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Traits have been widely used in plant ecology to understand the rules governing community assembly, and to characterize primary strategies that define community structure and ecosystem properties. Relatively little is known as to whether the traits that are ecologically important at macroecological scales are either variable, or of consequence at fine scales within species. We measured trait variation within populations of the grassland plants Festuca ovina L. and Koeieria macrantha (Ledeb.) Schult., to test the hypothesis that fine-scale intraspecific trait variation drives local community structure. Both species showed large genotypic variation for all traits. Size-related traits of genotypes of K. macrantha observed under monoculture predicted their performance in model grassland communities that possessed both genetic and species diversity. The same correspondence was much weaker for the experimental population of F. ovina. A trade-off in allocation between shoot mass and relative allocation to reproduction was evident in the experimental population of F. ovina. Furthermore, both species showed evidence of a positive relationship between specific leaf area (SLA) and allocation to culm mass. This trait covariation indicates the existence of intraspecific trade-offs in life-history and growth strategies similar to those used to define primary plant strategies, which potentially buffers both species abundance and species coexistence against environmental challenges. Key words: community ecology, genotypic diversity, perennial grasses, primary plant strategies, species coexistence, trait trade-offs. En ecologie vegetale, on a largement utilise les caracteres genetiques pour comprendre les regles regissant l'assemblage des communaules, el pour caracteriser les strategies primaires definissant la structure des communautes el les proprietes des ecosystemes. On ne sait pas vraiment si les caracteres ecologiquement importants aux echelles macroecologiques sont variables ou encore une consequence aux fines echelles dans l'espece. Les auteurs ont mesure la variation de caracteres a l'interieur de populations de plantes des prairies, les Festuca ovina L. et Koeleria macrantha (Ledeb.) Schult., pour verifier l'hypothese a savoir que la variation intraspecifique des caracteres aux fines echelles guide la structure de la communaute locale. Les deux especes montrent une importante variation genotypique pour tous les caracteres. Les caracteres lies a la grandeur chez les genotypes du K. macrantha observes en monoculture permettent de predire leur performance dans des communautes herbacees modeles possedant a la fois une diversite genetique et specifique. La meme correspondance s'avere beaucoup plus faible chez la population experimentale du F. ovina. De plus, les deux especes montrent l'existence d'une relation positive entre la surface foliaire specifique (SLA) et l'allocation a la masse des chaumes. La co-variatiou de ces caracteres indique l'existence de compromis intraspecifiques dans le cycle vital et des strategies de croissance, semblables a celles utilisees pour definir les strategies primaires des plantes, lesquelles tamponnent potentiellement l'abondance des deux especes et leur coexistence devant des defis environnemeniaux. Mots-cles : ecologie des communautes, diversite genotypique, herbes perennes, strategie primaire des plantes, coexistence des especes, compromis entre caracteres. [Traduit par la Redaction], Introduction Life-history theory predicts that species growing in specific environments will share similarity in certain key traits or combinations of traits (MacLeod 1894; MacArthur and Wilson 1963; Caswell et al. [...]