| The global mangroves have experienced rapid loss and the mangroves area in our country also continues to decrease. The high population density along the South China coastlines requires large lands for industrial and agricultural use, inducing the alteration of many mangrove suitable forestland to other use. In order to restore the mangrove resources and expand the area of mangrove in our country, efforts have been made on restoration afforestation at the edge of bare beach along coaslines. However, these beaches are low in elevation with deep-water flooding, and thus the mangrove seedlings are difficult to survive. To solve the problems of afforestation on these deep-water mudflat, we carried out a study at shuidong habor, Maoming City, Guangdong Province. An elevation gradient of-20 cm,-40 cm and-60 cm, relative to mean sea level, were selected for experiments. At each elevation, two experiments were set up. One is by lifting the part of beach surface with different sizes of PVC tube before planting mangroves, called mangrove tube test; and the other one is directly transplanting different sizes of seedlings, called seedling standard control test. Seedlings of Rhizophora stylosa, Kandelia obovata, Laguncularia racemosa, and Sonneratia apetala were selected as test plants. We aimed to select proper plant species and habitat manipulation technologies to aid the afforestation on deep-water mudflat. The main results are as follows:(1) The seedling survival rate of Rhizophora stylosa, Kandelia obovata, Laguncularia racemosa and Sonneratia apetala all decreased with the beach elevation decreasing, and eventually all died. This result suggests that afforestation is difficult to succeed on deep-water mudflats with conventional methods by transplanting seedlings on elevations lower than-20 cm. When the deep-water mudflat was lifted to the elevation of 15 cm by tubes, the survival rates and preservation rates after one year were significantly increased for all four mangrove species. This result suggests that partially lifting the elevation by tubes is helpful for the afforestation on the deep-water mudflats. In addition, by transplantating big-sized seedlings(tree height about 1.5 m) and saplings(tree height about 2 m) of Laguncularia racemosa and Sonneratia apetala on deep-water mudflats lower than-20 cm, the preservation rates also maintained at a relatively high level. Therefore, both lifting the beach elevation and directly transplanting big-sized mangrove seedlings are effective technology for afforestation on deep-water mudflats.(2) In the tube test, the size of the tubes and the original elevation of the beach have little effect on the survival rate and preserving rate of transplanted seedlings. But the tube size has significant effects on seedling growth rate, with seedlings in larger size tubes performing better than in smaller tubes. As seedlings grow bigger, the difference of seedling growth between different size tubes become more significant. For example, the tube size of 10 cm diameter inhibited the normal growth of mangrove seedlings due to space limitation. However, such inhibition was relatively weaker in tubes of bigger size such as 30 cm in diameter. In addition, the tube test showed that the original elevation of the beach had no significant effect on seedling growth, which further demonstrated that the tube method is helpful for afforestation on deep-water mudflats by lifting the local elevation.(3) In the tube test, the growth rates of seedling height, ground diameter and crown width showed dynamic seasonal changes, with the growth indexes rates being especially rapid in summer and autumn. The fast-growing species, Laguncularia racemosa, showed the most obvious seasonal pattern within one year, which can be divided into four obvious growth stages: from December to February as the growth stagnation period, February to April as the initial growth period, from April to October as the fast-growing period, and from October to December as the slow growth period. Correlation analysis showed that the growth rate of tree height and crown range were significantly positively related to climatic elements such as average maximum temperature, mean minimum temperature, the average temperature, rainfall and sunshine hours, indicating that the higher temperature, abundant rainfall, and the long sunshine hours can promote the growth of Laguncularia racemosa tube seedlings.(4) Among the four tested mangrove species in the tube test, Laguncularia racemosa performed best after one year, with the highest preserving rate and fastest growth rate. For Sonneratia apetala, the initial survival rate was lower, but the preserving rate falls slowly and the growth indexes grew fast. Rhizophora stylosa and Kandelia obovata are native species, and had relatively slow growth rates. Therefore, in order to restore mangroves on the deep-water mudflats, robust Laguncularia racemosa and Sonneratia apetala are the priorities. Apply these fast-growing species as pioneer species, then gradually replant native trees in the improved habitats can ensure mangroves recovery in deep-water mudflat area, but also ensure the mangrove community species diversity and richness of the population.(5) Size of seedlings had a great effect on the flooding tolerance. The smaller size seedings had weaker flooding resistance, but larger size seedlings show relatively stronger flooding resistance. For example, the survival rate of Laguncularia racemosa seedlings is low after planted for two months, and eventually all died. But the survival rate of saplings in the summer test is more than 90%, and remained relatively high preservation rate six months later, showing strong anti-inundation capacity.(6) The completeness of seedling root systems during transplantation has an important impact on seedling survival and preservation. Intact root systems tended to favor higher preservation rate. For example, big seedlings of Kandelia obovata cultivated in pots had intact root systems during transplantation, and the preservation rate remained as high as 32% at the elevation of-20 cm after one year. However, for Kandelia obovata saplings growing for more than three years before transplantation, the root system growing in the ground suffered some degree artificial destruction when lifting seeding, and all died after planting for one year.(7) The inundation tolerances of native species Rhizophora stylosa and Kandelia obovata are weak. However, the fast-growing species Laguncularia racemosa and Sonneratia apetala with the height about 1.5 ~2 m have strong anti-inundation capacity, and they are more suitable for mangrove afforestation in deep-water mudflat area. By comparing the performances of different size seedlings, we can determine that the fast-growing species Laguncularia racemosa and Sonneratia apetala with the height about 1.5 ~ 2 m are the best options for restoration on deep-water mudflats. Based on our afforestation experience and the current experiments, we developed a set of practical technologies for big seedlings transplantation including digging seedlings, pruning, planting, management and maintainence after planting, and so on. |
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