Conservation of Hong Kong Natural Coastline through Mangrove Replanting  

Project Title: Conservation of Hong Kong Natural Coastline through Mangrove Replanting

Applicant: City University of Hong Kong

Total Approved Grant: $1,311,638.73 (Jointly funded by ECF and WWGF on 50/50 basis)

Duration: 01/03/2001 to 30/09/2004

Project Status/Remarks: Completed


The present project aims to conserve the mangrove ecosystems along the coastline of Hong Kong through replanting of mangroves. A series of greenhouse experiments and field trials will be conducted to understand the scientific basis of mangrove replanting and to enhance it success. The success of any mangrove replanting will depend on the effectiveness and efficiency of the planting techniques, and the rate of recruitment of flora and fauna. This includes how are mangroves planted, how does a new mangrove ecosystem develop, how can healthy growth and establishment of mangrove plants and their associated animals be ensured, and how do the changes in environmental factors affect their growth, etc.

Mangrove swamps are unique inter-tidal wetland ecosystems found in sheltered tropical and subtropical shores. It represents a rich and diverse living resource with significant ecological, environmental, social and economic values. However, mangrove habitats are decreasing around the world. Losses in many tropical countries have exceeded 1% of mangrove area every year. The survival of mangroves is threatened by human activities around the world especially in Hong Kong and other cities along southeast coasts of China. In recent years, the government has recognised the importance of mangrove ecosystems and started to conserve it. Replanting of mangroves by government authorities has taken place in area where natural mangroves are destroyed due to infra-structural development or reclamation. Green groups in Hong Kong have also initiated some mangrove planting programmes in Sai Kung district since 1997. Similarly, in China, the government has been collaborated with academics and local people to replant mangroves in Xiamen and Shenzhen. However, it has been reported that the success rate was relatively low in many cases and it was not easy to have the planted individuals continue to grow successfully. For instance, in Hong Kong, the replanted young mangrove seedlings were suddenly dead in Yuen Long and Sai Kung after their initial establishment (personal communication). The reasons caused such death are not known, as routine monitoring and post-planting care are not commonly carried out in most of the replanting projects in Hong Kong.

To achieve a successful mangrove conservation and rehabilitation, extensive research on the ecology of the mangrove ecosystems, in particular the establishment of replanted seedlings and the associated animals are required for policy makers, government officials and the public. It is essential to answer the following questions related to mangrove establishment : What is the growth rate of seedlings? How are plants established after planting or transplanting? What is the difference between direct planting with propagules and transplanting of seedlings? What are the requirements for replanting of mangroves? How are animals colonised after tree planting? what are causes led to the death of young seedlings? How do they response to environmental changes?

Specifically, the objectives of the proposed project include:

  • To review the experience of the transplantation programme carried out in Shenzhen Futian Nature Reserve and in Xiamen, China
  • To test how long the propagules (or droppers) or seeds of true mangrove plants in Hong Kong can be stored after collection and the best way to store them without affecting their viability.
  • To determine the optimal conditions and pre-treatment required for germination of different species of mangrove propagules or seeds, and the suitable seasons for transplanting young seedlings from greenhouse to the coastal area where mangroves are disturbed by human activities.
  • To examine the performance of different mangrove planting techniques and their suitability in Hong Kong habitats.
  • To compare the differences between direct planting of propagules and transplanting of appropriate seedlings in terms of their survival and growth rates.
  • To investigate effects of different environmental factors on establishment of mangrove plants, including soil texture (physical make up of substratum soil), water logging and hypoxia (related to sea level changes), eutrophication and algal cover, salinity, tidal flushing and spillage of toxic chemicals such as toxic metals and organic compounds.
  • To compile the data obtained from this study in user-friendly multimedia CD-ROM for government officials, schools, green groups and public to promote their understanding on conservation and management of mangrove ecosystems in Hong Kong.

Summary of the Findings/Outcomes:

Mangrove swamp represents a rich and diverse living resource with significant ecological, environmental, social and economic values but they are destroyed by human activities around the world. Planting is one of the recommended conservation strategies. The present project reviewed the planting experiences in Mainland China and Hong Kong SAR. A series of greenhouse experiments and field trials were conducted on germination and planting of mangrove species. Through routine field visits, information on reproductive pattern, fruiting time and availability of propagules (droppers, fruits or seeds) of the eight true mangrove species in Hong Kong were obtained. Most propagules except Lumnitzera racemosa (a dormant species that requires three months storage) could not be stored as they were easily infected by moulds and lost viability during storage. The most suitable techniques for germination and raising seedlings of each true mangrove species were developed. The effects of various environmental factors, including salinity, tidal flushing, soil texture and pollution on germination and growth of mangroves were evaluated, and the sensitivity to each factor was found species specific. Field trials revealed that Luk Keng and Ma Wan swamps were not suitable for mangrove planting because of serious human disturbance and other limiting factors. Within Kei Ling Ha Lo Wai, the most suitable swamp for mangrove planting, the success was higher in location closest to the existing mangroves or at higher tidal position. The two common planting techniques, direct dropper planting and transplanting of seedlings was compared. Seedling transplanting could be done at any time throughout the year and had higher survival percentages than direct planting of droppers although the former technique demands more time and efforts. Problems encountered by mangrove plants including algal deposition, barnacle colonization, animal grazing, and human interference were observed. Data collected were compiled in a user-friendly interactive CD-ROM.