3.1 Other Tree Risk Cases – Cinnamomum camphora

3. Other Tree Risk Cases

While in section 2 the biotic factor, fungal colonization strategies on tree, were discussed. In this part, the abiotic defects would be investigated for their risk on trees.

3(a) Cinnamomum camphora with crack in pavement and cavity

One Cinnamomum camphora was located next to sitting area of Norwegian International School (Kindergarten campus) (Fig. 3.1.1) with 16m tall and Aggregated DBH 1335mm. Tree photo was shown in Fig. 3.1.2 and 3.1.3.

Fig 3.1.1 Tree located near Norwegian International School (Kindergarten Campus) (Google map, 2018).

Fig. 3.1.2 Location of tree next to sitting area of international school (Photo taken on 20^th Aug 2018).

Fig 3.1.3 Trunk base condition of the tree. It is located on slope crest with one of the trunks embedded with paved ground (Photo taken on 20^th Aug 2018).

As illustrated from general image of tree in fig. 3.1.2 and 3.1.3. Obvious issues can be figured out was the leaning angle of trunk 2 towards the slope, highly restricted area which conflict with trunk 1 and finally suspected crack of pavement due to root development. In addition, cavity with dimension of 450mm (width of cavity opening) x 750mm (horizontal depth) x 1350mm (vertical depth) was found on trunk 1 which will be illustrated in fig 3.1.4 with summarized defects of the tree in different period.

Fig. 3.1.4 Photo showing critical defects of the tree from 17^th Aug 2017 to 20^th Aug 2018. (Upper left) and (Upper right) showing condition of cavity at trunk 1 with wood wound formed. Increase in extent of crack on pavement was observed comparing (Bottom left) and (Bottom right).

In traditional arboricultural practices, cavity is often a critical concern regarding tree stability and thus safety. Several mathematical models have been developed to analyses probability of tree failure. In early time, 3 formulas had been developed which was collectively called “strength loss formulas” (Kane et al., 2001) to analyze loss in stem moment of inertia (I_STEM) to assess tree failure potential. However, the formulas assume the decay/cavity is in center of the tree where actual situation varies. Significant decrease in I_STEM is expected for off-centered decay/cavity but may not be reflected in formula (Kane and Ryan, 2004). Another famous formula was introduced with off-center decay/cavity taken into consideration is called t/R ratio, t represents sound wood thickness and R represents stem radius (Mattheck and Breloer, 1998). t/R ratio <0.3 was considered tree with high risk and mitigation measure should be considered.

In this case study, total diameter of the trunk 1 is 970mm with horizontal cavity width 750mm with sound wood thickness of 110mm at each side.

According to formula:

110/485 = 0.23

The tree is considered high risk with the formula applied, however, it only measured ratio of sound wood thickness to trunk radius. Development of wound wood which aims for closure of wound to prevent fungal infection (Eyles et al., 2003) was not considered, also tree form and crown loading of the tree. In addition, it assumes the tree has no other critical defects that contribute to its structural integrity (Kane and Ryan, 2004).

This theory has been challenged by another researcher that tree failure still occurred even without any hollow inside trunk (Gruber, 2008). The study pointed out that the t/R ratio was “mono-parametric” model and did not taken in account of other factors for example the slenderness of tree (Mattheck et al., 2002) and could not explain tree failure with t/R ratio larger than 0.3.

Gruber’s critics was later defended by other research that t/R ratio was applied for tree in ordinary wind load. Tree failure under extreme weather such as hurricane is not predictable, which was taken by Gruber as base for his argument, beyond the scope of tree risk assessment. The extra wind load exert force higher than the tree could withstand, even intact tree would fail. (Fink, 2009). Apart from t/R ratio, slenderness of tree did consider in assessment as shown in Fig. 3.2.

Fig. 3.2 Graph showing interaction t/R ratio and slenderness of the tree. Tree with high slenderness could fail even with high t/R ratio. Hollow tree is not the sole factor for tree failure (Fink, 2009).

Thus, during assessment of cavity of tree in case study. H/D ratio should also be considered. In addition, wind load should be measured to understand the force exerted on the tree in normal day and investigate its probability of failure. Crown reduction would be conducted to reduce slenderness of the tree.

The second condition of concern for the tree is leaning and crack on pavement which is suspected to be caused by the tree.

For assessment of change in leaning angle, clinometer was commonly used for regular measurement to look at any change of the tree. For more advanced and accurate measurement, tree motion sensor (Fig. 3.3) is introduced and click here for full description. This is a measurement of root plate tilt to record how tree sway in normal wind force.

Fig 3.3 Tree motion sensor installed on tree and connected to mobile apps for remote retrieval of data (argus-electronic gmbh, 2018).

There are several advantages of the sensor over conventional clinometer. Firstly, regular gathering of data can be achieved to every hour without intensive labor conduct on-site measurement. Secondly, it is direct measurement of tree sway motion in real situation compared with tree pulling test with artificial wind replacement load and fixed tree model which is criticized by Mattheck and Bethge (2005).

For crack caused by roots of the tree, an innovate method using Ground Penetration Radar (GPR) could be used to visualize tree morphology of underground roots, the details will be discussed in section 4 – tree defect detection system. Meanwhile, measurement of any change in dimension of crack is proposed.

To sum up, besides crown reduction to decrease slenderness of the tree to safe level, continuous monitoring of leaning angle and crack of pavement are proposed. For any change in leaning and crack observed, remedial action should be conducted based on degree of change.