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Background. In wild plant populations, genetic divergence within continuous stands is common, sometimes at very short geographical scales. While restrictions to gene flow combined with local inbreeding and genetic drift may cause neutral differentiation among subpopulations, microgeographical variations in environmental conditions can drive adaptive divergence through natural selection at some targeted loci. Such phenomena have recurrently been observed in plant populations occurring across sharp environmental boundaries, but the interplay between selective processes and neutral genetic divergence has seldom been studied.[br/]Methods. We assessed the extent of within-stand neutral and environmentally-driven divergence in the Neotropical tree [i]Eperua falcate Aubl[/i]. (Fabaceae) through a genome-scan approach. Populations of this species grow in dense stands that cross the boundaries between starkly contrasting habitats. Within-stand phenotypic and candidate-gene divergence have already been proven, making this species a suitable model for the study of genome-wide microgeographic divergence. Thirty trees from each of two habitats (seasonally flooded swamps and well-drained plateaus) in two separate populations were genotyped using thousands of AFLPs markers. To avoid genotyping errors and increase marker reliability, each sample was genotyped twice and submitted to a rigorous procedure for data cleaning, which resulted in 1196 reliable and reproducible markers.[br/]Results. Despite the short spatial distances, we detected within-populations genetic divergence, probably caused by neutral processes, such as restrictions in gene flow. Moreover, habitat-structured subpopulations belonging to otherwise continuous stands also diverge in relation to environmental variability and habitat patchiness: we detected convincing evidence of divergent selection at the genome-wide level and for a fraction of the analyzed loci (comprised between 0.25% and 1.6%). Simulations showed that the levels of differentiation for these outliers are compatible with scenarios of strong divergent selection

† Background and Aims In habitat mosaics, plant populations face environmental heterogeneity over short geographical distances. Such steep environmental gradients can induce ecological divergence. Lowland rainforests of the Guiana Shield are characterized by sharp, short-distance environmental variations related to topography and soil characteristics (from waterlogged bottomlands on hydromorphic soils to well-drained terra firme on ferralitic soils). Continuous plant populations distributed along such gradients are an interesting system to study intrapopulation divergence at highly local scales. This study tested (1) whether conspecific populations growing in different habitats diverge at functional traits, and (2) whether they diverge in the same way as congeneric species having different habitat preferences. † Methods Phenotypic differentiation was studied within continuous populations occupying different habitats for two congeneric, sympatric, and ecologically divergent tree species (Eperua falcata and E. grandiflora, Fabaceae). Over 3000 seeds collected from three habitats were germinated and grown in a common garden experiment, and 23 morphological, biomass, resource allocation and physiological traits were measured. † Key Results In both species, seedling populations native of different habitats displayed phenotypic divergence for several traits (including seedling growth, biomass allocation, leaf chemistry, photosynthesis and carbon isotope composition). This may occur through heritable genetic variation or other maternally inherited effects. For a sub-set of traits, the intraspecific divergence associated with environmental variation coincided with interspecific divergence. † Conclusions The results indicate that mother trees from different habitats transmit divergent trait values to their progeny, and suggest that local environmental variation selects for different trait optima even at a very local spatial scale. Traits for which differentiation within species follows the same pattern as differentiation between species indicate that the same ecological processes underlie intra- and interspecific variation.

Unveiling the genetic basis of local adaptation to environmental variation is a major goal in molecular ecology. In rugged landscapes characterized by environmental mosaics, living populations and communities can experience steep ecological gradients over very short geographical distances. In lowland tropical forests, interspecific divergence in edaphic specialization (for seasonally flooded bottomlands and seasonally dry terra firme soils) has been proven by ecological studies on adaptive traits. Some species are nevertheless capable of covering the entire span of the gradient; intraspecific variation for adaptation to contrasting conditions may explain the distribution of such ecological generalists. We investigated whether local divergence happens at small spatial scales in two stands of Eperua falcata (Fabaceae), a widespread tree species of the Guiana Shield. We investigated Single Nucleotide Polymorphisms (SNP) and sequence divergence as well as spatial genetic structure (SGS) at four genes putatively involved in stress response and three genes with unknown function. Significant genetic differentiation was observed among sub-populations within stands, and eight SNP loci showed patterns compatible with disruptive selection. SGS analysis showed genetic turnover along the gradients at three loci, and at least one haplotype was found to be in repulsion with one habitat. Taken together, these results suggest genetic differentiation at small spatial scale in spite of gene flow. We hypothesize that heterogeneous environments may cause molecular divergence, possibly associated to local adaptation in E. falcata.

International audience; Gravitropism is necessary for plants to control the orientation of their axes while they grow in height. In woody plants, stem re-orientations are costly because they are achieved through diameter growth. The functional diversity of gravitropism was studied to check if the mechanisms involved and their efficiency may contribute to the differentiation of height growth strategies between forest tree species at the seedling stage. Seedlings of eight tropical species were grown tilted in a greenhouse, and their up-righting movement and diameter growth were measured during three months. Morphological, anatomical and biomechanical traits were measured at the end of the survey. Curvature analysis was used to analyse the up-righting response along the stems. Variations in stem curvature depend on diameter growth, size effects, the increase in self-weight, and the efficiency of the gravitropic reaction. A biomechanical model was used to separate these contributions. Results showed that (1) gravitropic movements were based on a common mechanism associated to similar dynamic patterns, (2) clear differences in efficiency (defined as the change in curvature achieved during an elementary diameter increment for a given stem diameter) existed between species, (3) the equilibrium angle of the stem and the anatomical characters associated to the efficiency of the reaction also differed between species, (4) the differences in gravitropic reaction were related to the light requirements: heliophilic species, compared to more shade-tolerant species, had a larger efficiency and an equilibrium angle closer to vertical. This suggests that traits determining the gravitropic reaction are related to the strategy of light interception and may contribute to the differentiation of ecological strategies promoting the maintenance of biodiversity in tropical rain forests.

Previous studies in lowland tropical rainforests of French Guiana showed that, among non-N 2 -fixing trees, two groups of late successional species contrasting in their leaf 15 N natural abundance coexist, suggesting two different main ways of nitrogen acquisition. Two abundant late-successional species typically co-occurring in rainforests in French Guiana, namely Eperua falcata and Dicorynia guianensis, were chosen as representative of each group. Stable isotope techniques and measurements of potentials of microbial N transformation were performed to assess to what extent leaf 15 N natural abundance of these species could be related to (i) δ 15 N signatures of soil mineral N sources and (ii) the capacity of soil to express nitrification and denitrification (both processes being directly involved in the balance between NH 4 + and NO 3 – ). Soil δ 15 N-NH 4 + was roughly similar to leaf δ 15 N of D. guianensis (around 3.5‰), suggesting a preferential use of NH 4 + , whereas in E. falcata, leaf δ 15 N values were closer to root δ 15 N-NO 3 − values (0.2 and −2.0‰, respectively), suggesting a preferential use of NO 3 − . These differences in N source utilization were not accompanied by differences in availability in soil NO 3 − or in intensity of microbial functions responsible for soil N mineral evolution. However, (i) under both tree species, these functions showed clear spatial partitioning, with denitrification occurring potentially in soil and nitrification in the litter layer, and (ii) E. falcata fine roots colonized the litter layer much more strongly than D. guianensis fine roots. This strongly suggests that (i) the contrasted leaf δ 15 N values found in the two late-successional species reveal distinct N acquisition strategies and (ii) the ability of roots to predominantly exploit the litter layer ( E. falcata ) or the soil ( D. guianensis ) may constitute an important explanation of the observed differences. A complementarity between tree species, based on mineral N resource partitioning (itself resulting from a spatially structured location of the microbial functions responsible for the balance between NH 4 + and NO 3 – ), can thus be supposed.

On 10 trees from 10 species of French Guyana tropical rainforest in a clear active process of restoring verticality growth strains were measured in situ in order to determine the occurrence of tension wood within samples. Wood specimens were cut in the vicinity of the growth strains measurements in order to measure some mechanical and physical properties. As suspected, tensile growth strains was very much higher in tension wood zone, because longitudinal modulus of elasticity was slightly higher. Longitudinal shrinkage was also much higher in tension wood than in opposite wood., Des mesures de contraintes de croissance ont été réalisées sur 10 arbres en cours de redressement actif appartenant à 10 espèces de la forêt tropicale humide de Guyane Française afin de s'assurer de la présence de bois de tension. Des échantillons de bois, prélevés au voisinage des mesures de contraintes de croissance, ont permis de mesurer un certains nombres de propriétés physiques et mécaniques. Comme présumé les contraintes de croissance sont beaucoup plus élevées au niveau du secteur de bois de tension, car le module d'élasticité est légèrement plus élevé. Le retrait longitudinal est aussi plus élevé dans le bois de tension que dans le bois opposé.

En Guyane française où les sols sont généralement acides et pauvres en éléments minéraux, la forêt est caractérisée par sa plurispécificité. Une approche de type écologique a montré, dans les racines des principales espèces forestières, la dominance d'endomycorhizes à structure intracellulaire particulière : des hyphes de type Paris, spiralé. Leur rôle dans l'acquisition du phosphore a été étudié expérimentalement chez deux Césalpiniacées. Les semis d'Angélique, Dicorynia guianensis Amshoff, sont très dépendants, pour leur croissance et leur teneur en phosphore, des mycorhizes à mycélium spiralé ; ce type de mycélium représente plus de 90 % de la colonisation fongique. Par contre, les semis de Wapa (Eperua falcata Aublet) forment également des mycorhizes mais ces dernières améliorent le statut phosphaté et sont sans effet sur la croissance de cette espèce. Ces résultats contribuent à la compréhension du rôle des symbioses endomycorhiziennes dans le processus de régénération naturelle et ouvrent des perspectives de recherche quant à leur intervention éventuelle dans le maintien de la diversité forestière.

We inferred water acquisition patterns of two major tropical rainforest canopy tree species, during wet and dry seasons in different soil drainage conditions, based on the natural abundance of O-18 in soil and xylem water and on descriptions of the vertical extension of root systems. Vertical O-18 patterns in the soil were not monotonic and spatially distinct soil layers displayed similar O-18 values. Therefore, vertical patterns of water extraction could only be interpreted by combining the isotopic data with observed root and soil moisture vertical distributions. On sites with deep vertical drainage (DVD), Eperua falcata was able to absorb water down to at least -3.0 m depth, whereas Dicorynia guianensis depended solely on superficial layers. On sites with superficial lateral drainage (SLD), the rooting system of both species was less deep, but Eperua falcata was still able to extract water around -2.0 m depth. Despite these distinct patterns, there was no effect of seasonal soil drought on leaf water status. In terms of adaptation to seasonal soil drought, the strategy of Eperua falcata might be advantageous under occasional severe soil moisture stress, Nous avons estimé la distribution verticale de l'acquisition de l'eau chez deux espèces abondantes de la strate arborée supérieure en forêt tropicale humide, en saison sèche et en saison des pluies, pour des sols qui diffèrent par le type de drainage. Nous avons combiné une approche basée sur les mesures d'abondance naturelle en 18O de l'eau dans le sol et dans l'aubier, et une approche basée sur la description de l'extension verticale du système racinaire des arbres. Les variations de d18O en fonction de la profondeur ne sont pas monotones, des valeurs similaires de d18O sont observées pour plusieurs horizons de profondeurs distinctes. L'interprétation des profils verticaux d'extraction d'eau n'est possible qu'en combinant les données isotopiques et les données relatives aux profils de prospection racinaire et de variations saisonnières d'humidité du sol. Sur le site à drainage vertical libre (DVD), Eperua falcata est capable d'absorber de l'eau jusqu'à 3.0 m de profondeur au moins, alors que l'alimentation en eau de Dicorynia guianensis repose essentiellement sur les horizons supérieurs. Sur le site à drainage superficiel et latéral (SLD), le système racinaire des deux espèces est moins profond, mais Eperua falcata puise tout de même de l'eau jusqu'à –2.0 m de profondeur. Malgré ces différences de profondeur d'extraction de l'eau, l'état hydrique des arbres est maintenu constant en saison sèche et en saison des pluies. En termes d'adaptation à la sécheresse du sol, la stratégie d'Eperua falcata pourrait présenter des avantages lors de sécheresses exceptionnelles

Pour comprendre le comportement macroscopique du bois, il est interessant de connaitre ses proprietes a l'echelle microscopique. Pour cela, diverses techniques d'imagerie nous donnent des informations sur la structure geometrique en deux ou trois dimensions, la composition chimique, la densite ainsi que la presence de reseaux cristallins et leurs orientations. Les proprietes mecaniques sont souvent estimees, soit par des essais sur fibre isolee, soit par modelisation a partir des donnees de rigidite des differents constituants pris hors contexte cellulaire. Aujourd'hui, seules les techniques de nano-indentation permettent une estimation des proprietes mecaniques locales in situ, en un nombre limite de point de mesure. La microscopie acoustique permet la caracterisation des rigidites au niveau local pour des materiaux a forte rigidite (metaux, verres, ceramiques...). L'utilisation de cet outil pour des materiaux beaucoup moins rigides comme le bois, necessite la mise au point d'une procedure experimentale specifique. Les essais realises sur un microscope acoustique en reflexion ont montre que pour des images a fort grossissement, l'utilisation de coupes minces (~ 5 µm) apres inclusion dans la resine avec respectivement l'eau ou une solution d'iodure de potassium comme liquide de couplage a 600 MHz ou a 1 GHz donnait des resultats permettant d'envisager une analyse des rigidites locales a partir des niveaux de gris de l'image acoustique. ––––– To understand wood macroscopic behaviour, it is useful to know its properties at the microscopic level. For this, several imaging techniques can provide information about the geometrical structure in 2 or 3 dimensions, chemical composition, density, crystal lattice and crystal orientation. Wood mechanical properties are determined either by testing isolated fibre, or trough modelling based on stiffness data of the various cell wall components obtained outside the cellular context. Only nanoindentations methods can be used to estimate in situ mechanical properties on a limited number of sites. On the other hand, this can be done on high stiffness materials (metals, glasses, ceramics...) by using acoustic microscopy. To apply this technique to wood, experimental procedure should be assessed. Trials on a scanning acoustic microscope show that for high resolution images, thin slices (~ 5 µm) after resin inclusion with water or potassium iodure as coupling liquid respectively at 600 MHz or 1 GHz give good results giving way to quantitative measurement of cell wall stiffness by image analysis.