Tree Risk Assessment Case 3 Acacia confusa 臺灣相思

 Acacia confusa 臺灣相思

1. Tree species and size

 The assessed tree was a mature Acacia confusa. The tree had 242 mm diameter at breast height (measured 1.3mabove ground), 1 no. of trunk. Height was 9m and crown spread was 8m.

2. Target and location

The site was a public beach in Ma Wan, HongKong. Frequently use recreation area by citizens. It has a vegetated slope located next to the walking pathway. Frequently citizen was using this recreation area. A building was nearby to the tree. The occupation of this area by citizen and building was high property value. ‘Targets’, as well as the proximity of citizen and building, bring a high probability of risk. The existing target situation is summarized in below table 2.1:

Table 2.1 Existing target situation:

Target existing 
(Y/N) 		Target removable(Y/N) Site use restricted(Y/N) Frequency of use 
Y N N High

The existing Targets cannot be removed from the pathway area or be restricted in their access. Tree Risk Assessment on an Area Basis defines a high use area as having high risk and is considered under Category 2.

3. General Condition

Foliage density, leaves color and size were normal. Epicormics grew along the tree indicated average tree vigor. 20-degree leaning was observed. Crown load was fair with medium live crown ratio. Only 5% dieback and few dead branches were observed.

4.Tree Defects

An open decay was identified on trunk base in size 120mm length, 40mm width, 30mm depth, facing South at ground level. Trunk decay and active termites track were found on trunk. The trunk was crooked irregularly. Mature Fomes sp. fruiting bodies were found at root.  Half of root was restricted by drainage, some of root was exposed and rotted. 

5. Site Condition

The tree was planted in 1m area with sandy soil. The tree was at the edge of a group of tree and facing the prevailing of wind.

6. Conclusion and Mitigation Measure

Fungal fruiting body of Fomes sp. with moderate severity of decay was observed on trunk base. Such kind of wood destructive fungi would weaken the structure and affect tree health in the long run. Termites track and termites activities were found which would deteriorate the internal structure of the tree. The wound with abnormal bark crack on the upper portion of trunk would affect stability of the tree and lead to tree failure during adverse weather. The tree located near the entrance of public toilet (the building) and leaned towards the pedestrian road. In view of its poor tree form and its potential hazard to the public, tree removal is proposed. This case was reflected misdesigned planting problem. Acacia can be grown up to large than 500mm diameter at breast height. When the tree getting mature, the tree has not enough space for developing its roots. Planted the tree nearby the drainage who did not consider the tree size and how is the root growth. Roots damage caused the tree under stress and decline the tree health. Tree was attacked by pest and fungal disease. Overcrowded planting is a common problem in Hong Kong. The tree compete for the light in tree group and this cause the trunk crooked. Improper planting could increase the risk of tree failure. It was a tree risk management problem.

7. Tree photos

The tree was planted in insufficient area, root restricted by drainage. Whole trunk was crooked and posted strangely.
1 month later inspection was carried out, old wound was attacked by termites during raining season.

8.Tree Plan