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Detecting Fungal Decay in Palm Stems by Resistance Drilling

Frank Rinn, Heidelberg, Germany
Like trees, palms can deteriorate internally at the stem base from fungal decay, coming from the roots or bud. But visual detection is far more limited because palms do not show secondary growth and hence there is no outer response wood indicating compensation for strength loss due to internal decay. Tapping with a mallet thus is the first option to enhance defect detection. However, because of the bark structure and the internal mechanical design of palms, only extremely hollow stages [of decay] (more than 90%), may be detected through resonant sound reaction of the stem.

Because of the limitations of visual inspection and tapping with a hammer, technical devices can be of great help to detect the earlier stages of internal decay in palms. In principle, sonic tomography is able to detect internal decay in palms too, but due to the mechanical architecture of palm stems, only big cavities or decay pockets can be detected reliably. This is because the high-density outermost areas of palm stems lead the sonic waves around the naturally soft center, and the difference in stress-wave speed between the naturally soft center of palms and decayed wood is quite small.

Because sonic tomographs usually detect and measure the apparent speed of the first incoming wave, they generally do not detect any signals from the very soft center of palms (and some conifers, too). Therefore, conventional sonic tomography is not the most appropriate method for decay detection in palms and usually fails to detect incipient central decay.

Although resistance drilling is not completely non-destructive, it can help inspecting palms. Resistance drilling was originally developed for measuring latewood density of oak tree rings in order to reconstruct past climate (Rinn 1988). As a welcomed side-effect, the ability to detect defects in trees and timber was observed (Rinn, 1989). The first series of devices was sold to scientists and tree- and timber experts for testing suitability in 1987.
Results clearly showed that the key condition for evaluating the capability of the method for inspecting trees, timber structures and poles is to reliably measure and reveal wood density (Görlacher & Hättich 1990). Because of that, early versions of the resistance drills of 1984 with a spring-driven recording mechanism were abandoned because they did not allow for the reliable measurement of wood density: the profiles either over-tune in resonance or were flat-damped and did not show both the real levels and the real variation of the wood density.

In the naturally soft center of conifers or palms, such profiles often dropped down to zero although the wood was intact. In the soft sapwood of broad leaf trees they [resistance drills] often showed an expressionless plateau that is often misinterpreted as sapwood-decay. In narrow tree rings, the profiles often varied from zero to maximum scale, over- and underestimating the real values, making it impossible to interpret the result. Thus, both sensitivity and resolution did not fulfill basic requirements that a reliable measurement tool has to meet. Consequently, electronic regulation and recording was developed and shown to be critical and mandatory for obtaining profiles that can reliably be interpreted in terms of wood condition (Rinn 1990). Because of that, only resistance drills with electronic regulation and recording reveal wood density and its changes correctly (for details see: Basics of typical resistance-drilling Winter 2012).

Fungal decay in palms often arises from the base, originally coming from the roots. To detect this type of decay, drilling is usually carried out at the base of the stem, mostly horizontally as shown here, using a cordless resistance drill. The profile is printed simultaneously in 1:1 scale on a mobile printer connected to the machine via Bluetooth. It is very important to immediately interpret the profile on the spot and determine its meaning for the evaluation of the stability of the palm. Later, in the office, all the surrounding parameters that may influence the profile are not present. If the interpretation is not clear, an additional drilling may be required. All this can only be done efficiently on the spot.



Typical resistance drilling profile of an intact coconut palm (Cocos nucifera). Intact parts are marked green, the bark area is marked brown.

Typical resistance drilling profiles of palms

Interestingly, in coconut palms (Cocos nucifera), often approximately 1/3 of the radius shows a significantly higher density. This seems to be a mechanical design rule for plants of this architecture, weight distribution and wind load pattern. Real date palms (Phoenix dactylifera) often only show 1/10 of the radius with a significantly higher density. Other phoenix palms may show a nearly constant density level across the whole diameter of the stem.

Sometimes the profiles are symmetrically shaped, sometimes one side of the stem is higher in density than the other. The reasons for this are not yet fully understood, but may be correlated to the lean of the stem and prevailing wind direction. In all profiles from intact palms, the curves were found to oscillate along the whole drilling path. However, the magnitude of the density variations can be slightly smaller in the center.

Identification of decay

Decay is detected mainly by identifying profile changes in comparison to the typical pattern. If the density variations are significantly smaller, this indicates incipient decay, if they are absent and the profile level is lower, the decay is advanced. A flat and significantly lower line mostly indicates a void or completely decomposed wood.

If an inspector is unsure whether a profile at the base of a palm shows decay, a reference drilling further up the stem (in the same direction and angle) helps finding the natural density variation pattern to compare with.

Central fungal decay in palm stems mostly leads to profiles with significantly smaller oscillations and a lower mean profile level. Total decomposition would lead to a severe drop of the profile and a nearly flat line.


Due to the grade of the missing density variations, different levels of deterioration can be distinguished, such as incipient (yellow) and advanced (red).


Future extension of decay

Palms do not show the same reaction pattern to decay as other trees, described by Shigo in the CODIT model (Shigo 1979). However, experiences from repeated measurements indicate that the slope of the drilling resistance profile from decay to intact sections seems to represent the radial extension rate of the internal deterioration: the steeper the slope from decay to intact, the slower the radial extension rate.


Thick bark is not decay!

The thickness of the bark can vary significantly, even on the same palm, so it has to be carefully distinguished from potential (external) decay. The profiles shown here were measured on one single Phoenix palm that was not pruned for many years, thus in some areas there were many old fronts to drill through. It is important that such profiles are not misinterpreted as showing decay. This emphasizes the need for direct interpretation of the obtained profiles on the spot. Later, back in the office, the knowledge about the thickness of the bark or fronds is difficult or impossible to reconstruct. Without this knowledge, is is impossible to reliably interpret such profiles.


Shigo, A.L. 1979: Tree decay, An expanded concept. United States Department of Agriculture. Forest Service. Information. Bulletin Number 419.

Rinn, F. 1988: A new method for measuring tree-ring density parameters. Physics diploma thesis, Institute for Environmental Physics, Heidelberg University, 85pp.

Rinn, F. 1989: Eine neue Bohrmethode zur Holzuntersuchung. Holz Zentralblatt Nr. 34 (20.3.1989).S. 529 – 530.

Görlacher R., Hättich, R. 1990: Untersuchung von altem Konstruktionsholz. Die Bohrwiderstandsmehode. Bauen mit Holz, Juni 1990. Erneuerte Auflage: 1992, Holzbaustatik-Aktuell Juli 1992/2.

Rinn, F. 1990: Device for material testing, especially wood, by drill resistance measurements. German Patent 4122494.

Rinn, F., Becker, B., Kromer, B. 1990: Density profiles of Conifers and Deciduous Trees. Proceedings Lund Symposium on Tree Rings and Environment, Lund University.

Rinn, F. 1993: Catalogue of relative density profiles of trees, poles and timber derived from RESISTOGRAPH micro-drillings. Proc. 9th Int. Meeting Non-destructive testing, Madison 1993.

Winnistorfer, P.M., Wimmer, R. 1995: Application of a drill resistance technique for density profile measurement in wood composite panels. Forest Products Journal, Vol. 45, No. 6, P. 90-93, June 1995.

Rinn, F. 2012: Basics of typical resistance-drilling profiles. Western Arborist, Winter 2012, pp. 30-36.