Proceedings e report - Firenze University Press

(a)
(b)
IN THE HEART OF THE LIMBA TREE: DETECTION METHODS FOR HEART ROT AND FALSE HEARTWOOD
Fig. 2 – Resistance profiles of two infected trees: (a) Tree 8: both resistance profiles visualize a large rotten
zone and have low velocities (ca. 850 m s -1 ). (b) Tree 27: measurement B has a low velocity (917 m s -1 ), but the
decrease in resistance could be ascribed to natural density variations if velocity measurements were absent.
3.3. Qualitative assessment of heart rot and limba noir
Two stem disks from Tshela were selected (Fig. 3): the first stem disk of the third tree (3.1) containing
rot and a cavity and the first stem disk of the seventh tree (7.1) with limba noir. When looking at disk
3.1, it is clear that the rotten zone surrounding the cavity is also reflected by a decreased resistance.
Resistance is the highest at the outsides, possibly as an indicator of a higher density (caused by e.g.
narrow growth rings). Preliminary research on dry stem disks from Ivory Coast showed a peak value
in resistance just after entering the darker heart. Passed this point, the resistance decreased quickly. On
fresh stem disks like 7.1, this effect is less clear but not absent. A ring with a higher resistance value is
noticed but the peak values do not always occur after entering the dark heart; they more or less vary
around the border of the dark heart and do not decrease as fast as on dry stem disks. The
reconstruction based on travel times is in both cases problematical.
4. Discussion
Normally, mature limba trees are sensitive to heart rot. In this case, less than 5 % of the sampled trees
was infected, possibly the consequence of good planting material and favourable site conditions.
Rotten or hollow trees are mostly found next to little paths that are regularly used by the local
population. Using our detection methods, hollow and heavily rotten trees are detectable, where before
only hollow trees could be distinguished using velocity measurements [9]. With only two
measurements per tree, the resistance profiles are sometimes hard to interpret. Velocity measurements
give a more straightforward value that can be immediately compared to reference velocity values.
Therefore, velocity measurements are recommended for quick diagnosis. The range of allowable
velocity measurements is still large (650 m s -1 ) so we propose further research on the diagnosis of
small cavities or wood rot within the lower end of this range (1050-1200 m s -1 ).
We confirm earlier results on the influence of density [3]. Higher diameters generate higher velocities
in several tropical tree species, while previous research concluded the opposite [3]. We could expect
that even-aged trees with a larger diameter are fast-growing so less dense and with a smaller velocity.
Still, the trends in several African tree species are far more significant than in the previous study. The
reason for this phenomenon is unknown and could be linked to wood anatomical features.
Transforming resistance profiles into point values is not recommended. Resistance profiles are most
appropriate for mapping of the impact, shape and location of wood anomalies. When analyzing the
reconstructions, small resistance values often indicate rotten areas while peak values can be associated
with the boundaries of limba noir. Acoustic measurements do not provide clear-cut reconstructions
because they were made on stem disks, ignoring the influence of sound propagation in an axial way.
Since stem disks often show cracks, these anomalies are also detected by velocity measurements,
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