Impact of species mixture on the stiffness of European beech (Fagus sylvatica L.) sawn timber

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.