Tree risk assessment methodology

The assessment is undertaken in line with the assessment 
methodology as outlined by Greening, Landscape and Tree 
Management (GLTM) Section of the Development Bureau in Hong Kong.

Tree risk is assessed in two stages:

• Stage 1 – Tree Risk on an Area Basis;

• Stage 2 – Tree Risk on an Individual Basis

Stage 1 – Tree Risk on an “Area Basis”

First of all, identify the site area into three tree risk category areas, having regard to the intensity of use of the site and the probability of risk. Category 1 is designated as a priority tree risk area, whereas Category 2 and Category 3 areas are designated 
as secondary tree risk areas.

Stage 2 – Tree Risk on a “Individual Basis”

Once a Category 1 zone is identified, those trees which may be 
problematic/important are systematically assessed regarding 
their health and structural condition with particular emphasis on 
risk angle.

Below  Six categories of trees should be  carried  out tree risk assessment individually:

  1. Old and ValuableTrees;
  2. Stonewall trees;
  3. Outstanding tree complaints related to structural or health problem;
  4. Mature trees with poor health or structure that are of tree 
    species with brittle branches;
  5. Trees with notabledefects or health problems; and
  6.  Trees growing in very stressful site conditions.

After detailed assessment of the condition of the trees in Category 
1 areas, those trees identified with high risk are highlighted for further monitoring or risk management.

Factors Affecting Tree Risk

In the context of Hong Kong which is dominated by built-up areas, 
the demarcation of tree risk management zones will focus on:

  • the probability of tree failure due to broken branch or collapsed trunk
  • the probability of impact of tree failure on the target. 

The Target

Carry out target analysis to find out human-beings, human activity and their property that can be injured, damaged or disrupted by a tree failure.(reference to citation 2 Guides on tree risk assessment and management arrangement, Hong Kong). Tree possible falling within 1.5 times of its height which is called falling zone. If there is anticipated injury to human life or damage to his property in the event of tree failure, the tree can be categorized as having potential risks. There are 4 categories of target according to the occupancy rate include Constant, frequent, intermittent and occasional. Constant use is meant the areas such as busy traffic road, high flow of pedestrians, unmovable facilities and building. Frequent use is meant the normal traffic flow and pedestrian flow area, public parks. Intermittent use is meant the low flow of traffic road and pedestrian pathway, discontinuous use cycling path. Occasional use is meant the maintenance path with restricted access and limited to use by the maintenance parties only.
A tree cannot be a hazard without the presence of a target.

Branch Breakage and Trunk Collapse

A tree is considered hazardous if it is structurally unsound or suffers from a serious health problem. The tree hazard is usually caused by a branch fall or trunk collapse. The degree of impact is proportional to the size and weight of the trunk/branch. Branch breakage and trunk collapse occur when the strength of the wood is exceeded 
by a mechanical stress. Trunk collapse may be due to root failure. 
There are two types of root failure. The first type is related to the
weakening of root strength caused by disease, decay, and injury. 
The second type is related to soil failure that occurs when the soil 
strength is insufficient to anchor the tree.

The probability of branch breakage or trunk collapse is influenced 
by the following:

(a) Tree Defects

Tree defects include branch defect (e.g. excessive end weight, 
branch decay, etc), trunk defect (e.g. co-dominant trunk with 
included bark, excessive lean, etc) and root defect (e.g.root 
decay, disease and mechanical damage, etc). Please refer to the 
‘Pictorial Guide for Tree Maintenance to Reduce TreeRisk’ at 
Annex C for common tree defects.

(b) Tree Species

Five species tree are more prone to branch fall or trunk collapse. 
Delonix regia (鳳凰木), Erythrinavariegata (刺桐), Alstoniascholaris
(糖膠樹), Aleurites moluccana (石栗) and Celtis sinensis (朴樹) are 
generally recognized as “brittle” by cumulative experience.

(c) Tree Size and Age

Regular inspect on the mature, old and valuable tree is needed. Species and age of the tree also play a role in tree failures. All species have a failure profile that details issues commonly observed in that species. [Citation 3].

Mature trees have limited ability to recover from stress and damage
[Citation 5], proactive and preventive measures are critical elements for preserving them.

(d) Site Condition

Site factors include prevailing wind, soil condition, growth space, etc have a significant influence on the development of tree failure. Such as saturated or compacted soil, restricted root by drainage and concrete pavement. Small size planter for a large mature tree.

Citation

[1] https://www.greening.gov.hk/en/knowledge_database/brown_root.html [accessed on 14/12/2018]

 [2] GUIDELINES FOR TREE RISK ASSESSMENT AND MANAGEMENT ARRANGEMENT

GREENING, LANDSCAPE AND TREE MANAGEMENT SECTION DEVELOPMENT BUREAUNOVEMBER 2015 (8th Edition)

[3]Why Trees Fall

Bartlett – https://www.bartlett.com/blog/2015/08/tree-advice/why-trees-fall

[4] Clark, J.R. and Matheny, N. 1991. Management of Mature Trees. Journal of Arboriculture 17:173-184.

[5] Management Guidelines for mature tree

GREENING, LANDSCAPE AND TREE MANAGEMENT SECTION DEVELOPMENT BUREAU December 2014