4. Tree Defect detection system

4. Tree defect detection system

There is numerous tree decay detection tools available in market (table 4.1). In this study, resistograph for detection of internal condition of the tree and air spade for root inspection will be discussed for their advantages and limitation. Also, a relatively new technology Ground Penetrating Radar (GPR) will be introduced as a tool for root mapping.

Table 4.1 Comparison of different tree decay detection tools based on their resolution, accuracy and cost. (Leong et al., 2012).

4(a) Resistograph

Resistograph is a portable tool that consists of a drill needle with 1.5mm shaft diameter and 3mm tip diameter which is found to be obtained optimum stability and resolution, while produce least destruction to tree (Rinn, 2014) (Fig 4.1). Drilling resistance, the energy required for the micro-drill (drilling resistance) to penetrate wood at constant speed, is measured (Rinn et al., 1996). The drilling resistance is highly correlated with wood density. Thus, any variation of wood density was detected by resistograph. The drop would reveal presence of decay, cavity or crack (Mattheck et al., 1997).

Fig 4.1 Development of drill needle to achieve an optimum performance (Rinn, 2014).

Resistograph advanced other defect detection tools with ability to identify slight or incipient decay. The detection of early stages of decay is crucial as 10% decrease in density can cause 80% decrease in wood strength (Wilcox, 1978). Fig. 4.2 and 4.3 shows an example of incipient decay of Pinus sylvestris and was detected by drilling resistance profiles.

Fig. 4.2 Incipient decay of Pinus sylvestris caused by brown rot (Serpula lacrymans). Resistance drilling is applied to detect the drop in wood density. (Rinn, 2015).

Fig. 4.3 Drop in drilling resistance was identified as shown in bottom profiles (highlighted in yellow) as compared with top profiles without decay. The decay is identified due to change in pattern of tree ring (Rinn, 2015).

Interpretation of resistograph require solid knowledge of how tree control further wood decay through famous CODIT (Compartmentalization Of Decay In Tree) model (Shigo, 1979). Also, species-specific anatomical properties should be considered as different species groups comes out with contrasting results as shown in Fig. 4.4. Alteration of earlywood and latewood would affect the width of tree rings and thus the result of drilling resistance profile. Study shown that tree rings characteristics are highly correlated with climate (Young, 2011).

Fig. 4.4 Resistograph results of different species groups. This implies characteristics of the species should be considered during interpretation of profiles (Rinn, 2014).

Apart from knowledge of tree anatomy, different types of resistograph can make significant difference on drilling profiles due to their technical specification and types of drilling needles (Rinn, 2012). For environmental factors affecting the reading, another study shown that temperature and moisture content can alter the result significantly (Ukrainetz and O’Neill, 2010). Moisture content was observed to be directly proportional to drilling resistance in Fig. 4.5.

Fig. 4.5 Graph showing drilling resistance increased with moisture content (Lin et al., 2003).

Less experienced arborist would misinterpret drop of resistance as incipient decay due to limited knowledge on wood properties of the tree species and effect of climate on their growth pattern (Fig. 4.6).

Fig. 4.6 The drop of drilling resistance from around 230mm to 300mm represents early wood of conifer which experienced a period of cold winter. Unexperienced frontline assessor would wrongly define this as incipient decay while the tree is intact (Rinn, 2015).

In conclusion, reading of drilling resistance profiles is affected by the setting of different brands of resistance drilling machine such as the shape of drilling needle, drill bit flexion. Also, species properties affect the result by difference in wood density distribution of conifer, ring-porous and diffuse-porous types. The environmental factor also alters the drilling resistance.

There are some drawbacks that the drilling of trees left a channel for pathogen intrusion (Helliwell, 2007; Schwarze and Heuser, 2006). Although tree’s own defense can block external pathogens from colonization (Weber and Mattheck, 2006), it provides a pathway for pre-existing fungi to proliferate into wound produced by drilling (Kersten and Schwarze, 2005). Wood shavings was retained inside the drill hole with limited oxygen level and high carbon dioxide due to poor aeration. It favors some pathogens such as Inonotus hispidus shown in Fig. 4.7.

Fig. 4.7 The graph showing the difference in wood decay fungi infestation between hole produced from increment borer (above) and drilling (below). Studies has shown that higher percentage of Inonotus hispidus was observed in drilling than increment borer. Micro-climate produced from drilling hole favor the growth of this fungal species (Kersten and Schwarze, 2005).

The pathogenicity caused by resistograph, however, required more detailed investigation. This include the recovery ability of the tree in response to fungi invasion and growth characteristic of fungi and their colonization strategy (Schwarze et al., 2004). This should be thoroughly studied to avoid unwanted fungal decay that increase risk of the tree.

4(b) Root detection tools

Air spade is a tool used in arboriculture for root inspection. It consists of a supersonic nozzle connected to a hose with compressed air emitted into soil. Soil is loosened and removed, root is exposed for assessment as shown in Fig. 4.8.

Fig. 4.8 Photo showing operation of air spade. The nozzle is connected to hose and soil is excavated with the pressure of compressed air (ATP Tree Preservation, Ltd., 2016)

Air spade not only used for tree root inspection, it can be utilized for improving soil aeration, especially compacted soil, also avoiding tree roots damage during construction near tree. One company in Hong Kong is specialized for providing the product and service (please click the link). This video clip illustrate details of how air spade operates. It is claimed by the company that air spade does not damage to roots and underground utilities which advanced the conventional shovels digging method. However, there are limited studies to investigate effect of air spade on tree health and structure, thus the promise of the company is not strongly supported in scientific aspect. Also, the long-term impact to tree is uncertain as few evidences are available.

Another method of root detection tool being advocated in current arboriculture industry is ground penetrating radar (GPR). GPR consists of a portable computer, radar antenna (for trunk inspection) and scanning cart (Fig. 4.9). Theory of ground penetrating radar is based on emission of electromagnetic wave by transmitter antenna and return the signal when it hit an object and recorded by receiver (Barton and Montagu, 2004). The data is gathered and analyzed for root mapping (Fig. 4.10). The following video clip illustrate operation of GPR.

Fig. 4.9 Components of GPR for root inspection. Scanning cart for transmitting and receiving signal and computer for analysis of data (Tree Rader, Inc., 2018).

Fig. 4.10 GPR image showing root mapping of the tree. Soil was excavated by air spade for comparison of the root morphology between GPR and real situation (Ow and Sim, 2012).

Unlike air-spade which only able to inspect the root at certain depth, GPR is capable of root mapping that is useful for estimation of distribution of root system. Also, one study found that GPR can used for detection of roots below paved ground in urban (Nichols, P. et al., 2017).

Despite numerous positive results have been recorded in GPR root mapping (Stokes et al., 2002; Cermak et al., 2000), there is several limitations lead to inaccuracy. Soil texture and moisture content is crucial for function of GPR (Rizzo and Gross, 2000; Stokes et al., 2002). Soil with high water content and high proportion of clay was shown to have negative impact on resolution of root mapping due to interference by high electrical conductivity properties of clay (Satriani et al., 2010).

Study also found that only roots with diameter larger or equal to 0.05m can be detected (Ow and Sim, 2012). This implies fine roots might missing in root mapping. Same results were found in other researches (Hruska et al., 1999; Stokes et al., 2002).

In conclusion, both air spade and GPR is a less-invasive tools compared with traditional solid digging method which roots may be hurt accidentally. For air spade, it helps soil aeration to improve growth of tree apart from root inspection. GPR is a powerful for root mapping that helps investigate roots distribution and assessment of root support system. However, it is significantly affected by various factors which affect the accuracy of the result.

Despite several constraints found using GPR for root assessment, with improvement of the techniques, the accuracy and reliability can be increased (Hruska, et al., 1999). Due to its non-destructive nature and repeatability, there is great potential of future development as an innovative method of root inspection.