Drought affects the competitive interactions between Fagus sylvatica seedlings and an early successional species, Rubus fruticosus: responses of growth, water status and δ13C composition

* • Competitive interactions between European beech (Fagus sylvatica L.) seedlings and the early successional species Rubus fruticosus, and the role of water availability, are reported and discussed in relation to management practices and climatic changes in beech ecosystems of Central Europe. * • Responses of growth, water status, gas exchange and carbon isotope composition (δ13C) to two competition and three irrigation treatments were examined in a factorial-design glasshouse study. * • Under regular irrigation, coexistence with R. fruticosus did not significantly affect growth, water potential and gas exchange of beech seedlings. However, moderate water shortage caused a twofold reduction in beech biomass and changes in root : shoot ratios. Drought lowered transpiration rates and predawn water potentials (below the xylem embolism threshold) for F. sylvatica;δ13C of leaves and fine roots increased (discrimination was reduced). By contrast, significantly lower δ13C of R. fruticosus foliage indicated an improved water status. Competitive interference intensified the effects of reduced irrigation. * • Water availability regulates the competitive interactions between beech seedlings and R. fruticosus. Natural regeneration of beech seedlings may be inhibited by interference from a species such as R. fruticosus, especially during summer drought, as predicted by actual climate models.