Our Work

Wetland Conservation > Coastal Habitat Enhancement cum Education Project at Sham Wat > Sonneratia mangroves management

In Hong Kong, there are two known invasive mangrove species from the genus Sonneratia - S. caseolaris and S. apetala. Both are fast-growing, tough pioneer species commonly used to restore mangrove forests along the coast. After being accidentally introduced into Hong Kong in 2000, they first took root in Deep Bay, then expanded to some of the mangrove forests on Lantau Island, including Sham Wat, via water. Sonneratia trees can reach up to 10-20m tall and form groves within 4 years, competing with local mangrove species for space and blocking out sunlight, which inhibits the growth of other mangroves beneath them. They favor open mudflats as well, and by colonizing these originally plantless lands, they have reduced the feeding grounds of migratory birds and habitats of other mudflat organisms, negatively impacting the local mangrove ecosystem. Therefore, it is necessary to manage and control invasive species with methods like prevention, monitoring, research, and removal.

The Sonneratia apetala trees in Sham Wat have an average height of 8-10m,

dwarfing the 1m tall local mangroves. Not only does their canopy block out sunlight,

they also compete with local mangroves for space and resources

Leaves opposite and narrow	White stamens with mushroomshaped or peltate stigma	Green when young and becomes greyish when mature. Each fruit produces 100-130 seeds

We organised a seminar to explain to the public the background information of the invasive plants, the situation in Hong Kong and their management in December 2022.

We held 2 days of Sonneratia removal activities in January 2023 with about 50 participants in total to remove smaller Sonneratia apetala trees under the guidance of arborists. While removing the Sonneratia, the tree stubs should be cut as low as possible and use mud to cover the tree stubs immediately after cutting. It helps to suffocate the cambium, a layer of cells that promote growth in a plant. The method is considered effective to eradicate the Sonneratia. We worked together to improve the Sham Wat’s coastal ecosystem with a final tally of 40 Sonneratia apetala trees.

Tools like a handsaw can be used to remove Sonneratia, and the cut should be made close to the ground 

To inhibit the resprout of Sonneratia apetala by covering their stumps with soil

Our team has searched for new recruited of Sonneratia and resprouting circumstances of felled Sonneratia along the coastline to monitor the regeneration of Sonneratia

• In 12 months after the removal of Sonneratia, 104 new recruit were found in the mangrove of Sham Wat. Possible sources of the new recruits could be either seeds floating from somewhere outside Sham Wat or/and seed banks left in the local area.

A fruit of Sonneratia caseolaris was found on the mudflat of Sham Wat, which is likely floating from somewhere outside Sham Wat

• The rapid growth of the new seedlings was recorded in just one year. The maximum height of the new seedlings reached 250cm with a mean of 98.3cm, while the maximum basal diameter reached 120mm with a mean of 21.9mm.

The seedling of Sonneratia can grow rapidly 

• Only two tree stubs of felled Sonneratia were found resprouted as of 12 months after the removal.
• The “cut and covered by mud” method (Tang, 2009) effectively eradicate Sonneratia. The tree stubs were cut as low as possible, and mud was used to cover the tree stubs immediately after cutting. It helps to suffocate the cambium, a layer of cells that promote growth in a plant.

Most of the tree stubs of felled Sonneratia show no sign of resprout

Only two tree stubs of felled Sonneratia were found resprouted

*Tang, W. S. (2009). The distribution, ecology, potential impacts and management of exotic plants, Sonneratia apetala and S. caseolaris, in Hong Kong mangroves [Master's Thesis].