## 3 Results and Discussion
### Surface Roughness
The results of the current study also revealed that in all surface planes (transverse, radial, tangential) of the examined disks, when the measurement was implemented vertically to the wood grain, a statistically significant higher roughness value was recorded, compared to the measurement implemented in parallel with the wood grain, which corresponds to statistically significant differences in all the studied cases. It is characteristic that 67.6% of the variability of roughness is being influenced by the factor of measurement \"Direction\" (orientation of vertically/parallel to wood grain). This tendency could be easily explained, taking into account that vertical to the wood grain, higher height differences are encountered due to earlywood-latewood transition zone areas, different growth rings, etc. Chestnut, as a ring-porous hardwood species, also demonstrates differences in cell wall thickness between the earlywood and latewood areas and as is widely accepted [41], the surface roughness is strongly associated with cell wall thickness. More specifically, latewood fibers present thicker cell walls than those of earlywood [8]. The presence of  
Figure 8: Summary bar plot presenting the surface roughness index values (\(Ra\), \(Rz\), \(Rq\) in \(\upmu\)m) of tree 5 (including the 5 disks). earlywood vessels that are characterized by a much higher cell diameter compared to latewood results in density differences in each of the growth rings [8]. In this study, statistically significant differences between heartwood and sapwood roughness values were not demonstrated, although sapwood appeared in some cases to exhibit slightly a higher surface roughness than the corresponding heartwood sites (referring to the same direction of roughness measurement). The wood tissue found in the heartwood part of the trunk consists of cells that have ceased to serve as part of the tree's conduit system and the cells have been filled with storage/healing substances, extracts, etc., presenting a slightly higher density. The density of wood, in combination with its structural characteristics and chemical composition, are all strongly correlated with its roughness [9], with the higher density corresponding most of the time to smoother surfaces. Among the roughness values of the three different surfaces examined on the disks, it was observed that the highest roughness values (_Ra_) were detected, in most disk cases, on the tangential surface of the disks, then on the transverse disks, and finally, the lowest _Ra_ values were recorded on the radial surfaces (concerning the vertical-to-the-grain measurements). This finding could be attributed to the presence of radii on the radial surfaces that probably make the wood surface smooth, as well as to the fact that the tangential surfaces in the samples taken corresponded mostly to the sapwood part of the tree, and therefore contained a higher proportion of sapwood than heartwood, which probably contributed to an increase in the surface roughness. Additionally, concerning the vertical measurements of all the categories, statistically significant differences were not recorded among the different categories. Meanwhile,, regarding the parallel-to-the-grain measurements of all the categories, statistically significant differences were found, with the transverse surfaces of the examined wood specimens to reveal the highest roughness values (_Ra_) (Figure 9). Therefore, when the roughness measurements were conducted parallel to the wood grain, the highest roughness was observed in transverse surfaces, which corresponded to a statistically significant difference between transverse and the other two surface planes (tangential and radial), and these two did not differ significantly from one another.