Variability of leaf traits related to photosynthesis was assessed in seedlings from 14 tree species growing in the tropical rain forest of French Guiana. Leaf photosynthetic capacity (maximum rate of carboxylation and maximum rate of electron transport) was estimated by fitting a biochemical model of photosynthesis to response curves of net CO2 assimilation rate versus intercellular CO2 mole fraction. Leaf morphology described by leaf mass per unit leaf area (LMA), density and thickness, as well as area- and mass-based nitrogen (N) and carbon (C) concentrations, were recorded on the same leaves. Large interspecific variability was detected in photosynthetic capacity as well as in leaf structure and leaf N and C concentrations. No correlation was found between leaf thickness and density. The correlations between area- and mass-based leaf N concentration and photosynthetic capacity were poor. Conversely, the species differed greatly in relative N allocation to carboxylation and bioenergetics. Principal component analysis (PCA) revealed that, of the recorded traits, only the computed fraction of total leaf N invested in photosynthesis was tightly correlated to photosynthetic capacity. We also used PCA to test to what extent species with similar shade tolerances displayed converging leaf traits related to photosynthesis. No clear-cut ranking could be detected among the shade-tolerant groups, as confirmed by a one-way ANOVA. We conclude that the large interspecific diversity in photosynthetic capacity was mostly explained by differences in the relative allocation of N to photosynthesis and not by leaf N concentration, and that leaf traits related to photosynthetic capacity did not discriminate shade-tolerance ranking of these tropical tree species.

-We characterised the among species variability in leaf gas exchange and morphological traits under controlled conditions of seedlings of 22 tropical rainforest canopy species to understand the origin of the variability in leaf carbon isotope discrimination (∆) among species with different growth and dynamic characteristics (successional gradient). Our results first suggest that these species pursue a consistent strategy in terms of ∆ throughout their ontogeny (juveniles grown here versus canopy adult trees from the natural forest). Second, leaf ∆ was negatively correlated with WUE and N, and positively correlated with g s , but among species differences in ∆ were mainly explained by differences in WUE. Finally, species belonging to different successional groups display distinct leaf functional and morphological traits. We confirmed that fast growing early successional species maximise carbon assimilation with high stomatal conductance. In contrast, fast and slow growing late successional species are both characterised by low carbon assimilation values, but by distinct stomatal conductance and leaf morphological features. Along the successional gradient, these differences result in much lower ∆ for the intermediate species (i.e. fast growing late successional) as compared to the two other groups.13 C / functional diversity / leaf gas exchange / species grouping / tropical rainforest Résumé -Le statut successionnel des espèces de la forêt tropicale humide est associé à des différences de discrimination isotopique du carbone et de traits fonctionnels foliaires. Nous avons caractérisé la variabilité interspécifique des échanges gazeux et des traits morphologiques foliaires en conditions environnementales contrôlées de jeunes plants de 22 espèces d'arbres de la canopée en forêt tropicale humide afin de comprendre l'origine de la variabilité de la discrimination isotopique du carbone foliaire (∆) observée entre ces espèces présentant des caractéristiques de croissance et de dynamique distinctes (groupes successionnels). Nous montrons premièrement que les espèces tropicales possèdent une stratégie très conservée de ∆ au cours de leur ontogénie (juvéniles élevés ici versus arbres adultes de la canopée en forêt naturelle). Deuxièmement, ∆ était négativement corrélée à WUE et N, et positivement à g s , mais les différences de ∆ entre espèces sont principalement expliquées par des différences de WUE. Enfin, nous montrons que les espèces appartenant à des groupes successionnels distincts présentent des traits fonctionnels et morphologiques foliaires distincts. Nous confirmons que les espèces à croissance rapide qui s'installent en premier au cours de la succession écologique (FE) maximisent A avec de fortes conductances stomatiques. Les espèces climax (qui s'installent en second dans la succession écologique), à croissance rapide (FL) ou à croissance faible (SL), présentent des valeurs de A identiques, mais des valeurs de g s ainsi que des caractéristiques morphologiques foliaires distinctes. Dans la succession éc...