Abstract:
Decay and hollows in stems of Living Trees (LT) are responsible for some tree failures which
may lead to loss of lives and damage to property, especially during stormy weather. This
possible disaster and attendant economic loss could be prevented if such defects are detected
early for timely intervention in cutting down the affected trees. Several attempts have been
made to detect the Location, Extent of Decay and Hollows (LEDH) in LT using resistivity
method with minimal success owing to the electrode configuration adopted. Therefore, this
study was undertaken to detect LEDH in LT.
Eighty LT comprising forty candle trees (Senna alata L. Roxb.) and forty almond trees
(Terminalia catappa L. Roxb.) were purposively selected within the University of Ibadan
campus. The Resistivity Profiles (RP) were obtained from resistivity measurements for the
selected LT, freshly-cut healthy, decayed and hollowed tree stems. The resistivity method
implemented involved the use of an earth resistivity meter and a modified form of
Schlumberger electrode configuration, which employed tiny electrodes with the spacing scaled
down to centimetre range. A laboratory experiment was set up using three wood fabricated
hollow cylinders filled with compacted sawdust to mimic stems of LT. The correlation between
the RP of healthy LT and that of healthy tree replica was determined. The RP of healthy,
decayed and hollowed trees were replicated in the Laboratory Prototypes (LP). Wood decay
was modelled by inserting copper wire lumps into the LP at depths 5.00, 10.00, 15.00 and
20.00 cm from the centre of the modelled decay to the LP surface. Hollows were replicated in
the LP using a plastic cylinder, at depths 4.00, 12.00 and 20.00 cm from the centres of the
modelled hollows to the LP surface. The replicated RP were compared to those of selected LT
to detect LEDH in LT. Data were analysed using descriptive statistics and ANOVA.
The Mean Resistivity Values (MRV) of decayed, healthy and hollowed trees were. 13.52±1.11,
62.59±8.61, 7388.17±1564.58 Ωm for candle trees; and 14.23±1.78, 171.24±33.43,
12430.70±1410.79 Ωm for almond trees. The sharp decrease in MRV of decayed trees may be
due to mobile cations in the decayed region. The rapid increase in MRV of hollowed trees
could be attributed to the non-conductivity of electric current by the hollows. The RP of healthy
LT correlated strongly with that of healthy tree replica (r
2
mean=0.956 for candle trees and
r
2
mean=0.998 for almond trees). Hence, the LP was a true replica of healthy LT. The resistivity
values of the LP ranged between 45 and 80 Ωm. Wood decay replicated in the LP were detected
as resistivity anomalies of 11–17 Ωm representing a decrease by a factor of four compared with
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the healthy tree replica. The embedded hollows were detected as resistivity anomalies of 155–
271 Ωm representing an increase by a factor of three compared with the healthy tree replica.
The location and extent of the resistivity anomalies corresponded to LEDH in LT of similar
dimensions.
The resistivity method with modified Schlumberger electrode configuration detected the
location, extent of decay and hollows in living trees. The method would assist in non-invasive
urban tree management
