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International audience; AbstractKey messageIn a mixed,Fagus sylvaticaL.-Acer pseudoplatanusL., young plantation, trees of both species absorbed water from superficial soil layers despite the presence of roots and water in deeper layers. Admixture proportion and tree density were weak predictors of water acquisition depth, as well as fine root vertical distribution, although it might be due to distinct periods of root and isotope investigations.ContextPromoting mixed forests and reducing stand densities have been proposed as effective ways to maintain the productivity of temperate planted forest stands in a changing climate.AimsThe objective of this study was to analyse how stand density and the degree of admixture of European beech and sycamore maple interactively influenced the water acquisition profile of individual trees.MethodsWe used a stable isotope (deuterium) approach to determine the profiles of soil water acquisition of both species in a 16-year-old plantation where trees had been planted along crossed gradients of tree density and species proportion. The profiles were then compared with the vertical distribution of fine root of these species in the plantation.ResultsAll the target trees mostly absorbed soil water from the first few centimetres of soil despite homogenous vertical water availability and the fact that a great part of the fine root biomass was located below 10 cm. Admixture proportion and tree density had negligible effects on soil water acquisition depth.ConclusionNo vertical differentiation of soil water acquisition between the two species was observed, suggesting that mixing these species does not promote reduction of belowground competition for resource acquisition. The vertical distribution of fine root may be a weak predictor of water acquisition depth.

Species may respond in three ways to environmental change: adapt, migrate, or go extinct. Studies of latitudinal clines can provide information on whether species have adapted to abiotic stress such as temperature and drought in the past and what the traits underlying adaptation are. We investigated latitudinal trait variation and response to drought in North American populations of Arabidopsis lyrata. Plants from nine populations collected over 13° latitude were grown under well-watered and dry conditions. A total of 1,620 seedlings were raised and 12 phenological, physiological, morphological, and life history traits were measured. Two traits, asymptotic rosette size and the propensity to flower, were significantly associated with latitude: plants from northern locations grew to a larger size and were more likely to flower in the first season. Most traits displayed a plastic response to drought, but plasticity was never related linearly with latitude nor was it enhanced in populations from extreme latitudes with reduced water availability. Populations responded to drought by adopting mixed strategies of resistance, tolerance, and escape. The study shows that latitudinal adaptation in A. lyrata involves the classic life history traits, size at and timing of reproduction. Contrary to recent theoretical predictions, adaptation to margins is based on fixed trait differences and not on phenotypic plasticity, at least with respect to drought.

International audience; The relationship between forest productivity and tree species diversity has been described in detail, but the underlying processes have yet to be identified. One important issue is to understand which processes are at the origin of observed aboveground overyielding in some mixed forests. We used a beech-maple plantation exhibiting aboveground overyielding to test whether belowground processes could explain this pattern. Soil cores were collected to determine fine root (FR) biomass and vertical distribution. Correlograms were used to detect spatial arrangement. Near-infrared reflectance spectroscopy was used to identify the tree species proportion in the FR samples and spatial root segregation. An isotopic approach was used to identify water acquisition patterns. The structure and the composition of the ectomycorrhizal fungal community were determined by high-throughput sequencing of DNA in the soil samples. We found no spatial pattern for FR biomass or for its vertical distribution along the gradients. No vertical root segregation was found, as FR density for both species decreased with depth in a similar way. The two species displayed similar vertical water acquisition profiles as well, mainly absorbing water from shallow soil layers; hence, niche differentiation for water acquisition was not highlighted here. Significant alterations in the fungal community compositions were detected in function of the percentage of maple in the vicinity of beech. Our findings do not support the commonly suggested drivers of aboveground overyielding in species-diverse forests and suggest that competition reduction or between-species facilitation of belowground resource acquisition may not explain the observed aboveground overyielding.

International audience; 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.