Tree risk case 1

The following three tree risk assessment (TRA) was conducted according to the Guidelines for Tree Risk Assessment and Management arrangement (8^th Edition) developed by Greening, Landscape and Tree Management Section of Development Bureau of Hong Kong SAR Government (GLTMS, 2015a).   Objectives of these TRA is to assess the condition and potential risk of the concerned tree, to make recommendations for reducing the risk of the tree and to propose the residual risk. Overall hazard rating consists of ratings of failure potential (1-4), size of potential failure part (1-4) and target use (1-4) with the highest mark of 12.

Tree risk case 1 –
A mature Casuarina equisetifolia with topped history

Background

The concerned tree is a mature tree of species Casuarina equisetifolia with DBH of 615mm, Height of 17.0 m and crown spread of 6.5 m.  The tree is located on a lawn adjacent to an Apartments at the northeast and the other three sides are mainly the garden of the Apartments.  The soil of the lawn should be shallow as many woody roots are exposed which indicates a shallow root system in response to lack of soil.  One side of root should be cut long time ago to form the existing pavement of the garden.  The tree should have hard pruning history that 3 main branches were all found topped with multiple leaders growing near the pruning wounds.

Main defect one – three topped main branches

Three main branches were observed topped at the level of about 12m to 14m height from ground (Figure 4.2), which resulted in forming of many large pruning wound of about 150 to 250mm diameter.  Afterwards, new branches developed from the pruned branches were pruned again and led to new wounds of about 50 to 100mm in diameter.  Many weakly attacked epicormic shoots were then developed at the tissue below the pruning wounds (Figure 4.3).  The epicormics are about 2m to 4m long with diameter of mostly less than 50mm.  From the field observation, many wounds are swollen showing the wound wood formation, yet some wounds had observed with symptoms of decay.

Regarding the hard pruned main branches, potential hazardous should be branch failure.  As mentioned, the epicormic branches developed near the cut are weakly attached to the base, and decay developing for the open may further reduce the strength of the attachment over time (Dunster, 2013).  When these branches grows larger, they are readily broken during strong wind, and therefore the failure potential is considered as high and the falling zone should be within the Garden with frequent land use, and the size of failure should be less than 150mm and therefore the overall hazard rating should be 3 + 1 + 3 = 7.  

The remedial measure should be selectively pruning of the weak branches and allow the stronger branch to further develop the holding wood.  On the other hand, the branches near the decaying wound should be removed as they have high failure potential.  After the pruning, the overall hazard rating will be reduced to 1 + 1 + 3 = 5.

Main defect two – open cavity and crack at a topped main branch

One crack and an open cavity were observed at the base of one topped main branch of T1 (Figure 4.4).  The crack was growing along the direction of lean and reaction wood was observed indicates action of bending motion along the main branch.  An open cavity was observed about half meter below the crack (Figure 4.5).  Both the defects are located at the base of the topped main branches and they will potentially lead to failure of the large main branch.  Furthermore, many epicormics shoots are growing at the swollen tissue at the topped wound, which are weakly attached which are readily broken under strong wind.

Both the defects are located at the base of the topped branches and they will potentially lead to failure of the large main topped branch.  Yet the potential of failure is considered as medium (rating of 2) at the present.  The size of failure will be about 250mm and the falling zone should be within the Garden with frequent land use.  The overall hazard rating should be 2 + 2 + 3 =7.  Considered pruning of the branch at this size will lead to large wound at trunk that open for fungal infection and decay, re-inspection in every 6 months is recommended.

On the other hand, if the crack has developed further, it will probably connect to the open cavity and become high in potential of failure.  The overall hazard rating will become 3 + 2 + 3 =8.  Pruning of the whole topped main branch will be necessary and the residual hazard rating will be reduced to 1 + 2 + 3 = 6.