It is shown that four point resistivity measurements can be used to detect decay in living trees. A low frequency alternating current is injected into the trunk and the induced voltage is measured between two points along the trunk. With additional measurement of the cross section area, the effective resistivity of the trunk is estimated. A comparison within a group of trees shows that trees in decay have approximately a factor of two lower effective resistivity than sound trees. The method is tested on several different groups of spruce (Picea abies); in total more than 300 trees are examined. The tests show that the method can detect decay caused by Heterobasidion annosum with high accuracy. Finite element modeling and simulations are used to validate the method.
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