The Application of Sonic Tomography (PiCUS 3 Sonic Tomograph) to Detect and Quantify Hidden Wood Decay in Managed Norway Spruce Stands

Tree mortality plays a vital role in the accuracy of growth and yield calculations. Economic loss caused by Heterobasidion sp. and Armillaria sp. is a common issue in forestry. Estonian forests, which are mostly managed, are susceptible to fungal infection due to freshly exposed wood surfaces, such as stumps and mechanical wounds. External signs of infection are often scarce and may lead to incorrect stand vitality valuation. Modern devices, such as the PiCUS 3 Sonic Tomograph, can be used for non-destructive decay assessment. We assessed decay in two intensively managed Norway spruce (Picea abies L. Karst.) stands in order to identify the reliability of sonic tomography in tree vitality assessment. We hypothesize that the tomograph assessment is more accurate than the visual assessment for detecting the extent of decay damage in Norway spruce stems. The sample trees were first visually assessed without additional equipment. In the second phase, the same sample trees were measured with the PiCUS 3 Sonic Tomograph. In the last part of the study, the sample trees were assessed from the tree stumps following the clear-cut. We identified a relationship (p-value < 0.001) between the tomograph assessment and the stump assessment when major decay was present. We did not discover a relationship between the visual assessment and stump assessment, indicating that evaluating the decay from external signs is inaccurate according to our results. Our data also indicate that the tomograph is not able to detect the early stages of decay damage, since it has no substantial effect on the wood structure.