In this session, I will discuss 2 kinds of tree defect detection systems that I have used in Hong Kong for advance tree risk assessment. They are Resistograph system and Sonic Tomograph system.
Resistograph and application
Using resistograph system is an invasive method for defect detection. The resistograph system has a needle. By rotating the needle in certain speed, the needle can drill in the wood. When a tree has an internal decay, the decayed wood will be relatively soft with less density when comparing with healthy sound wood. It is because the lignin or cellulose was decomposed by fungi. When we drill in the aforesaid decayed wood, the resistance will be relatively low (Zahner et al, 2012). Referring Figure 29 and 30, this is an example of diagram generated by a resistograph system measured from a tree with internal decay. According to the resistance measurement during the process of microdrilling at a constant speed, we can determine the wood strength remains in the wood tissues.(Figure 28)
Advantage
The longer the needle of the resistograph system, the longer distance you can measure the diameter of the tree trunk. In general, the needle is about 30 cm but some latest model can provide a needle in 40 cm. The advantage of using resistograph system is quick set-up, rapid assessment can be obtained, relatively cheaper than using tomograph system and the accuracy is high (Johnstone et al, 2007). As the set-up is easy and much cheaper than a sonic tomograph, I usually use for the measurement of a problematic tree before I choose using sonic tomograph.
Limitation
The disadvantage of using resistograph system is decay may be found after drilling. Although the drilling hole is small, this physical damages will be exist in the trees and these holes can provide an ‘expressway’ for pathogen, borer as an example, to invade the inner parts of the trees (Leong et al, 2012) . Therefore, when we use this system, we should well evaluate where the suspected decay area is and minimize the frequency of drilling. Furthermore, each time of drilling, it is better to sterilize the needle to avoid spreading the hidden fungal fruiting bodies inside the tree or even pass the hidden fungal fruiting bodies to another tree.
Regarding the report of Mattheck et al. (1999) and further elaborated by ISIK et. al. (2003), the resistograph measures may have varies within species and location of measurement. Also, environmental factors will also affect the wood density and influencing the measurement. Furthermore, as the resistograph can only provide a linear data in each drilling, we cannot project the total area or volume of the decay that in the problematic tree by 1 or 2 measurements in the same measuring height. The more the drilling we take, the more data we can obtain to figure out the size of decay but in parallel, more physical damage we will create on the tree.
Future development
Regarding the limitation, the developer of resistograph system can further study the following aspect for improvement.
- To minimize the diameter of the drill tongue such that to minimize the size hole will be generated in each drilling.
- To improve the software by inputting the drilling statistics of different species under different temperature and humidity to see the correlation among each factors.
Sonic Tomograph
It is less invasive than resistograph system. The velocity of sonic waves in trees depnes on several parameters. These parameters depends on angel between propagation direction of the wave and annual growth rings, moisture content in different layers of the trunk and types on wood. By transmitting sonic signal inside the cross section of the wood, it can generate a 2D cross-section graphic which shows the level of decay, the percentage of decay and the approximated size of decay (Wassenaer & Richardson, 2009; Lin et al, 2013; Lin & Yang, 2015). (Figure 31)
In fact, this system is relatively expensive and time consuming in application. The set-up procedures is much complicated comparing with resistograph system. It is usually used for advanced tree risk assessment for a problematic tree pointed out by visual tree inspection. In a tree with internal decay, we may find sound wood, heartwood, decayed wood or hollow area. The aforesaid medium has different density. By measuring the velocity of sound travelling forth and back in the aforesaid medium, we can distinguish the healthy wood and defected wood (Wassenaer & Richardson, 2009).
Application
To install the tomograph system on the tree, we should first identify the measuring level and tap a certain numbers of nails round the tree evenly and in anticlockwise direction. In general, 8 nails are recommended. The larger the circumference of the measuring level, the higher the number of nails to be used. After tapping the nails and named them MP1 to MP8, we will put the main unit of the tomograph system on the tree and connect sensors, laptop and hammer cable to respective socket. Afterwards, we will use a caliber to input the distance between each nail before measuring. By using the default hammer to tap the sensor attached on each nail, a sound wave will then generated and recorded and the data will then transmit back to the laptop. With the software installed in the laptop, the sound wave will change to a tomogram. The defected and healthy wood will be distinguished by different colour. Referring to my assessment of a Crateva unilocularis, a tomograph with 3 layers at 20cm, 60cm and 115cm above ground level were measured (Figure 32 to 36). The colour in dark brown represent highest wood density (healthy wood or solid wood). The change in colour from dark brown, brown, green, purple to blue shows the decrease in wood density. Blue represents the lowest wood density (suspected defected wood or cavity)
Limitation
- Physical damage is unavoidable as tapping the nail on the tree will cause a damage on tree bark and cambium. In fact, the holes are not of the size that would affect the structural integrity of a tree (Gilbert & Smiler, 2004)
- As the sonic tomograph cannot distinguish among cracks, ring shakes, decay, cavity and defensive reaction zone, we will use other defect detective system such as resistograph to estimate the type of decay afterwards (Wang & Allison, 2008; Wassenaer & Richardson, 2009; Leong et al, 2012).
- The environmental factor is also important in collecting successful data. If the surrounding of the tree is near the road with heavy traffic or near a construction site, the sound wave collected will be affected by source of vibration. With my bitter experience, I had spent half of a day for generating a sonic tomograph for a Bombax ceiba near a road but finally I could not collect the data we wanted because the B. ceiba was located next to a run in/out of a petrol station and next to a carriageway with heavy traffic. The sonic data was affected by the vibration generated from heavy traffic nearby and no consistent and effective record could be collected on that day.
- Tomograph may be failed in showing the status of decay in some trees infected by soft rot. According to the research from Deflorio et al (2007), sonic tomograph was failed in detecting the decay of soft rot fungus Kretzschmaria deusta because of the soft rot did not highly reduced the wood density of the infected tree.
- For the decay found in underground, sonic tomograph will not be effective for assessment as the sound wave cannot be received in return as the sensor cannot be set in the underground.
Future development
As sonic tomograph is about collecting the data of sound wave back and forth among sensors, the development is required to study the feasibility for the sound wave transmit in one direction only and can collect these data for analysis. And the sonic tomograph cannot distinguish the decay and cavity in some scenario, further study is required to increase the accuracy. Lastly, for the nail tapping in the tree as setting the sensor, the developer should study if putting the sensors on an elastic belt or other medium and surround the tree is feasible such that no damage will be caused during the assessment.
Tree Risk Management 