Tree defect detection systems

Introduction

In general, the overall health and structure of a tree may be assessed visually (Matheny & Clark, 1994; Mattheck, 2007), but these assessments may not predict the extent of internal decay and defects accurately. Some trees did not have any external signs of decay while some others show misleading external symptoms for the defects.  Therefore, a large number of tools were developed to assist the assessment of the extent and location of the decays and defects. Among them, two devices, namely Resistograph and PiCUS Sonic Tomograph, were widely used in Hong kong and their use will be discussed in details.

Resistograph

Resistograph is a method to assess the presence of decay and it is in principle a powered electrical drill with very small diameter. The drill can be penetrated through the assessed wood and the penetration resistance of the drill can be recorded with the results printed on a dendrogram (Nicollotti & Miglietta, 1998).   Various devices applying similar principles are commercially available such as Resistograph® of Rinn Technology and IML FX and FD Series.  Resistograph was proved to be a highly successful tool to locate and predict the presence of decay in trunk (Costello & Quarles, 1999; Johnstone, et al., 2007). This page did not aim to compare which models or series is better, but rather, would like to address the limitations for their use and also suggest improvement in their use in the future.

The first limitation is that it is 1-dimensional device and it can only detect decays or cavities at the direction of drills (Figure 4.1).  This means that the accuracy of assessment is rely on the experience of the Arborist to locate the location of drilling.  On the other hand, it has also provided some rooms for the Arborist who do the assessment to choose the location of drills that more likely to provide the evidence that its favorable for his conclusion.  For example, the Arborist may choose the location which is predictable to have thin thickness of sound wood, and suggest the tree or stem should be removed or the crown to be reduced.  In other case, if the tree is preferable to be kept, the arborist may choose some location that have greatest thickness.

While others may suggest that more drills at different directions can be resulted in a more concise extent of the defect.  However, it would create a large number of holes and tubes that provide pathways for fungal infection (Schwarze & Heuser, 2006; Helliwell, 2007).  Kersten and Schwarze (2005) had also reported that wood shaving from penetration testing provided wood decay fungi with growth substrate.  It is no clear relationship of the number of drills to the relative early decline of the assessed trees, and some trees shows good recovery and with response growth to cover the drilled area, there is still some cases that the assessed trees are suffering from severe fungal attack and decay problem. Therefore, the balance between more complete picture of decay and number of drills should be considered before the assessment, and assessed locations should be based on external sign of decay and results of other tools such as mallet or tomograph.

Another problem is related to the wood density of the assessed trees.  Some species in Hong Kong have very high wood density and the older model of resistograph cannot drill into the assessed trees, especially for the mature specimen.  While some cases can be drilled did not show clear change of resistance at the later portion of the graph.  The information of these species should be communicated among the users.

It is also recorded that the drill bit will change direction when it had met the cavity and the result may be deviated (Nicolotti & Miglietta, 1998).  Furthermore, it is also recorded the drill bit will bend substantially within the trees when it meets harder wood structures and tend to follow the line of least resistance and hence give the assessment result of lower resistance.

To improve the accuracy of assessment of resistograph, the user should make good decision of proposed locations with the aid of detailed visual assessment, taking consideration of any external sign as well as the use of other tools such as mallet.  Furthermore, stronger drill bit that will reduce the chance of deviation and stronger drilling speed for hardwood species assessment should be use.

PiCUS Sonic Tomograph

Sonic tomography can produce a 2-dimensional image showing the internal structure of a solid object by recording differences in the speed of sound wave transmission (Gilbert & Smiley, 2004).  A set of sensors will be connected to the trunk by pins and wires.  Sound waves will be made by tapping each sensor with a small hammer, the Picus system will then measure the transmission time from each tap with the hammer to each sensor.  The distances between sensor and the apparent sound velocities are measured and calculated by the system.  These data are then analyzed and a 2-dimension image of tomograph will be resulted, which can demonstrate the cross section of the tree and present the approximate locations of decays or cavities.

When compared with the resistograph, this device gives a more accurate result if the pattern of decay is irregular, as Picus can gives an image across the whole cross-section.

The use of PiCUS system to assess the extent of defect, however, had some limitations.  It was found that though it is reliable to find out the significant decay of tree trunk, it cannot locate and figure out small borer holes and the smallest detectable defect is in the range of 5 to 10cm (Gilbert & Smiley, 2004).

Second, the reading of the tomograph is relative, not direct measurement of wood density.  For some case minor changes in density and integrity were resulting in large differences in the acoustics of the wood, but not major changes in structural stability.  Furthermore, the tomograph cannot distinguish between crack, decays, star crack or voids (Wang & Allison, 2008; Leong, et al., 2012).  Therefore, back-up check with resistograph would help to verify the result. 

Third, the PiCUS system is found not able to detect the aggressive soft rot in some species due to relatively few reductions in the wood density (Deflorio, et al., 2008) and the back-up check is also a measure to increase the accuracy.

Other limitations include that the device can only detect the decay at trunk at about 20cm height from ground to height of about 3m, which cannot defect the presence of decay for the roots, and its accuracy will be reduced if the tree trunk is not in circular or elliptical shape. Finally, the holes created when using the device will also provide entrance for fungal infection, yet it is suggested not creating large harm in stability by Gilbert & Smiley (2004).

As mentioned, the accuracy of tomograph can be enhanced by the follow-up check with resistograph so that the exact thickness of sound wood can be known for calculation.  Advanced protocol can be developed to assist the assessment of decay at trunk of irregular shape (Gilbert, et al., 2016).  Further development is also required to improve the accuracy of the tomograph to distinguish internal cracks, decay, ring shakes, voids and defensive reaction zone.