<I>Bruguiera Sexangula</I>

BULLETIN OF MARINE SCIENCE, 47(1): 139-148, 1990

STUDIES ON LITTER FALL AND DECOMPOSITION OF BRUGUIERA SEXANGULA (LOUR.) POIR, COMMUNITY ON HAINAN ISLAND, CHINA

Changyi Lu and Peng Lin

ABSTRACT The litter fall of two Bruguiera sexangula communities at Dongzhai Harbour of Hainan Island was studied for 4 years, from 1984 to 1987. The mean litter production amounted to 12.55 ± 1.62 Mg'ha-"yr-' and 11.04 ± 0.18 Mg' ha-I.yr-I (dry matter) respectively in two communities, artificial and natural forest. The monthly production fluctuated not only with the monthly mean air temperature but also with the typhoon and strong wind. The decomposition rate of leaf litter of B. sexangu/a was measured seasonally from spring to winter in 1984 and 1985 at a mangrove stand. The results showed that the leaflitter decomposition half-times in the four seasons were 21, 20, 26 and 45 days respectively, and that marine animals played an important role in the decomposition process. By providing organic detritus a great deal of mangrove litter is incorporated rapidly into estuarine and coastal ecosystems. Thus mangroves are significant contributors to the food chains of these ecosystems.

By being able to support many consumers the mangrove is considered a highly productive community and an important primary producer (Odum and Heald, 1975). In the southeast coast of China, the mangrove is also an important part of the ecosystem of estuaries and coasts. People living at the littoral found ob- jectively that mangrove swamps might have an active and significant relation to fishery catches, and fish breeding in mangrove areas is much more productive than in non-mangrove areas (Lu and Lin, 1987). For example, in 1963 there were thriving mangrove forests near the Qukou experimental farm of Aquatic Institute, Hainan Island, the fishery yield from this farm was high. In 1967 these mangroves were cleanly felled and the farm was abandoned; in 1978 this farm was started again and its fishery yield was no more than 7% as much as before (Lin, 1986). Despite this likely relationship observed between mangrove and fishery, there was no other information available in China to illustrate this relationship theo- retically. The mangrove, like all other plants, requires nutrients and minerals for growth. The overall significance ofthe movement and transformation of matter and energy in the community is that mangroves utilize imported inorganic matter and export organic matter as plant debris which support inshore food chains (Lear and Turner, 1977). So it is evident that the "litter production" of mangrove as debris and the "litter decomposition" are two critical links in the material circulation and energy flow of the mangrove ecosystem. Some overseas studies have been carried out on processes. In China litter fall and decomposition of mangrove leaves, Kandelia candel, in the Jiulongjiang River Estuary of Fujian, have been studied for more than 6 years (Lin et al., 1985; Lu et al., 1988; Lu and Lin, 1988). The present paper describes measurements of the litter dynamics by trapping litter fall in two communities of Bruguiera sexangula, and the results of the leaf litter decomposition experiments at a mangrove stand in Hainan Island. These data should be helpful in illustrating the function of the mangrove in the coastal ecosystem and its important role in the subtropical inshore food chain for China.

139 140 BULLETIN OF MARINE SCIENCE, VOL. 47, NO. I, 1990

'The Qiong zhou Hainan Island Channel 13

The Gulf TonKin N

o 30 60 90 Km a.....-J 1-----1

Figure I. Sketch map shawing the mangrove distributian in Hainan Island. In this map the arabic numerals represent lacalities .ofmangrave respectively as fallaws: I = Haikau (2/0.13), 2 = Qiangshan (20/1733), 3 = Wenchang (26/2000), 4 = Qianghai (18/8), 5 = Wanning (16/13), 6 = Lingshui (15/ 20), 7 = Yaxian (20120), 8 = Ledang (10/2), 9 = Dangfang (4/8), 10 = Changjiang (3/ I), II = Zhanxian (16/530), 12 = Lingaa (\2/168), 13 = Chengmai (12/333); the parentheses abave indicating (Number .of species/Number .ofarea (ha» (Lin and Lu, 1985).

STUDY AREA

Hainan Island is the secand largest island in China and the largest Special Ecanamic Zane .ofChina. An ideal habitat far mangrove is faund around the caast .of this island. There, 29 mangrave species belonging ta 19 genera in 15 families make up 94% .of the mangrave species in China. Furthermore, Hainan Island has the vastest mangrave resaurces, being the mangrove distributian centre .of China (Fig. I). Twa cammunities were chasen at Dangzhai Harbaur Mangrave Reserve in nartheast Hainan Island adjacent ta Qiangshan (19°51 'N, 110024'E), where the greatest number .of mangrave species .occurs. One site lacated at He-Gang Farest (HGF), 10-50 m inland .of He-Gang Bay. There B. sexangula is the daminant species assaciated with small numbers .ofB. gymnorrhiza and B. sexangula var. rhynchopetala. Alsa same shrubby Acanthus ilicifolius and A. ebracteatus graw clase ta the graund surface. Tree density was II trees· 100 m-2• LAI was 5.9. Canapy height ranged fram 9-14 m. This artificial cammunity was 55-years .old (by 1984) and was undisturbed and well-pratected as "Feng Shui" farest (divimHary farest) by the villagers. Anather site was lacated at Changning River Farest (CRF), 200- 250 m inland from the Changning River, where B. sexangula is alsa the daminant species assaciated with rare B. gymnorrhiza and B. sexangula var. rhynchopetala but there were almast na accampanying species an the graund. This secand natural cammunity was 48-years .old (by 1984) and was near the Management Centre Statian. It is well-protected, with a canapy height that ranged fram 7-10 m. Accarding ta the recards (1973-1987) .ofthe nearby Shanjiang Weather Stati.on, the caastal nartheast area .of Hainan Island is typical .ofa s.outhern subtropical maritime climate with an annual mean air temperature .of23. 7°C. The lawest manthly mean temperature was 17.3OC(in January); annual range LV AND LIN: MANGROVE LEAF DECOMPOSITION, CHINA 141

Figure 2. A part of study site HGF, showing the litter fall traps supported under the canopy of the B. sexangula forest.

was 11.2°C. Rainfall is seasonal (annual mean of 1,942 mm). Study site HGF is within the upper intertide zone, infrequently flooded by tides. The soil of stand HGF is mostly solid sludgy clay with a high organic content and the salinity of the substratum (20-40 cm depth) was 140/00(in February 1984). Study site CRF is within the middle intertide zone, with more frequent tidal inundation. The soil of stand CRF is softer than that of HGF. The salinity of substratum of CRF (20-40 cm depth) was 18-240/00.

METHODS

Litter Fall Production. - Litter fall was measured for 4 years (from 1984 to 1987) by using 15 (at HGF) and 10 (at CRF) litter traps which were baskets constructed by 1.2-mm mesh size plastic coated fibreglass mesh, forming a vertically projective area of 1 m2• These traps were supported ""2.5 m above ground beyond the reach of spring tides but under the canopy, and set out in a randomized pattern at each site (Fig. 2). Litter fall material was collected from each of these traps at 10- (at HGF) and 5-day intervals (at CRF); separated into leaf, dead-branch, flower and fruit (including hypocotyle) compartments and then promptly air-dried. The proportional random samples were oven-dried monthly at 105°C and weighed to obtain the monthly total dry weight of litter fall production. Leaf Litter Decomposition. - Experiments were carried out to examine the decomposition of leaf litter in mangrove stands during the four seasons, in 1984 and 1985. Freshly fallen litter leaves of B. sexangula were collected, removed of all dirt and associated animals, and divided into 66 similar samples, each sample consisting of ""17 g fresh litter leaves ("" 10 g of dry weight in each sample depending on the conversion factors between wet and dry weight). The samples were placed in 25 x 20 cm fibreglass screen bags: 33 bags with 1.2-mm mesh size and another 33 bags with 9.0-mm mesh size. Three samples in every different type of mesh size bags were selected at random, dried at 105°C and weighed to obtain initial weight for future comparison. The remainder were enclosed and then fastened to the prop-roots on the mud surface at randomly chosen positions in the CRF study site. As described above, site CRF with more frequent tidal inundation was flooded by all high tides. The litter bags and the mud surface there were always very wet. Three bags of each mesh size were collected randomly once a week following initiation of experiments in every season. After the bags were rinsed in water to remove soil and associated animals (if found), they were opened and the residual leaves were dried at 70°C for 72 h, and then weighed. The experiments began on 9 April 1984 and 23 April 142 BULLETIN OF MARINE SCIENCE, VOL. 47, NO. I, 1990

Table I. Monthly litter fall production of B. sexangula forest in Dongzhai Mangrove Reserve of Hainan Island during 1984-1987 (dry wt g'm-2)

He-Gang Forest (HGF) Changning River Forest (CRF) Month 1984 1985 1986 1987 Mean ± SO 1984 1985 1986 1987 Mean ± SO Jan 84 84 68 44 70 ± 19 35 46 29 25 34 ± 9 Feb 70 97 59 112 85 ± 24 45 84 55 69 63 ± 17 Mar 155 79 190 99 131 ± 51 120 47 208 103 120 ± 67 Apr 104 118 74 98 99 ± 18 72 141 75 110 100 ± 33 May 101 123 113 105 111 ± 10 104 160 110 131 126 ± 25 Jun 102 147 113 84 112 ± 27 119 128 106 103 114 ± 12 Jul 114 92 154 131 123 ± 26 97 83 78 129 97 ± 23 Aug 126 134 174 127 140 ± 23 91 101 133 131 114 ± 21 Sep 286 111 258 59 179 ± 111 230 124 167 95 154 ± 59 Oct 76 109 67 59 78 ± 22 77 99 72 81 82 ± 12 Nov 97 62 53 72 71 ± 19 83 59 45 75 66 ± 17 Dec 72 61 50 51 59 ± 10 42 44 29 26 35 ± 9 Total 1,387 1,217 1,373 1,041 1,255 ± 162 1,115 1,116 1,107 1,078 1,104 ± 18

1985 as the spring experiments, on 13 July 1984 and 26 July 1985 as the summer experiments, on 21 October 1984 and 8 October 1985 as the autumn experiments and on 23 January 1984 and 23 January 1985 as the winter experiments.

RESULTS AND DISCUSSION Litter Fall Dynamics. -The monthly data collected during the period January 1984 to December 1987 are shown in Table 1. Based on the data, it can be estimated that the litter fall production of B. sexangula forest was 13.87,12.17, 13.73,10.41 Mg·ha-1·yr-1 for HGF and 11.15, 11.18,11.07,10.77 Mg'ha-I'yr-1 for CRF in 1984, 1985, 1986, 1987 respectively. The mean values of annual litter fall production were 12.55 ± 1.62 and 11.04 ± 0.18 Mg·ha-l.yr-1 for HGF and CRF respectively. Analyses of every compartment in litter fall showed that over the whole observation, 68.9% of the litter fall collected was leaf litter. Litter production values for mangrove forests worldwide were found to range

Table 2. Annual litter fall production (dry matter) for mangrove forests in some region ofthe world

Liner fan Mangrove type (g·m-'·yr-1) Reference

Scrub mangroves, SE Florida. USA 120 Pool et aI., 1975 Avicennia germinans, SW Florida, USA 444 Twilley et al., 1986 A. germinans/Rhizophora mangle, SW Flori- da, USA 810 Twilley et al., 1986 R. mangle, S. Florida, USA 896 Lugo and Snedaker, 1974 Mixed forest, Pinones, Puerto Rico 970 Pool et aI., 1975 Mixed forest, S. Thailand 832 Aksornkoae and Khemnork, 1984 R. apiculata, S. Thailand 670 Christenson, 1978 R. apiculata, Matang, Malaysia 971 Gong et al., 1984 R. apiculata, Hinch. Is., Australia 1,113 Bunt, 1982 A. marina, Sydney, Australia 580 Goulter and Allaway, 1979 Kandelia candel, SE Fujian, China 921 Lu et al., 1988 Mixed forest, Matang, Malaysia 2,340 Ong et al., 1982 B. sexangula, He-Gang, Hainan Island, China 1,255 This study B. sexangula, Changning River, Hainan Is- land, China 1,104 This study LV AND UN: MANGROVE LEAF DECOMPOSITION, CHINA 143

Table 3. Litter fall production of B. sexangula forest compared with those of some other productive forests in southern China

Liller fall Locality Soil type Vegetation (g'ffi-"yr-') Reference XYN* Laterite Rain forest 1,155 Nanjing Inst. Soil ScL, 1978 HNI* Laterite Secondary forest 1,020 Nanjing Inst. Soil Sci., 1978 J E F* Beach saline soil Mangrove, 20-yr-old 921 Lin et a!., 1985 K. candel forest HGF* Beach saline soil Mangrove, 48-yr-old 1,104 This study B. sexangula forest CRF* Beach saline soil Mangrove, 55-yr-old 1,255 This study B. sexangula forest • x Y N = Xishuangbanla, Yunnan; H N I = Hainan Island; J E F = Jiulongjiang Estuary, Fujian; H G F = He·Gang Forest; and C R F = Changning River Forest.

from 120 g·m-2·yr-1 for scrub mangroves in south Florida to 2,340 g'm-2'yr-1 for a 20-year-old managed forest in Malaysia (Table 2). The comparison of mangrove litter fall production in the world and the comparison of litter fall production of B. sexangula forest with those of other types of tropical forest which were studied and found to be more productive at litter production in China revealed that the litter production of B. sexangula forest was quite high and its litter production value was very close to those productive forests or even higher (Table 3). Litter fall rates varied seasonally. Some other observations indicated that litter fall productions were correlated with the air temperature. Goulter and Allaway (1979) found that litter fall showed marked seasonal fluctuations, highest in summer and lowest in winter for Avicennia marina in Sydney of Australia. Lin Peng et al. (1985) found that the monthly peak litter fall of Kandelia candel occurred in July when the monthly mean air temperature was the highest. The tendency of monthly variety in K. candellitter fall was almost identical with the fluctuation of monthly mean air temperature in Jiulongjiang Estuary, Fujian, China, having a regression equation oflitter production (Y, g·m-2·mo-1) and monthly mean air temperature (X, 0C): Y = 5.677X - 41.67, (r = 0.81, N = 12) (Lu et al., 1988). Seasonality in litter fall was also found in this study (Fig. 3). The regression equation oflitter fall (Y, g·m-2·mo-1) and monthly mean air temperature (X, DC) was Y = 5.009X - 13.18 (r = 0.44, N = 48) for HGF, Y = 6.511X - 61.01 (r = 0.62, N = 48) for CRE In fact, seasonal variation relates not only with the air temperature but also with other meteorological factors, for example, wind, soil salinity (Twilley et al., 1986), air humidity, and phenology of the plant itself. Litter fall, as Brown (1984) discussed, is caused by senescence or stress, by me- chanical factors and by a combination of these. From Figure 3 it can be found that the monthly peak litter fall did not always occur in July when the monthly mean air temperature was the highest, but always occurred in the month when violent typhoons or continuous strong winds in the dry season hit the forests, for example, in February 1987, March 1984 and 1986, and September 1984 and 1986. If the data of wind effectingfactors were rejected, more correlative regression 2 1 equation on monthly litter fall production (Y, g·m- ·mo- ) and monthly mean air temperature (X, 0C) could be obtained as follows: Y = 4.608X - 15.27 (r = 0.66, N = 43) for HGF and Y = 6.497X - 69.01 (r = 0.80, N = 43) for CRE The resulted data (Table 1)also revealed that there were variations oflitter fall production between months within a year as well as between years of collection. Bray and Gorham (1964) suggested that the ratio of litter fall maximum value to minimum value (Max/Min) could be used for comparing yearly variation oflitter 144 BULLETIN OF MARINE SCIENCE, VOL. 47, NO.1, 1990

-20 o o

-10 Monthly mean air temperature

8 Total litter fall

6 for C R F

C) 2

~ 8 C

o + + I I t I. I I I. I I I I I I I I I I. I I I I I I I I J M s J M S J M S J M S J

1984 1985 1986 1987

Figure 3. Dynamics of total Htter fall production of B. sexangula forest and relative temperature and wind factors from 1984 to 1987 at Dongzhai Harbour Mangrove Reserve of Hainan Island. CRF = Changning River Forest site, HGF = He-Gang Forest site. Arrow indicates there was typhoon or continuous strong wind influence in this month. LU AND UN: MANGROVE LEAF DECOMPOSITION, CHINA 145

1Il 1Il 111 E ~ ii" c: 'c,.t: .•.o ..o ~D ...8 40 CD Q. f ,/. o 7 14 21 28 42 49 Time (days Figure 4. Loss of dry mass from litter leaves of B. sexangula in mangrove stand during spring (A, circles, solid line), summer (B, triangles, dashed line), autumn (C, dots, solid line), winter (D, squares, solid line) experiments. Data are expressed as percentage of mean dry weight in 4-group samples. Two groups were 1.2-mm mesh-size experiments and another 2 groups were 9.0-mm mesh-size experiments in 1984 and 1985, each group containing 3 decomposing leaf samples. fall. They summarized ratios of litter fall in many communities over the world except data for mangrove forests, finding the ratio was about 1.1-3.2, the highest was 5.1. Based on this 4-year observation, Max (1984)/Min (1987) was 1.33 for HGF and Max (1985)/Min (1987) was 1.04 for CRF, which reflected a more steady-state characteristic of this flux in these two mangrove stands, either in the artificial community or in the natural community. In general, however, longer periods of observation yield higher ratios (Bray and Gorham, 1964). So the study on litter fall dynamics is being carried out continuously in Hainan Island. Litter Leaf Decomposition. - The resulted data from the decomposition experiments during spring, summer, autumn and winter repeated in two years (1984 and 1985) were listed respectively. Based on the average of the experimental data, dynamics were shown in coordinate diagrams (Fig. 4). From Figure 4 the tendency of loss of dry mass from litter leaves of B. sexangula at a mangrove stand during four seasons can be seen easily. These diagrams also give the leaf decomposing half-times (in that time leaves lost half of their dry weight) from spring to winter as about 21, 20, 26 and 45 days. Leaf decomposition rates revealed the following trends: Summer 2: Spring 2: Autumn> Winter. The seasonal variation of decomposition rates and tendency (Fig. 4) except the value in winter experiments were not markedly different compared to the data obtained in more northern subtropical Jiulongjiang Estuary, where the decomposing half-times from spring to winter were respectively 26, 18,24, and 56 days for K. candel and 14,6, 11, and 25 days for A. marina (Lu and Lin, 1988). For comparison purposes two mesh sizes, smaller (1.2 mm) and larger (9.0 mm), of decomposition bags were adopted. Fell et al. (1984) pointed out that (smaller) mesh size should allow small invertebrates free access to the leaves. The large invertebrates will be excluded, which is a major drawback of mesh bags. 146 !BULLETIN OF MARINE SCIENCE, VOL. 47, NO. I, 1990

100 Began on 23 Jan 1984 Began on 23 Jan 1985 eo - ~ _60 - 40 ~ I: 020

~O 1 n 7 14 21 211 35 42 49 56 7 14 21 28 35 42 49 56 =100 I: .- eo Began on 9 Apr 1984 Began on 23 Apr 1985

0 - 40 "ii 20 0 'D= 7 14 21 28 -;;100 14 m Began on 26 Jul 1985 -eo Began on 13 Jut 1984 -m 60 ftI=40 'S2l m u 0 1 1 7 14 28lU35 42 7 14 21 28 meo ~- Began on 21 Oct 1984 Began on 8 Oct 1985 60

0 7 14 21 28 lh35 Time (days) Figure S. Comparison of percentage changes of dry weight of B. sexangu/a decomposing leaves in different mesh-size experiments at Changning River Forest site (CRF). Open bars are the data from 9.0-mm mesh-size experiments; hatched bars from 1.2-mm mesh-size experiments. All values are means (N = 3) of percentage changes (residual to initiation of dry weight).

They said they had not observed large invertebrates feeding on the leaves of Rhizophora mangle; however, this may be important in other mangrove species. In our studies it was found that a large number of grazing or saprophagous animals such as fiddler crab, mudwhelk and other larger invertebrates when permitted to enter the larger mesh size bag associated themselves with the decomposing leaves. There were relatively obvious differences in the decomposition rates between two comparative groups of litter decomposition bags (Fig. 5), most of the sample groups with 9.0-mm mesh size bags showing significantly greater rates (P < 0.05) than those with 1.2-mm mesh size bags. It is evident that besides microorganisms, some marine animals, either small invertebrates or larger invertebrates, even some fish, such as Boleophthalmus pectinirostris etc., feed on decomposing leaves, hence playing an important role in the decomposition and fragmentation oflitter leaves. So larger mesh size decomposition bags should be used for decomposition experiments in order to renect the natural state more fully, as revealed in our case. Mangroves are highly productive wood communities in the subtropical estuarine and coastal areas, being an important primary producer in food webs of these ecosystems by providing organic debris in litter fall. Leaves are mostly not eaten directly by herbivores, although a few insects eat them before they fall from LV AND LIN: MANGROVE LEAF DECOMPOSITION, CHINA 147 the tree. So the transference way of energy and matter in this ecosystem has to be mostly dependent upon the large part of litter fall which is subject to coloni- zation at mangrove swamp environments and partially broken down by microorganisms and some other lower marine animals before it is available to food chains of higher animals. In this study, it was contirmed that mangroves in Hainan Island have higher productivity on litter fall production and higher litter decomposition rates, con- stituting a highly mobile cycle, and that via decomposition, the energy and matter in mangrove litter fall are rapidly incorporated into the rest of the ecosystem. The cycle of nutrients in mangrove ecosystems in general increases the fertility of the littoral. Giving more attention to protect and exploit rationally this valuable mangrove resource will produce good results in the development of Hainan Island.

ACKNOWLEDGMENTS

This project was supported by National Natural Science Foundation of China. We thank Wang Gongli and Chen Huanxiong for their kind cooperation in the field work in Hainan Island. We are also indebted to Chen Rendong and Xie Yuexin for their assistance, and to Ding Mingzhao for his computer skills.

LITERATURE CITED

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the Research for Development Seminar, the Mangrove Ecosystem. Published by the Aust. Inst. of Mar. Sci. --- and ---. 1988. Litter leaf decomposition of two species of mangrove. J. Xiamen Univ. (Nat. Sci.) 27(6): 679-683. ---, F. Z. Zheng and P. Lin. 1988. Study on litter fall production of Kande/ia candel mangrove community in Estuary. J. Xiamen Univ. (Nat. Sci.) 27(4): 459-463. Lugo, A. E. and S. C. Snedaker. 1974. The ecology of mangroves. Ann. Rev. Eco!' Syst. 5: 39-64. Nanjing Institute of Soil Science, Academia Sinica. 1978. Soil in China. Science Publishing House, Beijing. Odum, W. E. and E. J. Hea.ld. 1975. The detritus based food web of an estuarine mangrove community. Estuar. Res. 1: 265-286. Ong, J. E., W. K. Gong and C. H. Wong. 1982. Productivity and nutrient status oflitter in a managed mangrove forest. Symposium on mangrove forest ecosystem productivity. BIOTROP-UNESCO, Bogor, Indonesia. Pool, D. J., A. E. Lugo and S. C. Snedaker. 1975. Litter production in mangrove forests of southern Florida and Puerto Rico. Pages 213-237 in G. E. Walsh, S. C. Snedaker and H. J. Teas, eds. Proceedings of the International Symposium on Biology and Management of Mangroves. Institute of Food and Agricultural Sciences, Univ. of Florida, Gainesville, Florida, U.S.A. Twilley, R. R., A. E. Lugo and C. Patterson-Zucca. 1986. Litter production and turnover in basin mangrove forests in southwest Florida. Ecology 67(3): 670--683.

DATEACCEPTED: July 31,1989.

ADDRESSES:(c. Y.L.) Institute oj Environmental Science, Xiamen University, Xiamen, Fujian, China, 36/005: (PL) Department oj Biology, Xiamen University, Xiamen, Fujian, China, 36/005.