A phylogenetic framework of the legume genus Aeschynomene for comparative genetic analysis of the Nod-dependent and Nod-independent symbioses

Background : Among semi-aquatic species of the legume genus Aeschynomene, some have the property of being nodulated by photosynthetic Bradyrhizobium lacking the nodABC genes necessary for the synthesis of Nod factors. Knowledge of the specificities underlying this Nod-independent symbiosis has been gained from the model legume Aeschynomene evenia but our understanding remains limited due to the lack of comparative genetics with related taxa using a Nod factor-dependent process. To fill this gap, we combined different approaches to perform a thorough comparative analysis in the genus Aeschynomene. Results: This study significantly broadened previous taxon sampling, including in allied genera, in order to construct a comprehensive phylogeny. In the phylogenetic tree, five main lineages were delineated, including a novel lineage, the Nod-independent clade and another one containing a polytomy that comprised several Aeschynomene groups and all the allied genera. This phylogeny was matched with data on chromosome number, genome size and low-copy nuclear gene sequences to reveal the diploid species and a polytomy containing mostly polyploid taxa. For these taxa, a single allopolyploid origin was inferred and the putative parental lineages were identified. Finally, nodulation tests with different Bradyrhizobium strains revealed new nodulation behaviours and the diploid species outside of the Nod-independent clade were compared for their experimental tractability and genetic diversity. Conclusions: The extended knowledge of the genetics and biology of the different lineages sheds new light of the evolutionary history of the genus Aeschynomene and they provide a solid framework to exploit efficiently the diversity encountered in Aeschynomene legumes. Notably, our backbone tree contains all the species that are diploid and it clarifies the genetic relationships between the Nod-independent clade and the Nod-dependent lineages. This study enabled the identification of A. americana and A. patula as the most suitable species to undertake a comparative genetic study of the Nod-independent and Nod-dependent symbioses

Nomenclature des genres néo-calédoniens Arthroclianthus Baill. et Nephrodesmus Schindl. (Fabaceae-Desmodieae)

International audienceLa flore vasculaire néo-calédonienne se caractérise par une grande richesse phanérogamique avec plus de 3000 espèces pour un taux d’endémisme spécifique dépassant les 75 %. Seulement 65 % des espèces actuellement décrites ont fait l’objet d’une révision récente, postérieure à 1967. Les Fabaceae ne font pas exception puisque seule la sous-famille des Mimosoideae (ex Mimosaceae) a été révisée en 1983. Depuis, seules quelques espèces de Fabaceae ont été décrites. La majorité des genres de Papilionoideae de Nouvelle-Calédonie possèdent moins de cinq espèces et seulement deux genres sont endémiques de l’archipel : Nephrodesmus Schindl. et Arthroclianthus Baill., situés dans la tribu des Desmodieae, et possédant respectivement six et 19 espèces. La seule monographie d’Arthroclianthus remonte à plus d’un siècle et la dernière espèce décrite au sein de chacun de ces deux genres remonte à plus de 80 ans. Actuellement, l’identification des espèces s’avère délicate en raison de confusions taxonomiques. Dans le but d’améliorer la connaissance botanique des Papilionoideae de Nouvelle-Calédonie, une clarification nomenclaturale et taxonomique d’Arthroclianthus et Nephrodesmus est ici proposée. Cet article constitue ainsi la première étape de la révision de ces genres. Nous réduisons à 12 le nombre d’espèces d’Arthroclianthus, suite à la mise en synonymie de six taxons et l’exclusion d’une espèce, et à quatre le nombre d’espèces de Nephrodesmus. Le genre Nephrodesmus est typifié. Trois lectotypifications spécifiques sont effectuées pour le genre Nephrodesmus et neuf pour Arthroclianthus

Alyxia Banks ex R.Br. in New Caledonia: a clarification of several species complexes, nomenclatural notes, and a description of three new species

International audienceThe genus Alyxia Banks ex R.Br. is partially revised for New Caledonia, with 31 species recognised. The species complex Alyxia tisserantii Montrouz. is discussed and divided into seven species with existing names, and the synonymy is updated accordingly. Alyxia loeseneriana var. macrocarpa Boiteau is elevated to species status due to new flowering material with the name A. paniensis Lannuzel nom. nov., stat. nov. created to accommodate it, due to the preexisting Alyxia macrocarpa Koord. Detailed study of Alyxia caletioides (Baill.) Guillaumin ex Däniker revealed it was in fact made up of two distinct taxa; a new separate species, Alyxia urceolata Lannuzel, sp.nov. is therefore described. Two new species are also described following their recent collection: Alyxia humboldtensis Lannuzel & Gâteblé, sp.nov. is restricted to the summit of Mount Humboldt, and Alyxia minimiflora Lannuzel, sp.nov. is known from schistaceous cliffs around Nouméa. Finally, several nomenclatural issues are discussed, and an updated key to the genus in New Caledonia is provided

Les forêts humides de la province Nord, Nouvelle-Calédonie : synthèse des travaux de recherche 2012-2015

International audienceEt si l'or vert de la Nouvelle-Calédonie n'était pas les roches nickélifères enfouies dans son sous-sol mais plutôt ses forêts luxuriantes qui habillent la Chaîne Centrale ? Installées depuis des millions d'années, que sait-on aujourd'hui de ces forêts ? Comment fonctionnent-elles ? Comment sont-elles organisées ? Comment se régénèrent-elles ? Et surtout, d'où vient leur originalité écologique et floristique ? Depuis 2012, la province Nord s'est engagée dans un partenariat pluriannuel avec l'équipe de recherche en écologie forestière de l'Institut Agronomique néo-Calédonien (IAC) pour mieux connaître ce patrimoine naturel et culturel aussi riche que vulnérable. Leur objectif commun : améliorer les connaissances sur les forêts denses humides de la province Nord afin d'optimiser leur gestion et leur préservation par les services du Développement Economique et de l'Environnement (DDEE). Pendant près de quatre ans, depuis le sol, les airs et même depuis l'espace, les scientifiques ont observé, inventorié, mesuré, décrit, analysé les forêts humides de la province Nord. Désormais, ils en savent davantage sur leur distribution géographique, composition, organisation, dynamique et leur fragilité. Ce travail de grande ampleur résulte d'une collaboration scientifique entre l'IAC, le Cirad, l'IRD, l'unité mixte de recherche AMAP, et le CNES, avec le soutien financier de la Province Nord. Cet ouvrage collectif propose une synthèse illustrée des connaissances acquises lors de cet ambitieux programme de recherche et vous invite à découvrir les étonnants secrets des forêts humides néo-calédoniennes, qu'il devient urgent de préserver

Environmental correlates for tree occurrences, species distribution and richness on a high-elevation tropical island

International audienceHigh-elevation tropical islands are ideally suited for examining the factors that determine species distribution, given the complex topographies and climatic gradients that create a wide variety of habitats within relatively small areas. New Caledonia, a megadiverse Pacific archipelago, has long focussed the attention of botanists working on the spatial and environmental ranges of specific groups, but few studies have embraced the entire tree flora of the archipelago. In this study we analyse the distribution of 702 native species of rainforest trees of New Caledonia, belonging to 195 genera and 80 families, along elevation and rainfall gradients on ultramafic (UM) and non-ultramafic (non-UM) substrates. We compiled four complementary data sources: (i) herbarium specimens, (ii) plots, (iii) photographs and (iv) observations, totalling 38 936 unique occurrences distributed across the main island. Compiled into a regular 1-min grid (1.852 × 1.852 km), this dataset covered ∼22 % of the island. The studied rainforest species exhibited high environmental tolerance; 56 % of them were not affiliated to a substrate type and they exhibited wide elevation (average 891 ± 332 m) and rainfall (average 2.2 ± 0.8 m year−1) ranges. Conversely their spatial distribution was highly aggregated, which suggests dispersal limitation. The observed species richness was driven mainly by the density of occurrences. However, at the highest elevations or rainfalls, and particularly on UM, the observed richness tends to be lower, independently of the sampling effort. The study highlights the imbalance of the dataset in favour of higher values of rainfall and of elevation. Projected onto a map, under-represented areas are a guide as to where future sampling efforts are most required to complete our understanding of rainforest tree species distribution

Regional rainfall and local topography jointly drive tree community assembly in lowland tropical forests of New Caledonia

International audienceAim: To understand how variations in precipitation and topographic wetness influence tree community assembly at both regional and landscape scales in tropical forests. Location: New Caledonia (SW Pacific). Methods: We sampled 40 tree communities in 0.04-ha plots laid along topographic gradients within two landscapes with contrasting precipitation. Within a dry (2,500 mm/year) landscape we used the Topographic Wetness Index (TWI) to sample communities in topographic position with low (e.g., ridges) and high (e.g., valleys) water accumulation. For each sampled species, we measured five functional traits involved in drought resistance and resource acquisition (wood density, leaf area, leaf specific area, leaf dry matter content, and leaf thickness). We first examined trait covariation across species. We then analysed how the functional composition of communities varied between landscapes (according to precipitation) and within landscapes (according to TWI), using trait-based statistics and null models. Results We identified two ecological trade-offs driving trait variation across species: (i) one opposing high hydraulic efficiency to drought resistance, related to a wood economic spectrum; and (ii) the other opposing resource acquisition to resource conservation, related to a leaf economic spectrum. Across landscapes, species with drought resistance strategies were favoured at lower precipitation. Within landscapes, drought resistant species were selected under low TWI in the dry landscape, while low TWI increased the abundance of species with conservative strategies in the wet landscape. Conclusions: Precipitation and topography jointly shape the functional composition of tree communities. At low precipitation, hydric constraints prevailed on ridges and upslopes by filtering drought resistant strategies along the wood economic spectrum. Contrastingly, higher precipitation relaxed the hydric constraints and resource availability became a primary driver of changing strategies along the leaf economic spectrum. Thus, the landscape scale influence of topography on processes driving community assembly and functional composition critically depends on the regional climatic context

Prédire la structure des forêts tropicales humides calédoniennes : analyse texturale de la canopée sur des images pléiades

National audienceLarge-scale characterization of tropical rainforest is a challenge for their conservation and management. Very high spatial resolution images as provided by the Pléiades satellites offer new opportunities to study the structural organization of heterogeneous rainforests with limited accessibility. In this study, we have evaluated the potential of Pléiades images to map structural parameters of New Caledonian rainforests by analyzing texture of forest canopies. We have applied the Fourier transform textural ordination (FOTO) method to very high spatial resolution images to compute texture indices of canopy grain (i.e. a combination of size distribution and spatial pattern of tree crowns). The results have showed that this method was promising to isolate the rainforest from other types of vegetation, and to highlight their structural diversity on a large scale. Finally, this case study showed that the use of Pléiades images is promising to predict the structure of rainforests.Cartographier et classifier les forêts denses humides (FDH) selon une typologie structurale objective est un enjeu majeur pour leur conservation et leur gestion. Les principales contraintes dans l'étude des FDH sont dues à leur forte hétérogénéité et à leur faible accessibilité. Les images satellitaires Pléiades offrent de nouvelles opportunités pour l'étude, à large échelle, de l'organisation structurale de ces forêts. Dans cet article nous avons évalué à travers une étude de cas en Nouvelle-Calédonie leur potentiel pour construire une typologie des FDH à partir d'une analyse de texture de la canopée. La méthode FOTO (FOurier-based Textural Ordination) a été appliquée à des images à très haute résolution spatiale pour produire des indices de texture du grain de la canopée, qui associent la distribution des tailles des couronnes des arbres et leur répartition spatiale. Les résultats ont montré que cette méthode permettait, d'une part d'isoler la FDH des autres formations végétales, et d'autre part de mettre en évidence leur diversité structurale à large échelle. Enfin, cette étude de cas a montré que l'utilisation d'images Pléiades est prometteuse pour prédire la structure des forêts denses humides. Mots-clés : Structure de la canopée, analyse de texture, forêt dense humide, Nouvelle-Calédonie, télédétection, image à très haute résolution Abstract Large-scale characterization of tropical rainforest is a challenge for their conservation and management. Very high spatial resolution images as provided by the Pléiades satellites offer new opportunities to study the structural organization of heterogeneous rainforests with limited accessibility. In this study, we have evaluated the potential of Pléiades images to map structural parameters of New Caledonian rainforests by analyzing texture of forest canopies. We have applied the Fourier transform textural ordination (FOTO) method to very high spatial resolution images to compute texture indices of canopy grain (i.e. a combination of size distribution and spatial pattern of tree crowns). The results have showed that this method was promising to isolate the rainforest from other types of vegetation, and to highlight their structural diversity on a large scale. Finally, this case study showed that the use of Pléiades images is promising to predict the structure of rainforests

Community variation in wood density along a bioclimatic gradient on a hyper-diverse tropical island

International audienceAims:Analyse the controversial relationship existing between wood density, precipitation, temperature and aridity, and its importance in shaping forest communities facing drought.Location: New Caledonia (SW Pacific).Methods: We sampled wood density in 1580 trees belonging to 175 species across nine sites (eight 1-ha plots) located in dry, mesic and humid forests. We tested whether wood density varied across species, communities and habitats, and depended on mean annual precipitation (MAP), mean annual temperature (MAT) and an aridity index (AI). We also tested whether the variations observed in community mean wood density (WDmean) and in wood density standard deviation differed from those obtained by a randomized species distribution across communities. We constructed a phylogenetic supertree at genus level and tested for the non-random distribution of WDmean.Results: We found that WDmean tended to increase with increasing aridity (i.e. decreasing MAP and increasing MAT) and was significantly higher than expected under randomized species distribution at the dry forest site only. However, some species with low wood density grew well at these dry sites. At genus level, wood density was not random within the phylogenetic supertree. Some lineages exhibiting higher wood density than expected by randomization were also the most represented at the dry sites.Conclusions: Although we did not observe an environmental filter sensu stricto, aridity associated with a low MAP and a high MAT at the dry and mesic forest sites was likely to favour species with a high wood density. Our phylogenetic results suggested that species belonging to a few lineages exhibiting high wood density were likely to be adapted to drought, hence favoured by increasing drought