## 3 Results and Discussion
### Ultrasonic Tomography  
3D Ultrasonic tomography (Figure 11) shows a high ultrasonic velocity variability between 500 m/s and 3500 m/s. As is known, the greater the propagation velocity of the \begin{table}
\begin{tabular}{c c} \hline \hline
**Parts of the Stem** & **Vp (m/s)** \\ \hline Pith & 3700 \\ Heartwood & 4550 \\ Sapwood & 4650 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mean velocity values (Vp) of longitudinal ultrasonic signals characterizing the different anatomic parts of a healthy holm oak sample. Figure 10: Different view of the reflectivity pattern of the CRP 3D model of the studied holm oak and represented in a 0–1 intensity color scale. The reflectivity is detected by the natural color radiometry of the tree. In this case, the lowest reflectivity is observed in the southern part of the tree (see blue areas in the figure). longitudinal wave, the better the quality of the wood [16]. Considering the tomographic acquisition scheme, the velocity distribution visible on the tomographic images was the result of the influence of the ultrasonic signal propagation mainly along radial and tangential directions within the holm oak stem. The presence of the bark was not evident in the tomographic reconstruction. This was probably due to its small thickness (0.5-1.0 cm) compared to the ultrasonic signal wavelength of \(\lambda\)-8 cm, considering an average velocity of 2000 m/s and the signal frequency. This means that the resolution of the acoustic tomography was not enough to detect very small defects in the stem. However, this study was aimed to detect the internal decay of the stem to prevent potential risk failure rather than small flaws inside it. A visible decrease in the longitudinal velocity appeared approximately from the middle to the apical part of the stem, where a significant amount of damage was concentrated. In this part, the velocity was slower than 1000 m/s (blue zones in Figure 11). Considering that the longitudinal wave propagation is strictly related to the elastic and mechanical properties of the wood, a decrease in velocity can be related to decayed and diseased zones. For instance, a lower velocity can be caused by fungal degradation of the cell walls, which in normal conditions provide support to the tree [52]. The distribution of the low velocity zones in the middle and upper part of the stem (Figure 11) suggests that the decay affected both heartwood and sapwood. Moreover heartwood decay is one of the most common defects in standing trees [53]. In our case heartwood and sapwood decay could be related to the presence of pathogenic fungi that not only produce tail disease and bark cancer but also internal wood rot. The tomography results showed that the low velocity zone mainly developed in the southern side of the stem, approximately from its middle to its apical part, and could be correlated to widespread decay prevailing in this side. This observation was also corroborated by visual inspection and CRP models. In the southern side of the stem, an extensive bark darkening together with low reflectivity values and serious worsening of its internal elastic characteristics indicated an evolution of the disease from the surface to the inner parts of the stem (Figure 12a-c). This could be related to the exposure of the stem to the most critical microclimate conditions in the part facing south, where an increase in temperature favors fungal growth [54; 55; 56; 57; 58; 59; 60]. The significant volume of the decayed wood suggests that the stem could be at risk of failure. Furthermore, the analysis of the 3D tomographic model highlights a fast transition from high to low velocity zones, where a high contrast of acoustic impedance occurred. This elastic discontinuity could represent a potential wood failure zone (Figure 12c).