International audience* Ecosystem functioning relies heavily on below-ground processes, which are largely regulated by plant fine-roots and their functional traits. However, our knowledge of fine-root trait distribution relies to date on local- and regional-scale studies with limited numbers of species, growth forms and environmental variation. * We compiled a world-wide fine-root trait dataset, featuring 1115 species from contrasting climatic areas, phylogeny and growth forms to test a series of hypotheses pertaining to the influence of plant functional types, soil and climate variables, and the degree of manipulation of plant growing conditions on species fine-root trait variation. Most particularly, we tested the competing hypotheses that fine-root traits typical of faster return on investment would be most strongly associated with conditions of limiting versus favourable soil resource availability. We accounted for both data source and species phylogenetic relatedness. * We demonstrate that: (i) Climate conditions promoting soil fertility relate negatively to fine-root traits favouring fast soil resource acquisition, with a particularly strong positive effect of temperature on fine-root diameter and negative effect on specific root length (SRL), and a negative effect of rainfall on root nitrogen concentration; (ii) Soil bulk density strongly influences species fine-root morphology, by favouring thicker, denser fine-roots; (iii) Fine-roots from herbaceous species are on average finer and have higher SRL than those of woody species, and N2-fixing capacity positively relates to root nitrogen; and (iv) Plants growing in pots have higher SRL than those grown in the field. * Synthesis. This study reveals both the large variation in fine-root traits encountered globally and the relevance of several key plant functional types and soil and climate variables for explaining a substantial part of this variation. Climate, particularly temperature, and plant functional types were the two strongest predictors of fine-root trait variation. High trait variation occurred at local scales, suggesting that wide-ranging below-ground resource economics strategies are viable within most climatic areas and soil conditions

Plants respond to resource stress by changing multiple aspects of their biomass allocation, morphology, physiology and architecture. To date, we lack an integrated view of the relative importance of these plastic responses in alleviating resource stress and of the consistency/variability of these responses among species. We subjected nine species (legumes, forbs and graminoids) to nitrogen and/or light shortages and measured 11 above-ground and below-ground trait adjustments critical in the alleviation of these stresses (plus several underlying traits). Nine traits out of 11 showed adjustments that improved plants' potential capacity to acquire the limiting resource at a given time. Above ground, aspects of plasticity in allocation, morphology, physiology and architecture all appeared important in improving light capture, whereas below ground, plasticity in allocation and physiology were most critical to improving nitrogen acquisition. Six traits out of 11 showed substantial heterogeneity in species plasticity, with little structuration of these differences within trait covariation syndromes. Such comprehensive assessment of the complex nature of phenotypic responses of plants to multiple stress factors, and the comparison of plant responses across multiple species, makes a clear case for the high (but largely overlooked) diversity of potential plastic responses of plants, and for the need to explore the potential rules structuring them.

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Summary 1.The link between species' functional traits and competitive abilities has been described as a major factor structuring plant communities. However, two diverging hypotheses have been proposed to explain this process: competition-trait similarity and competition-trait hierarchy. 2. We performed a greenhouse experiment to determine whether grasses' root foraging strategies, from acquisitive or conservative functional groups, are linked to plant competitive ability and to test which hypothesis better explains interactions during the early stage of grass establishment under contrasting growth conditions. 3. Two grass species of each functional group were grown with and without a neighbour under two levels of water and phosphorus supplies. Three functional traits related to plant competitive ability were measured on all plants grown without neighbours: specific root length (SRL), root phosphorus use efficiency and root length density. Above-ground biomass was measured on plants grown with and without neighbours to evaluate the intensity of plant interaction. 4. We demonstrated that for the three traits the intensity of interaction is driven mainly by hierarchical trait distance, that is, trait distance between target and neighbour, and not by trait similarity. Growth conditions strongly affected the significance of the relation between hierarchical distances and competition intensity. For the SRL hierarchical distance, this effect may be due to the most competitive species (with high SRL) being strongly impacted by water shortage, which modified the competitive hierarchy. Trait plasticity in response to stresses also appeared an important factor influencing the competitive ability of species, that is, species with the most plastic SRL in response to P stress were also the most competitive under P stress. 5. A strong hierarchy exists among grasses' competitive abilities in non-limiting growth conditions that is linked to their root functional traits and investment in the root system. Consequently, our results support the trait hierarchy hypothesis in its ability to describe competitive interaction among grasses during early stages of establishment. 6. Our study provides evidence that root functional hierarchical trait distance and plasticity explain how grasses interact with their neighbours. This distance enables species to be ranked according to their competitive ability; however, this ranking may be influenced by the growth conditions and traits considered.

However, biophysical and ecophysiological constraints during vegetative growth are also at play and can strongly impact crop phenotypes. It has been argued that a broadened examination of crop phenotypes through a functional trait-based lens should improve our understanding of the domestication syndrome.2. We used a collection of 39 genotypes representative of key steps during tetraploid wheat domestication and grew them in a common garden experiment. We quantified the vegetative phenotype of each genotype through the measurements of 13 functional traits related to root, leaf and whole-plant dimensions.3. In modern cultivars, compared to ancestral forms, leaf longevity was shorter, while net photosynthetic rate, leaf production rate and nitrogen content were higher.Modern cultivars had a shallower root system and exhibited a larger proportion of fine roots, preferring to invest biomass above-rather than below-ground. We found ancestral forms to be integrated phenotypes characterized by coordination between above-and below-ground functioning. Conversely, in modern forms, human selection appeared to have broken this coordination and to have generated a new type of network of trait covariations.

221. Deciphering the mechanisms that drive variation in biomass production across plant 23 communities of contrasting species composition and diversity is a main challenge of 24 biodiversity-ecosystem functioning research. Niche complementarity and selection effect have 25 been widely investigated to address biodiversity-productivity relationships. However, the 26 overlooking of the specific role played by key species have limited so far our capacity to 27 comprehensively assess the relative importance of other potential drivers of biodiversity effects. 28 2. Here, we conducted a grassland diversity-productivity experiment to test how four potential 29 facets of biodiversity effects, namely species richness, functional diversity, species identity and 30 the relaxation of intraspecific competition, account for variations in above and root biomass 31 production. 32 3. We grew six plant species in monoculture, as well as in every combinations of two, three and 33 six species. Plant density was kept constant across the richness gradient but we additionally 34 grew each species in half-density monoculture to estimate the strength of intraspecific 35 competition for each studied species. We characterized eight functional traits, including root 36 traits, related to nutrient and light acquisition and computed both the functional dissimilarity 37 and the community weighted mean (CWM) of each trait. We further partitioned aboveground 38 biodiversity effect into complementarity and selection effects. 39 4. We observed strong positive biodiversity effects on both aboveground and root biomass as 40 well as strong positive complementarity effect. These arose largely from the presence of a 41 particular species (Plantago lanceolata) and from CWM trait values more than from a higher 42 functional dissimilarity in plant mixtures. P. lanceolata displayed the highest intraspecific 43 competition, which was strongly relaxed in species mixtures. By contrast, the presence of 44 Sanguisorba minor negatively affected the productivity of plant mixtures, this species suffering 45 more from interspecific than intraspecific competition. 46 5. This study provides strong evidences that the search for key species is critical to understand 47 the role of species diversity on ecosystem functioning and demonstrates the major role that the 48 balance between intraspecific and interspecific competition plays in biodiversity-ecosystem 49 functioning relationships. Developing more integrative approaches in community and 50 ecosystem ecology can offer opportunities to better understand the role that species diversity 51 plays on ecosystem functioning. 52 53 Key words: biodiversity-ecosystem functioning, complementarity effect, functional trait, 54 functional distinctiveness, niche difference, roots, selection effect, species coexistence 55 56 57 58 59 60 61 62 63 64 65 Because ecological niches are theoretically linked to a suite of functional traits (Violle 91 & Jiang 2009), functional traits appear to be a promising tool for understanding diversity...

. We thank Pascal Chapon for his dedicated technical help, the experimental station 'INRA LA Fage' as well as the 'Terrain d'experience' and 'PACE' platforms at CEFE (technical facilities of the Labex Centre Mediterranean de l'Environnement et de la Biodiversite, CEMEB) for providing all the facilities and technical support. F.F. was supported by a grant from1. Understanding the water-use of plants is timely under increasing drought stress due to climate change. Despite the crucial role of roots in water uptake, relationships between water-use and root traits are seldom considered. 2. Combining a functional trait-based approach with a water balance model, we tested whether root functional traits are related to spatial and temporal water-use among 12 Mediterranean rangeland species grown in common garden monocultures. Soil water content was monitored for 10 months, and the dynamics of water uptake of each species was modelled at a daily time step. Root functional traits were measured at two soil depths (shallow and deep soil). 3. Species with fast resource acquisition strategies in shallow soil, i.e. thin roots, maximised water uptake in a short period and consumed large amounts of water during periods of low water availability. Conversely, species with a more conservative root strategy, i.e. coarse roots, took up less water during the peak-growing season, maintained water uptake over a longer period of time and consumed less water during periods of low water availability. Deep root traits are strongly related to species' ability to take up water from deep soil. Deep roots with large diameters and low specific root length improve species' ability to reach water from deep soil. Biomass investment in the deep soil layer was positively related to the amount of water consumed during periods of low water availability. 4. Our results highlight that root functional traits influence a range of spatial and temporal water-use among Mediterranean rangeland species. They account for the amount of water taken up during dry periods but not during the entire growing season

Cover crops can produce ecosystem services during the fallow period, as reducing nitrate leaching and producing green manure. Crop growth rate (CGR) and crop nitrogen acquisition rate (CNR) can be used as two indicators of the ability of cover crops to produce these services in agrosystems. We used leaf functional traits to characterise the growth strategies of 36 cover crops as an approach to assess their ability to grow and acquire N rapidly. We measured specific leaf area (SLA), leaf dry matter content (LDMC), leaf nitrogen content (LNC) and leaf area (LA) and we evaluated their relevance to characterise CGR and CNR. Cover crop species were positioned along the Leaf Economics Spectrum (LES), the SLA-LDMC plane, and the CSR triangle of plant strategies. LA was positively correlated with CGR and CNR, while LDMC was negatively correlated with CNR. All cover crops could be classified as resource-acquisitive species from their relative position on the LES and the SLA-LDMC plane. Most cover crops were located along the Competition/Ruderality axis in the CSR triangle. In particular, Brassicaceae species were classified as very competitive, which was consistent with their high CGR and CNR. Leaf functional traits, especially LA and LDMC, allowed to differentiate some cover crops strategies related to their ability to grow and acquire N. LDMC was lower and LNC was higher in cover crop than in wild species, pointing to an efficient acquisitive syndrome in the former, corresponding to the high resource availability found in agrosystems. Combining several leaf traits explained approximately half of the CGR and CNR variances, which might be considered insufficient to precisely characterise and rank cover crop species for agronomic purposes. We hypothesised that may be the consequence of domestication process, which has reduced the range of plant strategies and modified the leaf trait syndrome in cultivated species.