected considering the structure, density, andtopography of the forest to represent the whole forest. Ineach sampling plot, four 10 m × 10 m subplots at least20 m away from each other were randomly selectedfrom the river bank to inland upward for tree censusand litterfall collection. For each species, seedlings(diameter <4 cm, height <1 m), saplings (diameter >4 cm,but <10 cm), and mature trees (diameter at breast height,DBH >10 cm) were identified, counted, and measured.The DBH (cm) and total height (m) of each plant in allthe subplots were measured (Table 1).

Litterfall study

Twelve square litter traps of 1 m2 mouths (4 in eachsample plot) were set randomly in every 10 m × 10 msubplot above the tide-level height under forest canopyfollowing the principles of Kamruzzaman et al. (2012).The trapped litterfalls were collected at monthly intervalsfrom January to December 2013. The litterfalls of indi-vidual traps were sorted into leaves, twigs, stipule, flow-ers, propagules, and others, and air-dried for 24 h. Theair-dried litterfall components were then oven-dried at80 °C until constant weight was attained.

Climatic parameters

The corresponding climatic parameter (air temperature,wind speed, and rainfall) data for the study area werecollected from January to December 2013 from thenearby Meteorological Department of Malaysia, and usedfor the determination of correlation with litterfall produc-tion.

Statistical analysis

The mean values of monthly litterfall production of dom-inant (Rhizophora apiculata) and co-dominant (Xylocar-pus granatum) species along with different componentswere calculated. Considering the three seasons (Hossainet al. 2008), namely intermediate (January–April), dry(May–August), and wet (September–December), seasonalvariation of total litterfalls and species-wise differentcomponents of litterfall production were compared by a

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one-way analysis of variance, followed by the Duncanmultiple range test by using SAS 9.2 version. Pearson’scorrelation coefficient analysis was performed to deter-mine the influence of climatic parameters (temperature,wind speed, and rainfall) and forest structure (DBH,height, tree density, RD, and basal area) on total yearlylitterfall production and its various components. The sta-tistical distribution of different litter components,recorded in each month throughout the sampling period,for both the species (i.e. R. apiculata and X. granatum)

was illustrated by the box and whisker plot. The plotsshow minimum, 25th percentile, median (marked with adash), 75th percentile, and outlier of the data using Pale-ontological Statistics version 2.15C.

Results

Forest structure

The overall stand density of the Sibuti mangrove forest was1938.46 ± 482.24 trees ha−1, 1722.22 ± 254.58 saplings

Figure 1. Location of the study area showing sampling plots in Sibuti mangrove forest, Sarawak, Malaysia.

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Figure 2. Total monthly rainfall, mean monthly maximum, mean monthly, and mean monthly minimum temperature (°C) at Sibutimangrove forest from January to December 2013.

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ha−1, and 6222.22 ± 384.90 seedlings ha−1. The tree den-sity of Rhizophora apiculata recorded was 1541.67trees ha−1, followed by Xylocarpus granatum (333.33 tree-s ha−1). The RD of R. apiculata was 77.11%, significantlyhigher than that of X. granatum (16.92%; Table 1). Thedensity of X. mekongensis and Thespesia populnea wasrecorded to be very low with 23.07 trees ha−1 each. How-ever, RD of both the species was 1.19%. The forest wasmostly dominated by R. apiculata having an average diam-eter, height, and basal area of 26.62 ± 7.05 cm, 15.69± 2.75 m, and 103.69 ± 42.79 m2 ha−1, respectively(Table 1). X. granatum was the co-dominant speciesrecorded in the forest, having an average diameter, height,and basal area of 12.39 ± 8.94 cm, 8.44 ± 4.76 m, and4.63 ± 3.70 m2 ha−1, respectively.

Litterfall production

A relatively high leaf litter production was recorded inApril, followed by February, June, and July forR. apiculata (Figure 3(a)). Peak flower litterfall wasrecorded in June, whereas the highest fruit litterfall wasfound in September (Figure 3(a)). For X. granatum, peakleaf litterfall was found in July (Figure 3(b)). Flowers wererecorded only in April, and propagules were also found inrelatively high amounts in April (Figure 3(b)). Monthlytotal litterfall production of the forest did not fluctuatethroughout the year, except for a few months (Figure 4).

A total of 1640.82 g m−2 of dry weights of litterfalls, con-sisting of leaves, stipules, flowers, propagules, and twigswere produced annually (Table 2). Leaves were the highestcontributory component (57.21%), followed by propagules(11.89%), flowers (10.85%), twigs (8.56%), and stipules(8.45%). R. apiculata alone contributed 92.94% to totallitter productivity, followed by X. granatum (4.01%) andother species (3.05%). During the study period, R. apicu-lata alone produced 1525.05 g m−2, whereas X. granatumproduced only 65.77 g m−2 of litter annually (Table 2). Incase of R. apiculata litterfall, leaves alone contributed58.62%, followed by propagules (11.98%), flowers(11.66%), stipules (9.09%), and twigs (8.66%). Similarly,X. granatum leaves were the highest contributors(68.01%), followed by propagules (18.98%), twigs(12.75%), and flowers (0.31%). Considering the seasonalvariations, the highest litterfall production was recorded inthe intermediate season (January–April), followed by dry(May–August) and wet (September–December) seasonsfor both the species (Table 2).

In R. apiculata, flowers were recorded in highestamounts in the dry season (16.06%), followed by wet(10.22%) and intermediate seasons (6.48%). Propaguleswere recorded in higher amounts in the wet season(21.41%), followed by intermediate (6.80%) and dry sea-sons (5.72%). Flowers and propagules reflect phenologi-cal characteristics as regards productivity. Both theflowers and propagules were mature, and tended to fall

Table 1. Forest structure with dominant species of Sibuti mangrove forest, Sarawak, Malaysia.

Species DBH (cm) Height (m) Density (trees ha−1) Relative density (%) Basal area (m2 ha−1)

Rhizophora apiculata 26.62 ± 7.05 15.69 ± 2.75 1541.67 77.11 103.69 ± 42.79Xylocarpus granatum 12.39 ± 8.94 8.44 ± 4.76 333.33 16.92 4.63 ± 3.70Excoecaria agallocha 9.20 ± 3.56 7.00 ± 2.12 38.46 1.99 1.77 ± 0.70Intsia bijuga 5.75 ± 0.96 6.25 ± 1.26 30.77 1.60 0.69 ± 0.01Thespesia populnea 5.33 ± 1.52 5.16 ± 0.76 23.07 1.19 0.61 ± 0.01Xylocarpus mekongensis 11.00 ± 3.60 7.33 ± 2.08 23.07 1.19 0.47 ± 0.01

Figure 3. Month-wise average litter production of different components of (a) R. apiculata and (b) X. granatum in Sibuti mangroveforest.

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off in dry and wet seasons. The box and whisker plotillustrates that the median of leaf litterfall was over 18times higher in R. apiculata (75 g m−2 month−1) than inX. granatum (4.2 g m−2 month−1). Besides, the remain-ing litterfall components were also found to be in higheramounts in R. apiculata than in X. granatum (Figure 5).

In the case of R. apiculata, seasonal variations werefound in leaf (df = 2, f = 8.27, and p = 0.0092), flower(df = 2, f = 4.38, and p = 0.0470), and propagule (df = 2,f = 8.22, and p = 0.0093) litterfall. However, stipule(df = 2, f = 0.47, and p = 0.6391), twig (df = 2, f = 3.13,and p = 0.0929), and total litters (df = 2, f = 0.47, andp = 0.6374) of R. apiculata did not show any seasonalvariations throughout the year (Table 2). On the otherhand, seasonal variations were not found for total litter-fall and other litterfall components of X. granatum,except for twigs (df = 2, f = 20.81, and p = 0.0004;Table 2).

Pearson’s correlation coefficient matrix (Table 3) didnot show any significant correlation of the forest struc-tures (DBH, height, tree density, RD, and basal area)with total litterfall in this forest ecosystem. Almost noneof the litterfall components of R. apiculata and

X. granatum had any significant correlation with the for-est structures, except for the leaf litter of X. granatum,which showed a positive correlation with basal area(Table 3).

Total litterfall as well as almost all of the litter com-ponents of both R. apiculata and X. granatum did notshow any significant correlation with the monthly cli-matic variables (mean temperature, rainfall, and windspeed). However, rainfall governed (r2 = −0.62 andp = 0.030) the flowering of R. apiculata, while twig lit-terfall of X. granatum was influenced by mean tempera-ture (r2 = 0.67 and p = 0.017) and rainfall (r2 = −0.62and p = 0.030) in this mangrove forest (Table 3).

Discussion

The riverine and estuarine mangrove forest of Sibuti ishighly dominated by R. apiculata, followed by X. grana-tum and other species (Table 1). The percentage of litter-fall was influenced by tree density available in thisforest. Usually, litterfall production and its componentsvary with species, forest type, and geographical location(Day et al. 1996; Hossain and Hoque 2008). The total

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Figure 4. Relation between total monthly litter productivity of R. apiculata and X. granatum and monthly rainfall of Sibutimangrove forest.

Table 2. Seasonal (g m−2 season−1 ± SE) and annual litter production (g m−2 yr−1) of various components and dominant andco-dominant species of Sibuti mangrove during study period.

Species Season

Litter components

TotalLeaf Stipule Flower Propagule Twig

Inter 342.39 ± 0.76a 51.57 ± 0.56a 37.17 ± 0.49b 39.03 ± 0.55b 61.53 ± 0.71a 531.69 ± 0.98a

R. apiculata Dry 302.63 ± 0.73ab 37.17 ± 0.58a 86.80 ± 0.64a 30.94 ± 0.61b 42.92 ± 0.48a 500.46 ± 1.11a

Wet 248.90 ± 0.66b 49.90 ± 0.67a 53.80 ± 0.65ab 112.70 ± 0.62a 27.61 ± 0.40a 492.90 ± 0.62a

Total (yearly) 893.92 ± 0.29 138.63 ± 0.20 177.77 ± 0.23 182.66 ± 0.28 132.06 ± 0.20 1525.05 ± 0.31Inter 16.47 ± 0.25a 0 0.20 ± 0.08a 9.24 ± 0.43a 2.00 ± 0.16b 27.91 ± 0.50a

X. granatum Dry 16.30 ± 0.31a 0 0 3.22 ± 0.29a 5.43 ± 0.09a 24.95 ± 0.29a

Wet 11.95 ± 0.32a 0 0 0 0.96 ± 0.07b 12.91 ± 0.33a

Total (yearly) 44.73 ± 0.10 0 0.20 ± 0.02 12.45 ± 0.12 8.39 ± 0.06 65.77 ± 0.14Other species 50.00Grand total 1640.82

Note: Means within a column for each species with different superscript letters are significantly different (n = 48, Duncan multiple range test)(p ≤ 0.05).

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litterfall in the Sibuti mangrove was found to be1640.82 g m−2 yr−1, which was the highest in compari-son to several other studies in the world (Table 4). Thehigher production of litterfall in the Sibuti mangrove for-est could probably be due to the pristine condition of theforest, tree maturity, and/or frequent inundation by ade-quate fresh water flow from the upstream areas. Usually,litter production is found higher in those forests whichreceive fresh water supply and are situated in rivermouths (Lugo and Snedaker 1974). Gong et al. (1984)stated that the age of the forest may play an importantrole in litter production. Like in the Sibuti mangrove, ahigher litterfall production of R. apiculata was found in

a 25-year-old Matang mangrove forest, Malaysia, and6–36-year-old forest in Ca Mau province, Vietnam(Gong et al. 1984; Clough et al. 2000).

The percentage (43%) of non-leaf components of thetotal litterfall was remarkable. In a subtropical mangroveforest of Okinawa Island, leaf litterfall constituted thelargest component of total litterfall, ranging from 52.8 to67.9% in five subsequent study periods (Sharma et al.2012). Though non-leaf components decompose slowly,they nourish soil fertility, especially when leaf litterfall iscomparatively less (Bunyavejchewin and Nuyim 2001).R. apiculata showed a significant seasonality asregards to litterfall in many mangroves of the world(Bunyavejchewin and Nuyim 2001). In contrast, the sea-sonal variations of total litterfall and its various compo-nents for both R. apiculata and X. granatum in theSibuti mangrove were not significant. This could proba-bly be due to the geographical position of the study area,(tropical area) where climatic variations are very negligi-ble. Besides, no significant variations were noticedbetween the forest structures (DBH, height, tree density,RD, and basal area) and litterfall components for boththe species. Considering the pattern of litterfall and foreststructure, it may be believed that the forest is in maturestage, whereas productivity is almost stable throughoutthe year (Figure 4). The peak leaf litterfall of R. apicula-ta was observed in the intermediate season (January–April), and a similar observation was reported byAngsupanich and Aksornkoae (1994) from PhangngaBay, southern Thailand. Peak leaf litterfall in Februarywas also recorded by Duke, Bunt, and Williams (1984)in the north-eastern Australian mangroves. The floweringof R. apiculata was found profound in the dry season,especially in June–August, and following the sequenceof flowering, the fruiting of R. apiculata was observedhighest in the wet season (September–December). Simi-lar trends of flowering and fruiting of R. apiculata werealso reported in north-eastern Australia (Williams, Bunt,and Duke 1981) and Papua New Guinea (Leach andBurgin 1985). Though flowering and fruiting of R. api-culata was prominent in dry and wet seasons, a small

Figure 5. Box and whisker plots of different litterfall compo-nents recorded in each month throughout the sampling time forR. apiculata and X. granatum in Sibuti mangrove, Sarawak.Notes: The plots include minimum, 25th percentile, median(marked with a dash), 75th percentile, and outlier of the data.The outliers are shown as * and 0. (Legend: Ra = R. apiculata;Xg = X. granatum; Lf = Leaf; Fl = Flower; Pr = Propagule;Tg = Twigs).

Table 3. Pearson correlation coefficient and significance level for various components of litters with climatic variables and foreststructure of Sibuti mangrove, Sarawak.

Species Components Mean temp Rainfall Wind speed DBH Height Tree density Relative density Basal area

Leaf 0.548ns −0.017ns −0.179ns 0.434ns 0.480ns −0.440ns −0.065ns 0.354ns

Stipule −0.049ns 0.345ns −0.345ns 0.125ns 0.333ns −0.397ns −0.249ns −0.030ns

R. apiculata Flower 0.303ns −0.622* 0.354ns −0.093ns 0.207ns −0.078ns −0.011ns −0.100ns

Propagule −0.38ns 0.317ns 0.144ns 0.271ns 0.448ns −0.464ns −0.067ns 0.221ns

Twig 0.127ns −0.372ns −0.43ns 0.215ns 0.292ns −0.023ns −0.173ns 0.213ns

Total litter 0.204ns −0.063ns −0.078ns 0.348ns 0.517ns −0.362ns −0.101ns 0.272ns

Leaf 0.213ns −0.282ns −0.158ns −0.255ns −0.320ns −0.338ns −0.234ns 0.613*X. granatum Propagule 0.487ns 0.118ns −0.228ns 0.564ns 0.462ns 0.164ns 0.163ns 0.508ns

Twig 0.668* −0.624* 0.243ns 0.097ns 0.048ns −0.246ns −0.331ns −0.006ns

Total litter 0.538ns −0.231ns −0.167ns 0.290ns 0.178ns −0.099ns −0.066ns 0.055ns

**Significant at p ≤ 0.01; ns; not significant at p ≥ 0.05.*Significant at p ≤ 0.05; for litterfall components (leaf, stipule, flower, twig) of each species, n = 144; for climatic and forest structure variables,n = 12.

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quantity of flower and fruit litterfalls were foundthroughout the year, which coincided with the findings oflitterfall production in the Malay Peninsula (Sasekumarand Loi 1983). Generally, the rainfall throughout theyear and constant temperature in the tropical region maygovern the phenology of mangroves.

The rate of litterfall production is influenced by theclimatic and environmental variables, especially windspeed, temperature, rainfall, and evaporation (Hossainand Hoque 2008; Ye, Chen, and Chen 2013). Sasekumarand Loi (1983) also stated that temperature, wind speed,rainfall, and evaporation play as a driving force for lessthan 50% of variability of litter production in Sonneratia,Avicennia, and Rhizophora dominated mangroves inMalaysia. In subtropical mangroves of Japan and China,total litterfall production was found to be related withtemperature and wind speed (Mfilinge et al. 2005;Sharma et al. 2012; Ye, Chen, and Chen 2013). Windspeed had also a profound influence on litterfall produc-tion in semi-arid mangrove forests of Karachi, Pakistan(Farooqui et al. 2012).

Except for rainfall, temperature and wind speed inthis study area did not fluctuate throughout the year.Concerning seasonal variations of different componentsas well as total litter productivity of both the species,none of the climatic variables (temperature, wind speed,and rainfall) showed a significant positive correlation(Table 3). The overall monthly wind speed and fluctua-tions were also negligible, and varied between 1.8 and2.52 kph throughout the year (DOMSM 2013).

Insignificant correlation may be due to a negligible andlower fluctuation of climatic variables in the study area.Similarly, no correlation of rainfall and wind speed withlitter productivity was found in a Rhizophora mangleforest of Natal, Rio Grande do Norte, Brazil (Ramos-e-Silva, da Silva, and de Oliveira 2006). Though rainfallin the study area fluctuated throughout the year (101.3–691.25 mm; Figure 4), it did not show significant corre-lation with total litterfall and its major components ofboth the species, except for flowers of R. apiculata andtwigs of X. granatum (Table 3). Similar to our findings,Lopez-Portillo and Ezcurra (1985) did not find correla-tion between litter production and rainfall. Zhou et al.(2007) also did not find any significant relation betweenrainfall and litter production in the Dinghushan Bio-sphere Reserve, China. They opined that rainfall mainlycontrols vegetation growth by increasing the content ofsoil water as well as atmospheric humidity. Temperatureand rainfall were negatively correlated with leaf litterfallin a coastal plain forest (Marques and Oliveira 2004)and also in another rain forest of Atlantic (Morellatoet al. 2000). During the study, it was hypothesized thatclimatic variables might have a significant influence onthe rate of litter productivity, and after correlating thevariables with total litterfall and its different components,correlation was not found as important as initiallyexpected. Similar results were also found in some otherstudies throughout the world; in particular, no influenceof mean and minimum temperature on litter production(Zhou et al. 2007) and no correlation of temperature and

Table 4. Comparison of litterfall production estimates of R. apiculata from mangroves in the regional and other tropical coasts.

Species Location LatitudeDry wt of litterfall

(g m−2 yr−1) Source

Rhizophora apiculata Australia 1115.00 Bunt (1982)Rhizophora sp. Southwest Florida 25°02′N 810.30 Twilley, Lugo, and Patterson-

Zucca (1986)Rhizophora sp. Bonny estuary, Nigeria 4°22′N 846.80 Abbey-Kalio (1992)Rhizophora sp. Churute estuary, Ecuador 2°35′S 646.05–1065.8 Twilley et al. (1997)R. apiculata Mandovi-Zauri estuaries,

India1171.65 Wafar, Untawale, and Wafar

(1997)R. apiculata (Planted) Mekong delta, Vietnam 8°47′N 941.7–1737.4 Clough et al. (2000)R. apiculata (Planted) Camau province, Vietnam 8°50′N 886.95–1416.2 Nga et al. (2005)Rhizophora apiculata Camau province, Vietnam 1416.00 Nga et al. (2005)In MalaysiaR. apiculata Sungai Merbok, Malaysia 5°40′N 1007.40 Ong, Gong, and Wong (1980)Rhizophora sp. Kuala Selangor, Malaysia 3°15′N 1576.80 Sasekumer and Loi (1983)R. apiculata (Planted) Matang mangrove,

Malaysia4°50′N 697.15–1138.8 Gong et al. (1984)

R. apiculata Matang mangrove,Malaysia

762.85 Gong et al. (1984)

R. mucronata + R. apiculata Siar Beach, Lundu,Sarawak

1°45′N 572.00 Saberi (1989)

R. apiculata + X.granatum + Other species

Sibuti mangrove, Sarawak 3°59′N 1640.82 This study

Rhizophora apiculata Sibuti mangrove 1525.05 This studyXylocarpus granatum Sibuti mangrove 65.77 This studyOther species Sibuti mangrove 50.00 This study

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humidity with litter productivity (Farooqui et al. 2012)were found. Our findings coincided with the opinion ofPallardy (2008), who mentioned that heredity is theprime influencing factor for the shedding of leaf forsome tropical trees, instead of negligible variability ofclimatic changes.

Conclusion

The Sibuti mangrove is a highly productive ecosystem interms of primary productivity, especially through litter-fall. The dominant species R. apiculata is playing signifi-cant productive roles compared to co-dominant species.An insignificant influence of climatic variables (tempera-ture, rainfall, and wind speed) and forest structure onoverall litter productivity indicates that a higher produc-tivity of this mangrove forest may be due to other favor-able variables such as stand maturity, vegetationheredity, and nutrient availability. As a riverine–estuarineforest, it receives a high amount of nutrients as well asfresh water flow, which could be another attribute tocomplement its higher productivity. At the same time,the pristine nature of the mangrove forest also leads to ahigher productivity of this mangrove ecosystem. So, wesuggest further intensive researches on nutrient dynam-ics, rate of nutrient release through decomposition, can-opy closure, and other factors to establish acomprehensive scenario of the Sibuti mangrove ecosys-tem, which ultimately will be helpful for taking anappropriate decision in terms of silviculture, conserva-tion, and sustainable management of this pristine forest.

AcknowledgementsThe authors acknowledge the Department of Animal Scienceand Fishery, UPM Campus Bintulu Sarawak for providingfacilities, and Malaysian Government for providing fundsthrough E-Science, Project No. [04-01-04-SF1422]. Specialthanks to Sarawak Biodiversity Center (SBC) and ForestDepartment (FD) of Sarawak for giving permission to carry outthis field work. Thanks to Mr Khurshid Alam Bhuiyan for pre-paring the study area map.

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