## 4 Conclusions  
The paper considered the roller pressing applied to different types of wood to improve their mechanical and other characteristics. The conducted experimental research focused on analysing how the change in hardness, surface roughness, and elastic modulus depends upon the pressing force, i.e., upon the thickness reduction at various values of force and the corresponding increase in the density of the wood. The obtained results indicate that roller pressing has positive effects on the hardness, elastic modulus, and roughness of the processed surface. The ash sample exhibited the largest relative increase in hardness, while the most significant relative increase in the elastic modulus was measured on the linden sample. The correlation coefficients confirm the strongest relationship between the mentioned characteristics (and wood types) and the pressing force. After being modified by roller pressing, the types of wood with otherwise inferior mechanical characteristics become suitable for application wherever higher values of strength and hardness are required. They could be a proper alternative to higher-quality wood types which are becoming increasingly difficult to acquire. The largest relative decrease in surface roughness was achieved on the alder sample. The surface quality is one of the most important properties of solid wood, closely related to its strength characteristics as well as the joining and bonding quality [44; 56; 57]. The potential enhancement of mechanical and other properties is especially significant for lower-quality wood, considering the declining availability of high-quality lumber and its market price.  
Limitations of the research are related to the fact that initial conditions concerning the humidity were not identical for all samples. Therefore, absolute data comparison between different types of wood was not possible. However, all tests on one type of wood were carried out on the same day and under controlled conditions, which allows a relative comparison of the results.  
Future investigations should focus on optimising the pressing force to determine its critical value, i.e., the value that wood samples can withstand without being destroyed. The aim is to achieve the maximum increase in the density of the surface layer, that is, to increase the hardness and elastic modulus while reducing roughness. Furthermore, future research should deal with other parameters, such as rollers' diameters and rotation velocity that affect samples' mechanical and physical characteristics. Additionally, the research may be directed towards multi-criteria evaluation and multi-criteria optimisation of output parameters for different input parameters of the processing.  
In further research, the obtained results should be correlated with the chemical, physical and energetic properties of wood. As mentioned earlier, wood properties differ even within the same species and strongly influence wood performance. Due to the complex nature of wood, a single approach is not sufficient to completely define its properties. The correlation between the obtained results and the properties of wood would further elucidate and help optimize and improve the performance of investigated wooden workpieces [52; 58].  
Conceptualization, V.K., L.B., B.T. and M.P.; methodology, V.K., D.D., S.K. and D.V.; validation, L.B. and M.P.; formal analysis, D.D. and S.K.; investigation, V.K., B.T. and M.P.; resources, D.D., L.B. and D.V.; writing--original draft preparation, V.K., D.D., S.K., L.B. and M.P.; writing--review and editing, B.T. and D.V.; visualization, V.K. and S.K.; supervision, D.V. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The research presented in this paper was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia.  
The authors declare no conflict of interest.