With its yellow and gold flowers, Acacia confusa is the most easily recognised tree species in Hong Kong.
Sturdy and fast-growing, it was planted on roadside slopes across Hong Kong to prevent soil erosion amid rapid urban development in the 1950s and '60s.
However, the tree has a relatively short botanical life expectancy as it lives only 50 to 60 years on average.
It means the majority of Acacia confusa trees planted in the city over the past five decades are old and dying.
Revitalising roadsides
To solve the problem of dying trees endangering public safety, the Highways Department launched the Enhancement Programme of Vegetated Slopes last year to remove old Acacia confusa trees in phases.
Native and localised species are planted to replace them. Localised species differ from native ones as they were imported from overseas and have adapted to local conditions over many years.
The new vegetation can prevent soil erosion and enhance biodiversity.
To test the effectiveness of the programme, the department began to replace Acacia confusa on the slopes along Pui Man Street, Wong Tai Sin, with various plant species in 2012.
Highways Department Landscape Architect Jason Wong said the new plants have reduced soil erosion.
"Here we have planted a lot of native and localised plant species. For example, we've got Celtis, Bauhinia and Ilex species. For shrubs, we've got Rhododendron, Rhaphiolepis and currently the Ixora and Hibiscus are blossoming.
"The Highways Department wishes to create a safe highway landscape that has high ecological and aesthetic value for public enjoyment and wildlife use."
The department has implemented the programme at 11 sites so far.
It has consulted arborists, the Tree Management Office and District Councils, and conducted surveys and tree health assessments to identify dying trees needing to be replaced by other vegetation.
Tree troubles
Baptist University College of International Education Tree Management Programme Course Co-ordinator Alvin Tang said Acacia confusa is an evergreen tree species.
"Some of the trees in our urban areas (are) aging. They start to have numerous problems such as having fungal diseases.
"Because of their having diseases and poor structure, they will easily have broken branches. As a consequence, it will pose a threat to road users."
Old and dying trees not only pose safety risks to vehicles and pedestrians, they can also endanger the lives of tree removal workers.
Certified Tree Worker Climber Specialist Wind Tse said: "When Acacia confusa dies its trunk hardens, making it difficult for us to cut through it. Workers have to climb up the tree to cut it into pieces, but the branches of the tree become brittle when it dies.
"Unable to evaluate the state of the tree and how dry it is, the climbers might fall to the ground when the tree collapses."
Besides ensuring public safety, the vegetation programme also spreads environmentally-friendly messages.
Arboreal arts
The waste timber generated from the scheme is recycled.
Some of it has been sent to academic institutions which have turned it into wood sculptures for educational use.
Baptist University Academy of Visual Arts Studio Technician & Demonstrator Parry Ling said the hardness of Acacia confusa makes it an ideal furniture material.
"If we would like to get a certain size of the wood or timber or wood trunk, we can save money and time, and reduce the carbon footprint."
Mr Tang said the waste wood logs and branches can be shredded into small pieces for growing mushrooms.
"This could be utilised as a substrate to grow mushrooms, especially Ganoderma. We reckon this wood substrate is a good material because it has a very high density and is very hard."
There will soon be more wood for recycling as the scheme will be extended to more sites, including South Lantau Road, Tuen Hing Road, Tsing Yi Road West, Tai Tong Shan Road and Deep Water Bay Road.
More trees which have completed their mission will give way to a variety of plants, to continue the beautification of the city and boost biodiversity.
