Ekopreferensi Dua Jenis Avicennia Terhadap Parameter Lingkungan Di Tegakan Bakau Muara Sungai Wulan Demak

Studi mengenai ekopreferensi dan dua jenis mangrove (Avicennia marina dan A. alba) dilakukan di tegakan mangrove muara sungai Wulan, Demak. Tujuan penelitian ini diarahkan untuk melihat kemampuan ekopreferensi A. marina dan A. alba terhadap salinitas, jenis sedimen dan kandungan air. Data vegetasi diperoleh dengan menggunakan transek garis yang dikombinasikan dengan cara kuadrat yang dimodifikasi dari Cox (1967) dan Mueller-Dumbois & Ellenberg (1977). Dua stasiun pengumpulan data vegetasi ditentukan secara horisontal dari garis pasang terendah sampai garis pasang tertinggi. Pengukuran parameter lingkungan dilakukan pada transek yang ada dan tanah sampel dianalisis untuk melihat fraksi tekstur. Hasil analisis menunjukkan bahwa dalam hal keragaman vegetasi, tegakan mangrove yang ada di muara Sungai Wulan miskin dalam hal komposisi jenis dibandingkan dengan tegakan mangove yang ada di sepanjang pantai utara Jawa Tengah. Studi ini juga menunjukkan bahwa kehadiran A. marina cenderung menjadi species pelopor dibandingkan dengan A. alba. Diduga hal ini disebabkan oleh kemampuan A marina untuk beradaptasi pada kondisi salinitas dan tipe tanah yang marginal. Kata kunci: Bakau, ekopreferensi, A. marina, A. alba, Muara Sungai Wulan A b s t r a c t Study on ecological preferences of two Avicennia species was carried out in Wulan River, Demak, Central Java. The sudy was aimed at revealing the eco-preference toward salinity, sediment types and water content. Vegetation data were collected by implementing line transect approach combined with quadrate methods modified according to Cox (1967) and Mueller-Dumbois & Ellenberg (1977). Two sites for vegetation data collection were set horizontally from High Tide Mark toward Low Tide Mark. All ecological parameters were measured in situ on the established transects. The results showed that in term of mangrove species diversity this area is low or marginally poor compared to other mangrove belts along the northern coast of Central Java. The study also showed that the two Avicennia species are the pioneer species. These two species of Avicennia are adaptable to the salinity and soil types that are poor in term of nutrients content.


A b s t r a c t
Study on ecological preferences of two Avicennia species was carried out in Wulan River, Demak, Central Java. The sudy was aimed at revealing the ecopreference toward salinity, sediment types and water content. Vegetation data were collected by implementing line transect approach combined with quadrate methods modified according to Cox (1967) and Mueller-Dumbois & Ellenberg (1977). Two sites for vegetation data collection were set horizontally from High Tide Mark toward Low Tide Mark. All ecological parameters were measured in situ on the established transects. The results showed that in term of mangrove species diversity this area is low or marginally poor compared to other mangrove belts along the northern coast of Central Java. The study also showed that the two Avicennia species are the pioneer species. These two species of Avicennia are adaptable to the salinity and soil types that are poor in term of nutrients content. Key word: mangrove, ecological-preference, A. alba, A. marina, Wulan River estuary

INTRODUCTION
Mangrove ecosystem of Indonesian islands covers a wide area along the coast and the mouth of rivers. Based upon the data available from the Forestry Department, the total size of mangrove area in Indonesia is 4.25 million ha covering only 3.98 percent of the country forest. In economical senses, such size of the forest is hardly significant considering the economical role of terrestrial green forest. In spite of its minor role to the economic sense, it has an important role in a coastal zone ecosystem. Martusubroto and Naamin (1977) had reported that the total production of prawn fishery showed a positive correlation to the total mangrove area.
Northern coast of Central Java has its characterized features in comparison to other Indonesian coastal areas, that is in term of: the population density, the presence of huge areal of brackish water pond, and the limitation of mangrove area. According to Rahardjo et al. (1980) and Hendrato (1986) the information regarding the distribution of vegetation and the influencing environmental factors are still lacked. Therefore this study was designed to determine the possibility of the ecological preferences in A. marina and A. alba to the environmental: parameters and also the effects of those environmental parameters In controlling the distribution of vegetation within certain zonation. One of the mangrove characteristics is the zonation pattern of the vegetation observed in coastal line. Therefore organisms in that system ecologically have their own niche (Kartawinata et al. 1979) as shown on the variation developed within the communities and on various composition of vegetation from place to place (Chapman 1976. Lear & Turner 1977and Hedrato 1983. The ecological preference among the mangrove species, according to Steenis (1958) in Kartawinata et al. (1979) is caused by the combination of factors such as: soil type, salinity, the resistance to current and wave, and seedling condition to grow. Similar argument was reported by Chapman (1976) and Lear & Turner (1977). A specific preference to soil type was observed on genus Rhizophora Alrasjid (1977) observed that R. mucronata was commonly found in clay soil along the coastal area while R. stylosa was found in sandy and corally soils. The same condition was reported by Kartawinata et al. (1979).
As a controlling factor on the zonation development, salinity was found to be significant influencing the presence and growth of Rhizophora. Varied salinity concentrations lead to the variation in species. Despite the fact that all mangroves species can tolerate various salinity concentrations, it was observed that there are some species which do not need salt to grow (Richards 1964in Sukardjo 1984.

METHODS
Since this study was conducted in the field, a pre survey was conducted to get some information on both local topography and hydroceanography data led by secondary data. Data on vegetation were collected using 5 x.5 m quadrat modified from Cox (1967), Mueller-Dumbois & Ellenberg (1977 and Soerianegara and Inderawan (1982). Transects of 100 m were laid horizontally from the low tide mark (LTM) to the high tide mark (HTM) on the mangrove belt of Wulan river estuary. Vegetation samples were identified in the field and confirmed at Coastal Zone Ecodevelopment Laboratory, UNDIP, Jepara. Vegetation parameters of density, dominance, frequency and important value index; were calculated using the formula suggested by Cox (1967), andMuller Dumbois &Ellenberg (1977). Soil samples of 20 x 20 x 20 cm were collected concurrently from the sites along the transect line. A laboratory analysis of texture fraction and water content was done at Soil Mechanics Laboratory, the Faculty of Civil Engeneering, UNDIP.

Vegetation parameters
The Wulan river estuary is situated in the district of Demak, in the northern coast of Central Java. The illustration of Wulan river estuary is shown in Figure 1. In a succesive term, this mangrove belt has been considered to be in a secondary stage.
This study was able to identify six mangrove species which are Avicennia alba, A. marina, Acanthus ilicifolius, Cynometra ramiflora, Rhizophora conjugata, and R. mucronata. From those six members of mangrove vegetation community, A. ilicifolius and C. ramiflora are bushes. Compare to the other mangrove area along the northern coast of Central Java, this area is poor in term of vegetation composition (Rahardjo et al. 1980and Hendrato 1983, 1966. Table 1 shows the analysis for vegetation parameters determined in this study. Among the species Avicennia marina gives the highest density (372 trees per ha). This explains the ability of A. marina in occupying the space. The higher its ability to occupy the area the wider its chance to develop in the ecological zone. The highest frequency A. marina suggests that compared to A. alba, the former species is superior in occupying the ecosystem (Table 1). Besides, there has been no information whether these two species do share the same niche.
The summation of relative density, relative dominance, and relative frequency shows the important value index. Furthermore the important value index shows the ability of vegetation to adapt the interacted of environmental factors. Referring to this index, A. marina shows the highest value as well compared to A. alba and other species. The important value index of A. alba is lower than that of R. conjugata and this might be explained by the bigger trunk of the latter species.

Figure 1. The Illustration of Wulan River Estuary
Overall, A. marina is found dominant in the mangrove belt of Wulan river. All parameters revealed that the adaptive performance of A. marina which has the ability to grow in a marginal condition is influenced by the open water. Conversely, the distribution of A. alba was found to be limited only to the area that had been occupied by the pioneer species of A. marina.
The fact that seedlings of A. marina were found densely at the open water of Wulan river estuary has been merely caused by its high regenerative ability being a pioneer species. With a relatively calm water, the seed distribution will not be too far from the coastal area. Consequently, the seeds are able to grow to seedlings as was also found by Hendrato (1983) in Morodemak area.
Environmental Parameters Table 2 shows the overall results of the environmental parameters. The range of water content is 60.21-76.57 %. Such percentage is likely to have a correlation to the clay and silt content of the soil substrate. It was assumed that the distribution pattern of A. marina was influenced by the clay and silt content as well as water content of the soil. Study done by Hedrato (1983) on the mangrove ecosystem of Morodemak has proven that the presence of A. marina is correlated positively with both water content and silt concentration. Besides, it has been known that A. marina can withstand salinity as high as 90 0/00. The species also has pneumatophore roots that are useful in respiration process (Chapman, 1976).  The salinity value of 2.0 -15.00 o/oo is likely to be influenced by the minimum tidal spring at coastal areas due to the sedimentation occurred on the river estuary. Possibly, this is also caused by the huge volume of the inland freshwater. Salinity influence is clearty observed on the distribution of Avicennia marina, especially on the seedlings. Seedlings that were densed at the lower tide may obviously indicated a high tolerance ability of this vegetation to salinity.
Based on the distribution of sand, silt, and clay fractions it was found that the top soil texture of 20.0 cm depth was ranging from siltyclay loam to silty loam. The sand content tends to increase from the HTM (High Tide Mark) to the LTM (Low Tide Mark) while the silt tends to decrease. Vertical profile of the soil type was shown in Figure 2.
Analyses of environmental parameters (as shown graphically in Figure 3) demonstrate the roles of salinity, soil type and water content on the zonation and distribution of A. marina, which greatly differs from distribution and zonation of A. alba. This result was also confirmed by Hendrato (1983) in his study in Morodemak.

CONCLUSION
Based upon this study, it can be concluded that the presence of A. marina is strongly influenced by environmental factors such as salinity, soil type and water content. The distribution pattern of this species supports its presence which proves its pioneering role in the development of a new mangrove belt. The effect of other environmental factors than salinity, soil type and water content, need to be furtherly studied in relation to the distribution and zonation of mangrove ecosystems.