The mangrove is a characteristic vegetation in tropical zone, but so far as China is concerned it may also extend to Fuding district (27˚20′ N), situated at the north-eastern part of Fujian, South China.The subtropical mangrove of Fujian consists of six species belonging to five families. They are Bruguiera gymnorrhiza and Kandelia candel of Rhizophoraceae, Excoecaria agallocha of Euphorbiaceae, Acanthus ilicifolius of Acanthaceae, Aegiceras corniculatum of Myrsinaceae and Avicennia marina of Verbenaceae. In the southern coastal regions from Zhaoan to Xiamen all six species are present. The communities are composed of kandelia candel+Bruguiera gymnorrhiza-Aegiceras corniculatum association, Kandelia candel-Aegiceras corniculatum association, Avicennia marina association and Kandelia candel association. The most widely distributed community is Kandelia candel-Aegiceras corniculatum association. Kandelia candel is the dominant and the hightest trees reaching a height of eight metres. In the middle coastal regions from Xiamen to Putian, there are four species (Kandelia candel, Aegiceras corniculatum, Acanthus ilicifolius and Avicennia marina). In the northern coastal regions from Putian to Fuding, there is only one association dominated by Kandelia candel. The salinity of sea water may greatly affect the morphological and physiological characters of mangrove plants. It usually shows that as the osmotic pressure raises, the leaves become thick and coriaceous, whilst their stomata sink into the epidermis. In addition, it is also related with the growth and development of mangrove plant and coexisted condition of aquatic animals. Our observation indicates that when the salinity of sea water is low, for instance at 7.5–21.2%, Kandelia candel can grow normally and bring forth blossoms and fruits; the five-year-old trees may reach 1.6-2 metres in height and Balanus adhere to it in much less in number or may be absent. When the salinity of sea water is high, at 25.6–37.4‰ Kandelia candel grows weakly and the five-year-old trees are only about one meter in height and their leaves become small and succulent; they do not bloom normally, some may even die (about 30% in the area surveyed) and there often are large number Balanus attaching to the trunk. The growth in height of Kandelia candel may be related with the edaphic nitrogen content(%) of the second layer (20–40cm.), since the content of the first layer soil (0–20cm.) is not stable, and is readily influenced by the waves, the contents from the second layer were determined. There exist semilogarithmic linear correlation between them, its formula is as follows:
log y = 0.9397 + 3.124x
where y is the tree height (m.), x is the edaphic nitrogen content(%). r=0.8875, df=7, determined range: tree height in 0.5–2.2m., edaphic nitrogen content in 0.3–0.43(%).
The correlation between the growth in the height of Kandelia candel and the soil salinity appears to be a parabolic curve.
In the mangrove ecosystem, the ecological function of animals is also significant, but former investigators paid little attention to this. The insect could eat more than 30% of the leaves of mangrove plants, for example, in the community of Kandelia candel of Sanjiangkou of Putian, 95% of the leaves were found eaten by a species of tortrix moths. There are two kinds of animals adhering to the trunk and leaves of tidal forest, i. e. Chithamalus sp., which adhere to blade and Balanus amphitrite albicostatus which affix on the trunk and branches. Since they are so large in number that even the whole surface of the trunk can be covered with them. The amount of animals on the trunk of a Kandelia candel tree (height 53 cm., diameter 2 cm.) was calculated, which totals to 506 Balanus. This explains why the growth of the plant is seriously affected.
There are benthic animals under the tidal forest, such as Sesarma sp., Pachygrapsus crassipes and Littorinopsis melanostoma and so on. They dig holes into the ground. For example, there are 122 holes, each of 0.5–1.2 cm. diameter in a 70× 70cm2 quadrat; this porous ground is greatly in favour of edaphic ventilation, favourable to the growth of plants. The mangrove is a natural shelterbelt, which has the effect of decreasing the tidal force, so it provides the tidal establishment and the building of the sludge beach. It is also of great economic value, such as the utilization of hypocotylar starch, corticose tannin and trunk wood etc. Therefore, care should be taken to protect such plant communities
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