LCQ15: Factors of consideration in selecting the tree species to be planted

Following is a question by the Hon Chan Hak-kan and a written reply by the Secretary for Development, Mrs Carrie Lam, in the Legislative Council today (October 20):

Question:

In mid-September this year, an Acacia confusa tree located on the slope along Fung Mo Street in Wong Tai Sin collapsed, crushing two passing taxies.  Moreover, a tree expert has recently relayed to me that the average life span of an Acacia confusa tree is about 40 years only and since a large number of such trees have been planted in Hong Kong since the 1980s, he estimated that these trees would gradually develop illness or even collapse in the foreseeable future, posing danger to the public.  In this connection, will the Government inform this Council:

(a) of the total number of Acacia confusa trees planted in Hong Kong at present, broken down by District Council district and, among them, the number of such trees which are located in high-risk areas with high pedestrian and vehicular flows, as well as the health conditions of such trees at present;

(b) apart from Acacia confusa trees, which other species of trees with a similar life span of about 40 years were planted in large numbers in Hong Kong during the 1980s, and list the distribution of such trees by District Council district;

(c) whether it will draw up specific plans for the removal of Acacia confusa trees; if so, of the details; if not, how it ensures that they are in good health condition;  

(d) of the factors to be considered by the authorities at present for selecting the tree species to be planted; and

(e) given that the Development Bureau has stated in its paper submitted to the Panel on Development of this Council on July 27 this year that the Tree Management Office would commission research covering different areas (including selection of suitable tree species for greening), in order to build up its professional knowledge base, of the latest progress of the research?

Reply:

President,

Since the 1970s, government departments concerned have carried out large-scale afforestation in the countryside to prevent soil erosion, Acacia (Acacia confusa) as one of the chosen tree species.  Acacia is widely planted in the rural areas of Hong Kong for its rapid growth and effectiveness in improving infertile soil.  It is also planted in the urban areas for greening the environment quickly.

Government is very concerned about the recent tree failure incident.  The Tree Management Office (TMO) has followed up with the departments concerned immediately after the incident to look into the cause of the incident, so that appropriate measures will be taken to protect public safety.

My reply to the five parts of the question is as follows:

(a) Most of the Acacia in Hong Kong is found in the countryside.   Government does not have statistics on the total number of Acacia in the territory nor a breakdown by district.  According to the tree risk assessment arrangements implemented by the tree management departments this year, there are about 147,000 Acacia in total at locations with high pedestrian or vehicular flow.  147 Acacia were assessed to have health or structural problems. Departments concerned have taken appropriate risk mitigation measures, such as pruning, treatment of pests and diseases, cabling and propping, and will continue to monitor those trees.

(b) A number of government departments carried out large-scale planting for various reasons in the 1980s.  For instance, the Agriculture, Fisheries and Conservation Department (AFCD) widely planted Acacia (Acacia confusa), Brisbane box (Lophostemon confertus), Paper-bark tree (Melaleuca leucadendron) and Horsetail tree (Casuarina equisetifolia), among others, in country parks with infertile soil to prevent soil erosion.  The Civil Engineering and Development Department planted Eucalypts (Eucalyptus spp.) extensively on slopes and remote hills to protect the slopes and prevent soil erosion.  The Housing Department planted Bauhinia (Bauhinia blakeana) and Chinese banyan (Ficus microcarpa) in many public housing estates for greening purpose.  Generally speaking, the trees planted in the 1980s have not shown any widespread obvious health problems.  As they are extensively planted in the countryside and urban areas, the departments concerned do not have a detailed breakdown of their distribution by district.

(c) We note that some Acacia in the territory are aging.  TMO will remind the tree management departments to carry out routine tree maintenance in a professional manner, with particular attention being given to ageing trees. On detecting any sign of health problems, appropriate risk mitigation measures will be taken promptly.  In absence of other feasible means to improve their health, departments will consider removing them as a last resort to protect public safety.

TMO is drawing up tree care measures for ageing trees, including guidelines for departments concerned on tree replacement.  In view of the declining health of ageing Acacia planted in country parks years ago, the AFCD launched the Hong Kong Country Park Plantation Enhancement Scheme in 2009.  Under the Scheme, Acacia and other exotic species in poor health are replaced gradually with diverse species of native trees and shrubs of higher ecological value with the objective of enhancing the overall ecological and landscape value of the woodland in country parks.  

(d) In carrying out tree planting, we select suitable tree species taking account of the planting objective, the environment of the planting site as well as the characteristics and maintenance requirements of different tree species.  For instance, for tree planting in country parks, the AFCD cultivates seedlings of native plant species and adopts a mixed-species planting strategy in order to enhance faunal and floral diversity in the woodland and avoid problems arising from mono-species planting such as a monotonous landscape, pest outbreaks and simultaneous ageing of trees.  In selecting tree species for urban planting, our focus is on the environment of the planting sites as well as the characteristics and maintenance requirements of different tree species.  We will select the most suitable tree species in the light of the design concept, environmental factors (such as planting space, the micro-climate of the planting area, soil quality, visual impact, traffic flow and landscape features) as well as the market supply of tree seedlings and the project budget.

(e) To raise the professional standard of tree management work in Hong Kong, TMO will commission four consultancy studies in 2010-11, including a study on the selection of suitable tree species for greening. TMO is now carry out the preparatory work (including drafting the scope and details of the studies and drawing up relevant documents for commissioning the consultancy studies), and will conduct tender exercises shortly.

Ends/Wednesday, October 20, 2010
Issued at HKT 14:04

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