Effect of tree mixture on the humic epipedon and vegetation diversity in managed beech forests (Normandy, France)

Using a space-for-time substitution procedure, we assessed the patterns of variation during stand development of the humic epipedon and vegetation diversity in a pure European beech (Fagus sylvatica L.) forest and in a mixed beech–hornbeam (Carpinus betulus L.) forest. The humic epipedon was investigated through macro morpho logical characteristics (thickness of the OL, OF, OH, and A horizons) and pH measurements (pHKCl, pHH20, and pH). Vegetation diversity was assessed using correspondence analysis and hierarchical cluster analysis to identify the main plant community succession groups. The groups were compared using three diversity measures (species richness, Shannon index, and evenness index) and three indices describing the community composition (similarity index between stands and index of heterogeneity at the stand and herbaceous layer levels). The pH measurements revealed an acidification of the A horizon during pure beech stand maturation. Macromorphological characteristics showed an alternation of an organic matter accumulation phase and organic matter disappearance along the two silvicultural cycles. Vegetation diversity measures showed small differences between the two forests. Indices accounting for community composition revealed a homogenizing effect of the pure beech silvicultural type on species composition of plant communities occurring during stand development. They also revealed a strong heterogeneity at the herbaceous level. Comparison of species composition indicated an increasing occurrence of acidophilous species in the pure beech forest. We discuss the interest of these results for future sustainable management decisions.