The importance of biodiversity in supporting ecosystem functioning is generally well accepted. However, most evidence comes from small-scale studies, and scaling-up patterns of biodiversity-ecosystem functioning (B-EF) remains challenging, in part because the importance of environmental factors in shaping B-EF relations is poorly understood. Using a forest research platform in which 26 ecosystem functions were measured along gradients of tree species richness in six regions across Europe, we investigated the extent and the potential drivers of context dependency of B-EF relations. Despite considerable variation in species richness effects across the continent, we found a tendency for stronger B-EF relations in drier climates as well as in areas with longer growing seasons and more functionally diverse tree species. The importance of water availability in driving context dependency suggests that as water limitation increases under climate change, biodiversity may become even more important to support high levels of functioning in European forests.

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Summary• 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 ( δ 13 C) 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 ; δ 13 C of leaves and fine roots increased (discrimination was reduced). By contrast, significantly lower δ 13 C 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.

We assessed the role of water availability as a factor regulating the ability of beech seedlings to cope with competitive interference for nitrogen resources by an early successional species (Rubus fruticosus). A glasshouse experiment was performed with two levels of interference (beech with and without R. fruticosus) and three levels of irrigation (high, intermediate, none). 15N uptake and partitioning of both species, and composition of N pools in leaves, roots and phloem of beech, were determined. Under all irrigation regimes, 15N uptake by beech seedlings decreased when grown together with R. fruticosus. R. fruticosus had higher 15N uptake rates than beech, under all water supply levels. When irrigation was reduced, a substantial decrease in 15N uptake of beech seedlings and a concurrent increase in 15N uptake by R. fruticosus were observed. Interference by R. fruticosus and low irrigation also affected the 15N partitioning in beech seedlings and resulted in reduced allocation of 15N to the roots. The combination of competitive interference and lack of irrigation led to an increase in soluble non‐protein N in roots and leaves of beech, due to protein degradation. This response was attributed to an increase in levels of amino acids serving as osmoprotectants under these conditions. The concentration of proline in leaves of beech was negatively correlated to shoot water potential. A competition‐induced reduction of total N in leaves of beech under high and intermediate irrigation was found. These results illustrate (1) the advantage of R. fruticosus in terms of N uptake when compared to young beech, particularly under inadequate water supply, and (2) the changes in N composition of beech seedlings in order to cope with reduced soil water and interference by R. fruticosus.

Effects of water stress on phenology, growth, stomatal activity and water status were assessed from April to November 1996 in 2-year-old seedlings of Quercus frainetto Ten. (Quercus conferta Kit.), Quercus pubescens Willd., Quercus macrolepis Kotschy (Quercus aegilops auct.) and Quercus ilex L. growing in containers in northern Greece. All four species developed more than 50% of their total leaf area before the beginning of June--an adaptation to arid climates. Well-irrigated plants tended to develop greater individual leaf area, number of leaves per plant, total plant leaf area, height and root:shoot ratios than water-stressed plants, but the difference between treatments was not significant for any parameter in any species. Quercus macrolepis appeared to be the most drought-tolerant of the four species. It maintained the highest number of leaves of the smallest size and increased the proportion of fine roots during drought. In all species, drought caused significant decreases in stomatal conductance and predawn and midday water potentials from mid-July until the end of August, when the lowest soil water content and highest mean daily air temperatures and midday leaf temperatures occurred; however, the responses were species-specific. Among the four species, Quercus macrolepis sustained the highest stomatal conductance despite very low water potentials, thus overcoming drought by means of desiccation tolerance. Quercus ilex decreased stomatal conductance even before severe water stress occurred, thereby avoiding desication during drought. Quercus pubescens had the highest water potential despite a high stomatal conductance, indicating that its leaf water status was independent of stomatal activity. Quercus frainetto was the least drought-resistant of the four species. During drought it developed very low water potentials despite markedly reduced stomatal aperture.