European beech (Fagus sylvatica L.) is the most commonly occurring deciduous tree species in Central Europe. Because of its competitiveness and (morphological) variability, it occurs in different species mixtures. The aim of the present study was to determine whether, and to what extent, the internal wood properties of European beech are affected by species mixture. Logs obtained from 100 beech trees from the low-mountain Spessart region in Germany were sampled and subdivided into five equal groups representing different mixture types: pure beech stands and four beech stands with the following tree species, respectively: Pseudotsuga menziesii, Picea abies, Pinus sylvestris and Quercus petraea. After logging and sawing, the boards were graded according to strength. For this purpose, the dynamic modulus of elasticity (MOE_dyn) was determined. The MOE_dyn, which is a relevant property for wood quality in the construction sector, was regressed against various independent variables, such as cambial age or mixture type. Relevant differences in wood quality were observed between mixture types. Beech trees in monocultures had the highest stiffness. In particular, species mixture with the light-demanding Quercus petraea or Pinus sylvestris yielded a significant reduction in the stiffness of the beech timber (–10% and –14%, respectively). Therefore, beech glued laminated timber of higher strength classes (≥GL 40c) was not viable using beech from these mixture types. The findings of this study also revealed significant effects of axial board position and cambial age on MOE_dyn. The present transformation of forests into mixed forests affects the quality of the wood. Future research should focus on the dependencies between varying growth conditions—also caused by climate change—and wood formation.

In Central Europe, European beech (Fagus sylvatica L.) is the most frequently occurring hardwood species. An efficient grading method has the potential to promote its utilisation as construction material. Wood density, eigenfrequency and length were measured in 99 European beech logs for calculating the dynamic modulus of elasticity (MOE_dyn) obtained by longitudinal vibration (resonance). In addition, the log taper was measured. Of those logs, 867 boards were cut using a bandsaw. The MOE_dyn in green condition was determined on 505 of the boards and the MOE_dyn in dry conditions was determined on all of them. The r² value between the MOE_dyn of a log and the mean of MOE_dyn of its boards was 0.72 in the wet condition. The MOE_dyn,12% of boards significantly increased by 88 N mm^-2 for each centimetre away from the pith. The negative effect of log taper on MOE_dyn of boards was barely significant (P-value = 0.050). The MOE_dyn,12% was highly dependent on the MOE_dyn,wet (r² = 0.83) and was 17% higher than the MOE_dyn,wet. The mechanical properties of European beech timber exceed those of European softwood species. However, the relationships regarding MOE_dyn between different grading levels in the processing chain appear to be similar to those of softwoods.