## 4 Conclusions  
The volatile composition of _Quercus_ alternative woods compared to traditional oak species only differs quantitatively, as the same compounds have been found. The volatile composition concentration of _Q. pyrenaica_ varies widely depending on cooperative treatments, origin and other factors. The volatile composition of _Q. pyrenaica_ wood can be considered similar to traditional oak species used in winemaking and is probably very interesting for its contribution of coconut, spice, clove, caramel to almond and clove notes due to its higher content of _cis_ lactone, eugenol, 4-VG and furanic compounds. These results have also been confirmed in studies carried out on wines aged in barrels and with pieces of wood with _Q. pyrenaica_. The volatile composition of _Q. faginea_ wood and of the wines aged seems to be qualitatively and quantitative similar to that of the traditional oak species. However, the wines aged with _Q. faginea_ were the worst evaluated from a sensory point of view, so that wood seems to be of no interest for wine aging. _Q. humbolttii_ wood seems to be an interesting wood due to its high contents of 5-MF, guaiaicol, 4-VG, _cis_ isoeguenol and syringol, but poor in furfural, 5-HMF, _cis_ and _trans_ lactones. Wines aged with this wood corroborated the results of the wood.  
Acacia, chestnut, cherry and ash show differences not only quantitatively but also qualitatively, presenting compounds that are not found in _Quercus_ woods and not having in their composition compounds as important as _cis_ and _trans_ lactones. All these woods except ash show some compounds that are not found in any other woods. Toasting causes an increase in the concentrations of most of the volatile compounds, and this increase is especially important in acacia, chestnut and ash woods. Thus, toasted acacia wood shows high contents of guaiaicol, eugenol, isoeugenol, syringol, furanic compounds, syringaldehyde and acetovanillone. Chestnut wood after medium toasting is rich in volatile phenols, furanic compounds and phenolic aldehydes/phenyl ketones. In addition, ash wood after medium toasting has high contents of guaiaicol, isoeguenol, syringol, maltol, vanillin, syringaldehyde and acetovanillone; however, it is rather low in furfural. American ash appears to be more aromatic than European ash. Acacia, chestnut, cherry and ash woods after medium toasting have higher lignin derivatives than _Quercus_ wood. In addition, acacia and chestnut woods are also richer in lipid and carbohydrate derivatives than traditional oaks woods, while cherry and ash woods are relatively poor in these compounds. In the case of wines, those made from acacia and chestnut wood are richer in eugenol. In addition, those made from chestnut wood also stand out for their high vanillin content and those made from acacia wood for their higher 4-EG content. On the other hand, wines aged in ash wood are characterised by being richer in vanillin and acetovanillone. Mulberry wood does not seem to be of great interest for the aging of wines, since it decreases the fruity notes and increases the compounds that contribute negative notes.  
Conceptualization, A.M.M.-G. and M.d.A.-S.; formal analysis, A.M.M.-G., M.d.A.-S., I.N. and R.d.B.-G.; investigation, A.M.M.-G., M.d.A.-S., I.N. and R.d.B.-G.; writing--original draft preparation, A.M.M.-G.; writing--review and editing, A.M.M.-G., M.d.A.-S., I.N. and R.d.B.-G.; supervision, M.d.A.-S. and I.N.; project administration, M.d.A.-S. and I.N.; funding acquisition, M.d.A.-S. and I.N. All authors have read and agreed to the published version of the manuscript.  
This study was financed by Junta de Castilla and Leon VA028U16, MINECO (AGL2014-54602-P and AGL2017-87373-C3-2-R) and Interreg Spain-Portugal for Iberhenol Project.  
For data request, the contact of the corresponding author is present in the affiliation part of each referenced manuscript.  
The authors declare no conflict of interest.