Case 2
Another case to be introduced is also a Ficus microcarpa. It is located at Tak Shing Street, Kowloon, Hong Kong. It is funny that the tree locates on a carriage way next to a public pavement. And there is Fortune Terrace, a residential building (Figure 7), next to the tree. It was planted in an irregular tree pit at grade.
The tree had a single trunk. Its diameter at breast height (DBH) measured at 1.3 m (AFCD, 2006) was 500 mm and the age class was mature. It was about 15 m tall with around 12 m crown spread (Figure 8). The tree crown distribution was not even due to the restriction from surrounding buildings. Although the distribution is uneven, the tree crown was still widely spread on other three directions.
There were several mechanical wounds found on the trunk and branches (Figure 9 to 12). These mechanical damages were likely caused by high vehicles. Cavities were also found on the trunk at 1.5m, 1.7m and 2m height above ground level. The largest one formed an opening with 60mm in width, 150mm in length and with 180mm in-depth. A poor CODIT was found next to the remaining stem. Further decay would be carried out from time to time.
It leaned towards the carriage way about 15 degrees from vertical and towards southeast. According to the growing direction, it was self-corrected with an abnormal bending (Figure 13).
Mechanical damages and wounds were also found on root flare and collar. Surface dead wood were exposed (Figure 14 to 15). As the tree was planted on a carriage way, the damage of root flare was caused by vehicle parking for years. Bulges were developed near the basal cavity which indicate a response growth against the internal decay. And the compression roots formed knots along the exposed roots. Girdling roots is another common defect observed due to the restricted tree pit. According to the size of tree crown, there were about 80 per cent of root covered by cement. The tree pit was relatively small but it was commonly found in the roadside in Hong Kong. Besides, a cavity was found with two-side opening at the root flare. This was the largest cavity found on this tree with 180mm in width and 250 mm in length. Litters were found inside the cavity and wood decays were observed.
According to the observation after the above visual assessment, the risk rating of branch, trunk and root flare are summarized in Figure 16.
With the risk categorization as above, the following mitigation measures are recommended.
- A short term measurement is to remove one of the movable targets. As the parking vehicles along Tak Shing Street is illegal, the management department can request the Transport Department to block the carriage way underneath the tree crown as a restricted zone of vehicle and seek the assistance from Police Force to step up enforcement action on illegal parking. The likelihood of impacting target will be changed from high to medium. And the residual risk and root flare will drop down to low and moderate respectively.
- Crown thinning and cleaning can reduce not only the likelihood of failure but to enhance the tree structural integrity by removing the dead twigs to minimize possible drops of tree debris and dead twigs. Those thinning works should be scheduled before typhoon season in Hong Kong. With the guideline from Tree Management Office, no more than 25 per cent of pruning works will be conducted. This can also help in reducing the tree crown loading.
- Furthermore, protection should be made in order to prevent further damage and new wound formed. Installation of tree guard is recommended to cut off the source of mechanical damage (mainly from vehicle) to the lower trunk and root flare.
- Enlarging the tree pit will be a long term measures to improve the situation of the tree in both structural and health extent. This can allow more oxygen and water dissolve in the top soil which encourage root growth. Furthermore, the adventitious roots can extend to the soil and form extra support to the tree. Guiding tubes can also be installed in order to protect the adventitious root development and direction on the supporting root we would like to setting up.
- Finally, closely monitoring is important for updating the condition of tree and take remedial action when necessary. Regular monitoring in every six month is recommended.
Tree Risk Management 