Assessment of wood decay in middle-aged Norway spruce (Picea abies) stands in Estonia using PiCUS 3 Sonic Tomography

Tree mortality due to wood decay significantly affects the accuracy of growth and yield calculations, especially in forest management context. In Estonia, middle-aged and mature Norway spruce (Picea abies) stands are highly susceptible to Armillaria sp. and Heterobasidion sp. infections. This study aimed to assess the spread of wood decay while contributing new data to the Estonian Network of Forest Research Plots (ENFRP) database. We hypothesized that wood decay is more frequent in stands with a higher proportion of spruce in the upper canopy layer. The study was conducted on 40 plots across mainland Estonia, with 10 trees per plot assessed both visually and with the PiCUS 3 Sonic Tomograph. The results showed no significant relationship between the share of spruce in the upper canopy layer and the presence of decayed trees (p ​ > ​0.05), nor were there significant differences across sites or forest types (p ​ > ​0.05). However, more decayed trees were detected in mesotrophic and meso-eutrophic forests. No significant association was found between prior thinning and decay occurrence (p ​ > ​0.05). The significant difference between the visual and tomographic assessments (p ​ < ​0.05) suggests that external decay indicators, such as cavities, may be unreliable. Significant differences at 30, 80, and 130 ​cm above ground level (p ​ < ​0.05) suggest, as expected, that decay progresses vertically along the trunk. The Hegyi index indicated that competition was significantly influenced by the presence of decayed trees (p ​ < ​0.05), implying that some competition-related mortality may be linked to fungal infection.