## 3 Results and Discussion
### Surface Roughness  
The three examined roughness indexes, \(Ra\), \(Rz\), and \(Rq\), follow a parallel path to one another, a trend that is also highlighted in the provided summary/clustering diagrams of the five trees (Figures 4-8). Based on this and the available literature, the \(Ra\) index was used in the statistical analyses of the current study to detect correlations among different wood structure factors. Figure 4: Summary bar plot presenting the surface roughness index values (\(Ra\), \(Rz\), \(Rq\) in \(\mu m\)) of tree 1 (including the 5 disks). Figure 5: Summary bar plot presenting the surface roughness index values (\(Ra\), \(Rz\), \(Rq\) in \(\mu\)m) of tree 2 (including the 5 disks). Chestnut wood, characterized by its medium value of density (approximately 0.57 g/cm\({}^{3}\)) and relatively medium to high hardness values (approximately 41.07 N/mm\({}^{2}\) tangentially and 40.29 N/mm\({}^{2}\) radially), was expected to show medium surface roughness values [37; 38; 39]. In general, density and porosity are inversely correlated with each other; therefore, both highly affect smoothness, since high-density samples show lower roughness, though sometimes, also a higher resistance during the sanding process [38; 39]. [PERSON] et al. [40] found a correlation of surface roughness with the hardness and scratch resistance of the wood, which applied in the presence of different moisture contents. They concluded that the effect of wood structures such as earlywood and latewood, and sapwood and heartwood were crucial factors. For instance, the sapwood zones in softwood species show a higher scratch resistance (compared to heartwood), with higher scratch resistance values recorded in the tangential direction. Finally, the scratch resistance is lower in earlywood locations compared to latewood. Figure 6: Summary bar plot presenting the surface roughness index values (_Ra_, _Rz_, _Rq_ in \(\mu\)m) of tree 3 (including the 5 disks). Figure 7: Summary bar plot presenting the surface roughness index values (_Ra_, _Rz_, _Rq_ in \(\mu\)m) of tree 4 (including the 5 disks). In the current study, statistically significant differences in roughness were not detected among the values of the different heights of the tree (among the different tree disks, 1-5, of each tree) when comparing the roughness values in the same variables (measurement direction, specimen's surface planes, areas of sapwood/heartwood). More specifically, according to the statistical analysis, the surface roughness (\(Ra\)) was affected by only 4.5% by the height of the specimen location in the trunk (different disks), presenting higher roughness in the higher tree heights. Statistically significant differences were recorded only between the lowest disks (1-2) and the highest disk (disk 5), proving that there is a slight difference in the surface roughness of the wood depending on the location of the wood specimen in the trunk longitudinally (trunk height). This could be probably attributed to the fact that in the lower tree heights, near the base of the tree, the wood is more mature and the density is higher due to the higher proportion of heartwood, among other factors. The surface roughness, in general, was affected by 72.9% by the factors \"Measurement Direction\" (vertically/parallel), \"Planes\" (transverse, radial, tangential surfaces), and the factor \"Area\" (sapwood/heartwood) in combination. The interaction of the factor trunk \"Height\" with the abovementioned factors of \"Direction\", \"Planes\", and \"Area\" affected the variability of \(Ra\) only by 8%, providing more evidence that the impact of trunk height can be considered to be of lower significance. Nevertheless, significant differences appeared among different trees (comparing the values of measurements of the respective variables of direction, plane types, disks, etc. ), with \"Tree 3\" revealing the statistically significant and lowest roughness compared to the rest of the trees, while the described within-tree trends that were detected apply to each of the tree cases.