Nothing is without risk. The concept of risk management is to manage the identified risk to an acceptable level. A company manages its risk - such as bad debt or fluctuating material price. People manage their own risk such as disease and accident by regular body check and doing protection measures.
Trees have risk too. Even for a healthily-growing trees planted along roadside may create damage to structures directly or block the view of driver / street light, which in turn creates risk indirectly. Pedestrians may kick on the roots protruding over the ground or the uplifted paving block and get injury.
Management of the risks above requires regular evaluation of our business, body status and tree condition respectively, and then formulating a plan of management such as provision of monetary buffer for the business, buying an insurance with body check for ourselves, and conducting regular 'tree risk assessment' with risk mitigation.
With the examples above regarding trees, we understand that a full tree risk assessment should not cover only the intrinsic feature (i.e. health and structure) of the tree itself, but should evaluate its interaction with the environment as a whole. However, in general the public uses to adopt a narrow sense, which is, 'tree risk assessment' being equivalent to the assessment on mainly the structure and health of the tree.
In this context, a proper tree risk assessment shall include the evaluation of the existing tree features and projecting a bit further of their development. In a systematic way, an arborist with relevant tree risk assessment qualification inspects around a tree from a far to a close distance, and from the top of the tree to the trunk base. If possible or necessary, he/she may also do some investigation on the ground and look for special feature of the root or the growing medium. He/She should have a standardized record form for taking notes of all important findings.
For health, the arborist looks for any abnormality on the tree crown, leaf size, foliage colour and density, as well as any sign of defoliation or dieback. These are evaluated with consideration of species properties, specific site condition, seasonal factor, comparison amongst tree of the same species at the same region is necessary.
He/She shall also check whether the crown weight is unbalance or excessively heavy, whether teh tree is leaning with signs of recent movement such as displaced root plate, fresh uprooting or crack on soil/ground features. If there is abnormal defoliation, the arborist shall further investigate the existence of pests, soil water availaility or any construction/disturbance nearby.
After the overall evaluation above, he/she will go into details of different tree parts, from the branches through trunk to roots, or vice versa. Structural problems, to list a few below, will be look for - cracks, wounds, decays, cavities, bark cracks, included bark, tapering problem, presence of fungal fruiting bodies or pests (or their signs).
Most of the above is also applicable to roots, though checking of roots growing underground is usually not practical or may create damage or stability problem if diffing of soil is involved. As long as there is no suspected basal problems or significant decline in tree health, tree risk assessment on roots usually covers the inspection of exposed and buttress ones only. This is also why the information obtained in tree risk assessment always has their limitation, even when proper procedure is followed. Limitations shall be listed and accepted sometimes in all kinds of risk assessment as well as management - not limited to tree.
With all the information obtained above, it goes to a critical step of tree risk assessment - evaluation of the risk.
Tree risk is rated according to the likelihood of failure, the size/weight of the part(s) likely to fail and the existence of target which may be hit if the part(s) failed. The arborist bases on these and suggest risk mitigation measures. Such measures may be as simple as moving the target away (no target no risk, but this option as a permanent measure may not be practical to many scenarios in a dense city like Hong Kong).
Other options include pruning, adding support to the tree or the defective parts, or even condemning the whole tree of the risk cannot be mitigated to an acceptable level with other alternative methods.
As tree risk assessment is about the life of trees and people, recruiting of a competent person knowledgeable to trees is therefore essential, so that the judgement will only be based on facts and science, without guess or subjective views.
[The author is an Board Certified Master Arborist, currently the President-Elect of the International Society of Arboriculture Hong Kong Chapter]
[筆者Chiky Wong為國際樹木學會資深樹藝師（Board Certified Master Arborist）並具樹木風檢評估資格，時任國際樹木學會香港分部候任主席]
