Effect of species composition, stand density and site index on the basal area increment of oak trees (Quercus sp.) in mixed stands with beech (Fagus sylvatica L.) in northern France

We investigated the relationship between tree size and radial growth of oak in mixed oak-beech stands where beech basal area ranged from 0.0 to 98.2%. We analysed 30 long-term permanent oak-beech plots in northern France with a total of 167 growth periods between 1904 and 2000. The study was based on a nonlinear segmented model of individual tree basal area increment as a function of diameter at breast height, which is implemented in the French forest growth simulator Fagacees. We investigated variations of slope. from the linear branch of the segmented model for oak. We found stand density, species composition, and site index to have a significant influence. The basal area increment of oak decreased with rising admixture of beech and increased with site quality and lower stand density. The effect of beech admixture found in the present study corresponds with findings from tree physiology on below- and aboveground competition. Our results underline differences in competitiveness and long-term species dynamics in mixed oak-beech stands.