Proceedings e report - Firenze University Press

IN THE HEART OF THE LIMBA TREE: DETECTION METHODS FOR HEART ROT AND FALSE HEARTWOOD
below the measuring point. Notes on the health status of trees were written down. The general study
took place on 87 trees in the plantation. None of them showed signs of limba noir. Next to limba trees,
other tree species were analysed with the acoustic method to investigate the influence of density and
diameter (n = 10 trees per species).
The detailed study is reconstructing the inside of the tree, providing two-dimensional visualisations.
This more intensive case study was performed on freshly cut stem disks, where disk surface and
detection measurements could be compared. In total, 8 disks from 5 trees were analysed.
2.3. Detection methods
Both detection methods are standard methods for the detection of cavities and wood anomalies.
However, these two methods are not often used on tropical tree species.
All resistance measurements were performed with a resistograph of the type IML-Resi B400. This
instrument has a needle that penetrates the wood and has a precision of 0.04 mm. Sound wood has a
higher resistance than infected wood. The measurement is semi-destructive, leaving only a small
channel where the needle passed. The result is a resistance profile from the outside of the tree (without
bark for the general study, with bark for the detailed study) to the pith.
A cheaper method consists of the determination of the velocity of sound within stems. Two sensors
were placed upon the tree on opposite sides. With a small hammer, one of the sensors was hit,
transmitting a sound pulse to the opposite sensor. The result has a precision of 1 μs. Every travel time
is the result of five repetitions that were recorded with the Fakopp Microsecond Timer. This
measuring device only leaves two small holes in the tree where the sensors were placed and is less
destructive than the resistograph. When the distance between the two ends of the sensors is measured
with a calliper (precision of 0.5 cm), the velocity can be calculated. The result is one velocity value
per measurement.
2.4. Data analysis
Comparing one velocity value with a complete resistance profile is complicated. After the first trials, a
transformation of the resistance profiles into variables that are comparable with velocity values, took
place. These variables are all values instead of profiles which makes comparison easier. Every
resistance curve shows a similar pattern (Fig. 1): an increasing trend as the needle enters the stem
(zone A) followed by a more stable trend within the main stem (zone B). Using a double linear
regression, we converted the resistance profiles into point values: the breakpoint or C (distance from
the start of the profile to the intersection of the two linear regressions with the best cumulated
determination coefficient, R²), the slope of the first (zone A) and second linear regression (zone B), the
maximum, minimum and range of resistance values after the breakpoint (within zone B, zone A is not
taken into account since resistance values are still in the transition stage after entering the stem). All
these values were compared with velocity and diameter values with special attention for the outliers of
velocity (wood anomalies!).
Fig. 1 – Example of the transformation of resistance profiles to point values.
The result is a breakpoint at point 407, corresponding with a distance of 16.28 mm.
During the detailed study, interpolation of the resistance profiles in order to visualize the internal
distribution of resistances is based on the principle of inverse distance weighing. For every unknown
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