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International audience; To limit anthropogenic impact on ecosystems, regulations have been implemented along with global awareness that human activities are harmful to the environment. Ecological risk assessment (ERA) is the main procedure which allows to assess potential impacts of stressors on the environment as a result of human activities. ERA is typically implemented through different steps of laboratory testing. The approaches taken for ERA evolve along with scientific knowledge, to improve predictions on ecological risks for ecosystems. We here address the importance of intraspecific variability as a potential source of error in the laboratory evaluation of pollutants. To answer this question, three aquatic macrophyte species with different life-history traits but with their leaves directly in contact with the water were chosen; Lemna minor and Myriophyllum spicatum, two OECD model species, and Ceratophyllum demersum. For each species, three or four genotypes were exposed to 7-8 copper concentrations (up to 1.9 mg/L, 2 mg/L or 36 mg/L for C. demersum, L. minor and M. spicatum, respectively). To assess species sensitivity, growth-related endpoints such as Relative Growth Rate (RGR), based either on biomass production or on length/frond production, and chlorophyll fluorescence F-v/F-m, were measured. For each endpoint, the effective concentration 50% (EC50) was calculated. Almost all endpoints were affected by Cu exposure, except F v /F n , of M. spicatum, and resulted in significant differences among genotypes for Cu sensitivity. Genotypes of L. minor exhibited up to 35% of variation in EC50 values based on F-v/F-m, showing differential sensivity among genotypes. Significant differences in EC50 values were found for RGR based on length for M. spicatum, with up to 72% of variation. Finally, C. demersum demonstrated significant sensitivity differences among genotypes with up to 78% variation for EC50 based on length. Overall, interspecific variation was higher than intraspecific variation, and explained 77% of the variation found among genotypes for RGR based on biomass, and 99% of the variation found for F-v/F-m. Our results highlight that depending on the endpoint, sensitivity can vary greatly within a species, and that pollutant- and species-specific endpoints should be considered in ERA.

Ant-associated microorganisms can play crucial and often overlooked roles, and given the diversity of interactions that ants have developed, the study of the associated microbiomes is of interest. We focused here on specialist plant-ant species of the genus Allomerus that grow a fungus to build galleries on their host-plant stems. Allomerus-inhabited domatia, thus, might be a rich arena for microbes associated with the ants, the plant, and the fungus. We investigated the microbial communities present in domatia colonised by four arboreal ants: Allomerus decemarticulatus, A. octoarticulatus, A. octoarticulatus var. demerarae, and the non-fungus growing plant-ant Azteca sp. cf. depilis, inhabiting Hirtella physophora or Cordia nodosa in French Guiana. We hypothesized that the microbial community will differ among these species. We isolated microorganisms from five colonies of each species, sequenced the 16S rRNA or Internal TranscribedSpacer (ITS) regions, and described both the alpha and beta diversities. We identified 69 microbial taxa, which belong to five bacterial and two fungal phyla. The most diverse phyla were Proteobacteria and Actinobacteria. The microbial community of Azteca cf. depilis and Allomerus spp. differed in composition and richness. Geographical distance affected microbial communities and richness but plant species did not. Actinobacteria were only associated with Allomerus spp.

Alders (Alnus spp.) represent keystone species trees of riparian and mountainous habitats of the northern hemisphere. Previous genetic studies have suggested a complex intrageneric diversification with numerous events of interspecific hybridization and polyploidization. Here, we first aim to test the present taxonomical treatment of Alnus by generating phylogenetic hypotheses based on plastid and nuclear data obtained from species belonging to the three main alder subgenera (Alnus, Alnobetula, and Clethropsis). A genome-skimming strategy was used to assemble the complete plastome and the nuclear ribosomal DNA cluster of 22 Eurasian and American alder individuals. Phylogenies based on these data strongly support an early diverging subgenus Alnobetula, while members of the subgenus Clethropsis do not constitute a monophyletic clade and are embedded within the subgenus Alnus. Incongruent topologies also sustain reticulate evolution within this group. Our results thus suggest considering the subgenera Clethropsis and Alnus within the same taxonomical unit. Our second aim is to test for the utility of highly variable plastid markers (microsatellites) to investigate the phylogeographic patterns of Eurasian alder species. Fifty-two polymorphic plastid microsatellite markers were developed and tested on 33 populations of the subgenus Alnus in western Eurasia. On average, 4.3 alleles per locus were revealed in 131 individuals of Alnus glutinosa, allowing the identification of 30 chlorotypes (multiloci profiles). Strong phylogeographic signals and recurrent cytoplasmic captures between co-occurring species are revealed, demonstrating that our plastid microsatellite profiling method is suitable for tracing the post-glacial spread of maternal lineages among alder species. All these results finally support the use of nuclear genomic regions for species identification and of plastid markers for phylogeographic aspects and origin certification in genetic resource management.

Summary Global-scale analyses of ectomycorrhizal (ECM) fungi communities emphasize host plant families as the main drivers of diversity. This study aims to test, on Alnus–ECM communities, which fungi are said to be ‘host-specific’, to what extent host species, habitat and distance explain their alpha and beta diversity variations, and their specificity. In France, ECM communities associated with two subgenera and five species of Alnus, were sampled on 165 trees from 39 lowland to subalpine sites. In all, 1178 internal transcribed spacer (ITS) sequences of ECM fungi clustered in 86 molecular operational taxonomic units (MOTUs). The species richness was low but still variable, and the evenness of communities was lower on organic soils and in Corsica. Similarity between communities was influenced both by host, soil parameters, altitude and longitude, but not by climate and distance. A large majority of ‘specific’ fungi were shared between host species within a subgenus, and showed habitat preferences within the subgenus distribution range. Our study confirms that Alnus ECM communities are low in diversity, highly conserved at a regional scale, and partly shared between congeneric host species. A large part of alpha and beta diversity variations remained unexplained, and other processes may shape these communities.

This synthesis has as main objectives to gather knowledge concerning the mycorrhizal status of Juniperus species and more particularly that of J. thurifera. The results highlight the obvious lack of scientific knowledge on the role of mycorrhizal symbiosis in the ecology of this forest species. However, despite these scientific gaps, some observations can be considered to introduce the mycorrhizal symbiosis in afforestation programs with Juniperus either through controlled mycorrhization or accompanying vegetation management by identifying the most effective nurse plants. Examples of validation of these ecological engineering techniques are presented with another Cupressacée, Cupressus atlantica, highlighting the possibilities of transferring these cultural techniques to the model Juniperus. To ensure a positive impact of the mycorrhizal symbiosis in Juniperus management plans, it is necessary to learn much more concerning the structure of the mycorrhizal communities associated with J. thurifera and their ecological roles in the different stages of tree development., Cet article de synthèse a pour principaux objectifs de rassembler des connaissances concernant le statut mycorhizien des espèces du genre Juniperus et plus particulièrement celui de J. thurifera. Les résultats présentés mettent en exergue le manque évident de connaissances scientifiques sur le rôle de la symbiose mycorhizienne dans l’écologie de cette essence forestière. Toutefois et malgré ces lacunes, quelques observations permettent d’envisager une valorisation performante de la symbiose mycorhizienne dans les opérations de gestion des formations à Juniperus que ce soit via la mycorhization contrôlée ou la gestion de la flore accompagnatrice en identifiant les plantes nurses les plus efficaces. Des exemples de validation de ces techniques d’ingénierie écologique sont présentés avec une autre Cupressacée, le Cyprès de l’Atlas, en soulignant les possibilités de transfert au modèle Juniperus. Afin de valoriser au mieux la symbiose mycorhizienne dans la mise en place de plan de gestion des formations à Juniperus, il sera nécessaire d’acquérir des connaissances beaucoup plus conséquentes au niveau de la structure du cortège mycorhizien associé à J. thurifera et de son rôle écologique dans les différents stades de développement de l’arbre., Sanguin Hervé, Khoulassa Siham, Zarik Lamia, Gryta Hervé, Boumezzough Ali, Ouahmane Lahcen, Hafidi Mohamed, Ouhammou Ahmed, Prin Yves, Dreyfus Bernard, Cambecèdes Jocelyne, Miché Lucie, Gauquelin Thierry, Duponnois Robin. Rôles potentiels de la symbiose mycorhizienne dans la conservation des populations méditerranéennes de Genévrier thurifère (Juniperus thurifera L.). In: Ecologia mediterranea, tome 39 n°1, 2013. Actes du IVe Colloque international sur le Genévrier thurifère. 5-8 octobre 2011 Mont-Dauphin et Saint-Crépin (Hautes-Alpes, France) pp. 99-107.

Population studies have revealed that the fungal ectomycorrhizal morphospecies Tricholoma scalpturatum consists of at least two genetically distinct groups that occur sympatrically in several geographical areas. This discovery prompted us to examine species boundaries and relationships between members formerly assigned to T. scalpturatum and allied taxa using phylogenetic analyses. Sequence data were obtained from three nuclear DNA regions [internal transcribed spacer (ITS), gpd and tef], from 101 carpophores collected over a large geographical range in Western Europe, and some reference sequences from public databases. The ITS was also tested for its applicability as DNA barcode for species delimitation. Four highly supported phylogenetic clades were detected. The two previously detected genetic groups of T. scalpturatum were assigned to the phylospecies Tricholoma argyraceum and T. scalpturatum. The two remaining clades were referred to as Tricholoma cingulatum and Tricholoma inocybeoides. Unexpectedly, T. cingulatum showed an accelerated rate of evolution that we attributed to narrow host specialization. This study also reveals recombinant ITS sequences in T. inocybeoides, suggesting a hybrid origin. The ITS was a useful tool for the determination of species boundaries: the mean value of intraspecific genetic distances in the entire ITS region (including 5.8S rDNA) was

Tricholoma scalpturatum is an ectomycorrhizal fungus that forms symbioses with roots of diverse trees and shrubs. It is commonly encountered in a wide range of habitats, across temperate ecosystems. A previous study has revealed a high genetic diversity at a local scale, and ruderal abilities. To examine genetic structure at a large geographical scale, a total of 164 basidiocarps were collected from 30 populations located in Western Europe, from Spain to Scandinavia. These samples were analyzed by three molecular methods with different levels of resolution: inter-simple sequence repeats (ISSRs), restriction fragment length polymorphisms (RFLPs) in the rDNA internal transcribed spacer (ITS), and ITS sequence analysis. Considerable genetic variation was found, and the morphospecies was separated into two genetic groups that were distinct from each other. The ISSR data and the relatively low percentage value (96%) of shared sequence polymorphisms in the ITS between isolates from the two groups, strongly suggest cryptic species and long-lasting separation. No geographical exclusion was detected for these two widely distributed taxa. However, high estimates of population differentiation were observed in each group, including between populations less than a few kilometers apart. This result provides evidence for limited gene flow and/or founding effects. It also indicates that T. scalpturatum does not constitute a random mating population, and the hypothesis of endemism cannot be excluded for this cosmopolitan wind-dispersed fungus.

Paxillus rubicundulus P.D. Orton has been shown to be a complex of at least three ectomycorrhizal taxa strictly associated with alders (Alnus) in Europe, P. rubicundulus s. str., and two undescribed clades. To assess the taxonomic status of these three clades and their phylogenetic relationships, phylogenetic analyses of two independent gene regions (ITS and gpd), combined with macro- and micromorphological comparisons of genetically identified specimens, were carried out. A total of 85 sequences were successfully obtained from basidiomata and alder mycorrhizae collected in France and Algeria and combined with GenBank and UNITE sequences. The phylogenetic results and estimates of genetic diversity confirmed that the three clades are distinct species, often found in sympatry. As a result, P. rubicundulus s. str. was redefined based on the revision of type material, and Paxillus adelphus and Paxillus olivellus are introduced as new Linnaean names. The often used name Paxillus filamentosus is rejected since it could not be applied to any of the new species. The three species are distinguished micromorphologically by spore size and shape. They are widely distributed in Europe, North Africa and western Asia; P. rubicundulus is rare, and all species have a limited host range.

Because of their ecological characteristics, slow growth rates and the presence of contaminants, Chaetothyriales fungi associated with structures built by tropical plant-ants can be difficult to isolate with standard procedures. Here, we describe an easy-to-use protocol for obtaining pure cultures by using cotton as a first substrate. We have further found by means of fluorescent stains that nuclei concentrate either in young hyphae or in the tips of the hyphae.

Paxillus involutus is a model species for ecological or physiological studies of ectomycorrhizal agaricomycetes. Three to six groups or species linked to it have been ecologically and morphologically distinguished. Phylogenetic studies have revealed the existence of four species in Europe: Paxillus ammoniavirescens, Paxillus obscurisporus, P. involutus, and a fourth as yet not described species. We studied 47 collections from 24 French and Italian locations, supplemented with GenBank data, in order to genetically and taxonomically delineate these species. Phylogenetic analyses of three nuclear DNA regions (rDNA internal transcribed spacer (ITS), tef1-α, and gpd) confirmed the four European species. Morphology, culture, and ecology features allowed us to delineate species boundaries and to describe the fourth species we named Paxillus cuprinus since it turns coppery with age. As there is no existing original herbarium specimen for P. involutus, one of our collections was chosen as the epitype. The low genetic diversity found in P. cuprinus correlates with stable morphological traits (basidiome colour, ovoid-amygdaliform spores with an apical constriction) and with ecological preferences (association with Betulaceae in open and temperate areas). In contrast, P. ammoniavirescens is characterized by a high genetic diversity and a high variation of its morphological and ecological features.

Because of their ecological characteristics, slow growth rates and the presence of contaminants, Chaetothyriales fungi associated with structures built by tropical plant-ants can be difficult to isolate with standard procedures. Here, we describe an easy-to-use protocol for obtaining pure cultures by using cotton as a first substrate. We have further found by means of fluorescent stains that nuclei concentrate either in young hyphae or in the tips of the hyphae.

Ectomycorrhizal (EM) fungi are major microbial components of boreal, temperate and Mediterranean forests, as well as some tropical forest ecosystems. Nearly two decades of studies have clarified many aspects of their population biology, based on several model species from diverse lineages of fungi where the EM symbiosis evolved, i.e. among Hymenomycetes and, to a lesser extent, among Ascomycetes. In this review, we show how tools for individual recognition have changed, shifting from the use of somatic incompatibility reactions to dominant and non-specific markers (such as random amplified polymorphic DNA (RAPD) and amplified fragment length polymorphism (AFLP)) and, more recently, to co-dominant and specific markers (such as microsatellites and single nucleotide polymorphisms (SNPs)). At the same time, the theoretical focus has also changed. In earlier studies, a major aim was the description of genet size and popul/ation strategy. For example, we show how some studies supported or challenged the simple, classical model of colonization of new forest stands by ruderal (R) species, propagating by spores and forming small genets, progressively replaced in older forests by more competitive (C) species, propagating by mycelial growth and forming larger genets. By contrast, more recent studies give insights into some genetic traits, such as partners’ assortment (allo- versus autogamy), genetic structure of populations and gene flow that turn out to depend both on distance and on whether spores are animal- or wind-dispersed. We discuss the rising awareness that (i) many morphospecies contain cryptic biological species (often sympatric) and (ii) trans- and inter-continental species may often contain several biological species isolated by distance. Finally, we show the emergence of biogeographic approaches and call for some aspects to be developed, such as fine-scale and long-term population monitoring, analyses of subterranean populations of extra-radical mycelia, or more model species from the tropics, as well as from the Ascomycetes (whose genetic idiosyncrasies are discussed). With the rise of the ‘-omics’ sciences, analysis of population structure for non-neutral genes is expected to develop, and forest management and conservation biology will probably profit from published and expected work.

Plants often rely on external, mutualistic partners to survive and reproduce in resource-limited environments or for protection from enemies. Such interactions, including mycorrhizal symbioses and ant-plant associations, are widespread and play an important role at the ecosystem and community levels. In ant-plant mutualisms, the plants may benefit from both the protection provided by the presence of ants and from the nutrients absorbed from insect debris. However, the role of third partners in plant nutrition, particularly ant-associated fungi, has never before been demonstrated. 2. We investigate this issue in the ant-plant Hirtella physophora. In this model system, Allomerus decemarticulatus ants are involved in two, highly specific interactions: first, with their host plant, and, secondly, with a fungus that they actively manipulate. Moreover, the ants combine both plant trichomes and fungal hyphae to make a trap to capture prey. 3. We empirically demonstrate the existence of a third type of interaction between the fungus and the plant through the use of both experimental enrichments with stable isotopes (15N) and histological approaches. The fungus growing in the galleries plays a role in providing nutrients to the host plant, in addition to the structural role it plays for the ants. Fungus-facilitated nitrogen uptake occurs mainly in old domatia, where abundant hyphae are in close contact with the plant cells. Whether the fungi inside the domatia and those in the galleries are the same is still uncertain. 4. Synthesis. Together, our results show that a fungal partner in an ant-plant mutualism can benefit the plant by improving its nutrient uptake, and they demonstrate the existence of a true tripartite mutualismin this system. Our results add further evidence to the notion that interpretations of some ant-plant symbioses as purely protective mutualisms have overlooked these nutritional aspects.

Population studies have revealed that the fungal ectomycorrhizal morphospecies Tricholoma scalpturatum consists of at least two genetically distinct groups that occur sympatrically in several geographical areas. This discovery prompted us to examine species boundaries and relationships between members formerly assigned to T. scalpturatum and allied taxa using phylogenetic analyses. Sequence data were obtained from three nuclear DNA regions [internal transcribed spacer (ITS), gpd and tef], from 101 carpophores collected over a large geographical range in Western Europe, and some reference sequences from public databases. The ITS was also tested for its applicability as DNA barcode for species delimitation. Four highly supported phylogenetic clades were detected. The two previously detected genetic groups of T. scalpturatum were assigned to the phylospecies Tricholoma argyraceum and T. scalpturatum. The two remaining clades were referred to as Tricholoma cingulatum and Tricholoma inocybeoides. Unexpectedly, T. cingulatum showed an accelerated rate of evolution that we attributed to narrow host specialization. This study also reveals recombinant ITS sequences in T. inocybeoides, suggesting a hybrid origin. The ITS was a useful tool for the determination of species boundaries: the mean value of intraspecific genetic distances in the entire ITS region (including 5.8S rDNA) was0.2%, whereas interspecific divergence estimates ranged from 1.78% to 4.22%. Apart from giving insights into the evolution of the T. scalpturatum complex, this study contributes to the establishment of a library of taxonomically verified voucher specimens, an a posteriori correlation between phenotype and genotype, and DNA barcoding of ectomycorrhizal fungi.

[EN] Ant-fungus associations are well known from attine ants, whose nutrition is based on a symbiosis with basidiomycete fungi. Otherwise, only a few non-nutritional ant-fungus associations have been recorded to date. Here we focus on one of these associations involving Allomerus plant-ants that build galleried structures on their myrmecophytic hosts in order to ambush prey. We show that this association is not opportunistic because the ants select from a monophyletic group of closely related fungal haplotypes of an ascomycete species from the order Chaetothyriales that consistently grows on and has been isolated from the galleries. Both the ants' behaviour and an analysis of the genetic population structure of the ants and the fungus argue for host specificity in this interaction. The ants' behaviour reveals a major investment in manipulating, growing and cleaning the fungus. A molecular analysis of the fungus demonstrates the widespread occurrence of one haplotype and many other haplotypes with a lower occurrence, as well as significant variation in the presence of these fungal haplotypes between areas and ant species. Altogether, these results suggest that such an interaction might represent an as-yet undescribed type of specific association between ants and fungus in which the ants cultivate fungal mycelia to strengthen their hunting galleries., We are grateful to D. Reynolds for his help with the determination of the fungi; to R. Boulay for providing us with the Monomorium subopacum sample; and to A. Yockey-Dejean, K. G. Dexter and M. A. Fares for proofreading the manuscript. We would also like to thank the Laboratoire Environnement de Petit Saut for furnishing logistical help. Financial support for this study was provided by a research programme of the French Agence Nationale de la Recherche (research agreement no. ANR-06-JCJC-0109-01), by the ESF-EUROCORES/TECT/BIOCONTRACT programme, by the Programme Amazonie II of the French Center National de la Recherche Scientifique (CNRS), by the Programme interdisciplinaire 'Interface Physique Chimie Biologie: soutien a la prise de risque' of the CNRS, and by a Nouragues research grant from the CNRS.

Dispersal and establishment are fundamental processes influencing the response of species to environmental changes, and the long-term persistence of populations. A previous study on the symbiotic ectomycorrhizal fungus Tricholoma scalpturatum revealed strong genetic differentiations between populations in Western Europe, suggesting restricted dispersal for this wind-dispersed cosmopolitan fungus. Two distinct genetic groups (genetic groups 1 and 2), co-occurring in some locations, were also identified and could correspond to cryptic species. In the present work, we examine the reproductive strategy and dispersal biology of the two T. scalpturatum's genetic groups. Variable molecular markers (intersimple sequence repeats and intergenic spacer 2-restriction fragment length polymorphisms) and spatial autocorrelation analyses were used to examine fine-scale patterns (< 140 m) of genetic structure, in an effort to determine the physical scale at which genetic structure exists. A total of 473 fruit bodies were mapped and collected over 3 years from two plots located in the south of France, including 219 and 254 samples from group 1 and group 2, respectively. High genetic diversity and the presence of numerous small genets were observed in both groups. Autocorrelation analyses revealed significant positive spatial genetic structures of genets at close distances (up to few metres for both groups). Mantel tests confirmed this isolation-by-distance pattern. These results clearly demonstrate high sexual reproduction and spatial structuring of genets at very small geographical scales in this wind-dispersed ectomycorrhizal fungal species, a pattern consistent with restricted contemporary dispersal of spores.

Summary Fungi combine sexual reproduction and clonal propagation. The balance between these two reproductive modes affects establishment dynamics, and ultimately the evolutionary potential of populations. The pattern of colonization was studied in two species of ectomycorrhizal fungi: Tricholoma populinum and Tricholoma scalpturatum . The former is considered to be a host specialist whereas T. scalpturatum is a generalist taxon. Fruit bodies of both basidiomycete species were mapped and collected over several years from a black poplar ( Populus nigra ) stand, at two different sites. Multilocus genotypes ( = genets) were identified based on the analysis of random amplified polymorphic DNA (RAPD) patterns, inter-simple sequence repeat (ISSR) patterns and restriction fragment length polymorphisms (RFLPs) in the ribosomal DNA intergenic spacer (rDNA IGS). The genetic analyses revealed differences in local population dynamics between the two species. Tricholoma scalpturatum tended to capture new space through sexual spores whereas T. populinum did this by clonal growth, suggesting trade-offs in allocation of resources at the genet level. Genet numbers and sizes strongly differ between the two study sites, perhaps as a result of abiotic disturbance on mycelial establishment and genet behaviour.

R. 2002. Forest habitat characteristics affect balance between sexual reproduction and clonal propagation of the ectomycorrhizal mushroom Hebeloma cylindrosporum.-Oikos 99: 25-36. We investigated how habitat characteristics and in particular the level of disturbance was affecting colonisation processes and population dynamics of an ectomycorrhizal fungus. The basidiomycete Hebeloma cylindrosporum associated with Pinus pinaster trees occurs in two distinct habitats within the forest dune ecosystem along the Atlantic coast of France. The 'dune habitat' (D habitat) is characterised by scattered trees growing in almost pure sand at the extreme fringe of the forest marking the transition with the bare dune. The 'forest habitat' (F habitat) is within the mature forest in places of high human frequentation which prevents litter and humus accumulation. We studied over several years (up to ten) populations of H. cylin-drosporum present in three geographic sites. Within each site we studied two areas corresponding each to one of the two possible habitats. In all sampled areas fruit bodies occurred within discrete patches of ground. Although an interaction between habitat and geographic site existed, habitat had significant effects on patch sizes and patch internal densities. Compared to the F habitat, patches in the D habitat were significantly larger and presented a lower density of fruit bodies. Patches very seldom occurred at the same place from one year to the next in the F habitat when in most cases they were perennial and increased in size in the D habitat. Fruit body size, which reflects reproductive output of the genets, differed between the two habitats (means of the cap diameters of 25.6 and 40.4 mm in respectively the F and D habitats). Almost all patches in the F habitat were made of several annual genets each producing on average 1.6 fruit bodies while most patches in the D habitat were dominated by one perennial genet which could simultaneously produce up to 60 fruit bodies. Under-representation of small sized genets in the D habitat suggests a low level of sexual recruitment in this habitat despite the large number of fruit bodies (and therefore of spores) produced by each of the existing genets. We therefore suggest that H. cylindrosporum could be considered as a fugitive species. Genets of this species survive in the D habitat in which few ectomycorrhizal fungal competitors occur. This habitat could represent the optimal habitat for this species. In the F habitat frequent recruitment could occur in 'empty patches' resulting from the local and transient elimination of competitors induced by human disturbance.

International audience

La fragmentation et la perte d’habitat font parties des menaces actuelles qui pèsent sur les écosystèmes forestiers. Ces modifications du paysage affectent la richesse spécifique et la composition des communautés. Cependant, une hypothèse récente (Fahrig, 2013) suggère que la proportion d’un habitat donné affecte plus les communautés que la surface et la fragmentation de cet habitat. Dans cette étude, l’influence de la proportion d’un habitat dans le paysage est analysée, en ciblant un habitat devenu rare : les hêtraies anciennes. Ces dernières ont la particularité d’avoir un sol boisé continu, datant au moins du minimum forestier de 1850. Etant donné que les champignons du sol occupent un rôle majeur dans les écosystèmes forestiers, nous supposons que leurs communautés vont être particulièrement sensibles à des modifications du paysage. Plus précisément, nous supposons que i) les sites contenant la plus importante proportion de hêtres anciens seront aussi les plus riches en espèces, ii) que l’augmentation de cette proportion s’accompagnera aussi de changements de composition des communautés et que iii) les champignons spécialistes du hêtre ou associées aux racines des arbres seront plus sensibles à ce facteur paysager. Afin de tester nos hypothèses, des carottes de sol ont été prélevées sur 28 sites représentant un gradient de quantité d’habitats anciens dans le paysage, au sein du Parc National des Cévennes. Le séquençage haut débit de l’ADN environnemental a permis d’obtenir 3832 OTUs (unité taxonomique opérationnelle), dont 88% sont identifiés à des Fungi. La proportion de hêtres anciens dans le paysage a un effet sur la diversité alpha et béta des communautés de champignons, mais ne permet pas d’expliquer à elle seule toutes les variations. En effet, la répartition des champignons est affectée par une combinaison complexe de plusieurs variables environnementales et bioclimatiques, notamment la pente et l’altitude. Nos résultats apportent des perspectives nouvelles pour la gestion des écosystèmes forestiers, et soulignent l’importance de la proportion d’habitats anciens, et pas uniquement leur fragmentation ou leur surface. Notre étude serait à prolonger avec d’autres espèces cibles, ou dans un paysage encore plus fragmenté, comme les hêtraies relictuelles situées à basse altitude et souvent protégées.