Interactive effects of competition and water availability on above- and below-ground growth and functional traits of European beech at juvenile level

European beech (Fagus sylvatica) is one of the most important commercial tree species in Europe. This species is known to suffer from strong intraspecific competition for above- and below-ground resources and recent studies suggest that mixing beech with other species enhances its growth. European Beech is also known to be very sensitive to drought and natural regeneration of this species is therefore threatened under expected climate change scenarios. Whether beech regeneration under future climate conditions could be favoured in mixed forests is still unknown. In the present study, we analysed how species interactions affect the response of beech seedlings to drought. We tested the followings hypotheses: (i) competitor presence and identity impact seedling performance, and (ii) water availability influences the strength and nature of the interactions. In a greenhouse pot experiment, beech seedlings were grown either alone or in the presence of one competitor seedling (beech, sessile oak, Scots pine). A drought treatment was applied to half of the pots and the other half was well-watered. After one growing season, the seedlings were harvested and above- and below-ground growth and functional traits were measured. Under well-watered conditions, functional traits and growth allocation favouring roots indicate that competition occurred mainly for soil resources. Specific fine root length as clearly altered by the presence of pine only, probably through allelopathy or soil acidification. Drought, however, had a strong impact on beech seedling performance whatever the treatment. Drought decreased the importance of negative interactions between seedlings and a positive interaction even occurred between beech and oak. Our results suggest that under the drier climatic conditions expected in the future, naturally regenerating European beech in mixtures with oak may be advantageous for beech seedling survival and growth. In contrast, favouring regeneration or developing plantations which include beech and pine seems less advantageous.