D. Bouchard

Survival of inoculated Laccaria bicolor in competition with native ectomycorrhizal fungi and effects on the growth of outplanted Douglasfir seedlings

The survival, development and mycorrhizal efficiency of a selected strain of Laccaria bicolor along with naturally occurring ectomycorrhizal fungi in a young plantation of Douglas fir was examined. Symbionts were identified and their respective colonization abilities were determined. Eight species of symbiotic fungi, which may have originated in adjacent coniferous forests, were observed on the root systems. Mycorrhizal diversity differed between inoculated (5 taxa) and control (8 taxa) seedlings. Ectomycorrhizal fungi which occurred naturally in the nursery on control seedlings (Thelephora terrestris and Suillus sp.) did not survive after outplanting. Both inoculated and naturally occurring Laccaria species, as well as Cenococcum geophilum, survived on the old roots and colonized the newly formed roots, limiting the colonization by other naturally occurring fungi. Other fungi, such as Paxillus involutus, Scleroderma citrinum and Hebeloma sp. preferentially colonized the old roots near the seedlings collar. Russulaceae were found mainly in the middle section of the root system. Mycorrhizal colonization by Laccaria species on inoculated seedlings (54%) was significantly greater than on controls (13%) which were consequently dominated by the native fungi. Significant differences (up to 239%) were found in the growth of inoculated seedlings, especially in root and shoot weight, which developed mainly during the second year after outplanting. Seedling growth varied with the species of mycorrhizae and with the degree of root colonization. Competitiveness and effectiveness of the introduced strain on improving growth performances of seedlings are discussed.

Temporal persistence and spatial distribution of an American inoculant strain of the ectomycorrhizal basidiomycete Laccaria bicolor in a French forest plantation

Selected strains of ectomycorrhizal fungi, such as the basidiomycete Laccaria bicolor, are currently being used as inoculants in nurseries to improve growth of forest trees after outplanting. Information is needed on the survival of these introduced strains in forests and their impact on indigenous biodiversity. Dissemination and persistence of an American strain, L. bicolor S238N, were studied 10 years after outplanting in a Douglas fir plantation located at Saint‐Brisson (Morvan, France). About 430 Laccaria spp. sporophores were collected over 3 years. Inheritance of nuclear ribosomal DNA, as well as RAPD markers, was characterized in L. bicolor S238N, using a haploid progeny set of 91 monokaryons. More than 50 markers were identified (19 heterozygous and 33 homozygous or cytoplasmic markers), which unambiguously confirmed that the introduced strain was still present in the inoculated plots. Neither selfing (P < 0.0008) nor introgression with indigenous strains was detected although in vitro interfertility between the American strain and indigenous L. bicolor was identified. No ingress of the introduced genet into adjacent uninoculated plots colonized by various local Laccaria genets was detected. It is proposed that the spatial distributions identified have developed through mycelial propagation of the introduced strain and intraspecific competition with native genets. Although longer‐term data is still lacking, the stability of the inoculant strain and the limited disturbance to indigenous populations described support large‐scale nursery production of this host‐fungal combination.

Monitoring the persistence of Laccaria bicolor as an ectomycorrhizal symbiont of nursery-grown Douglas fir by PCR of the rDNA intergenic spacer.

The large‐scale inoculation of selected beneficial ectomycorrhizal fungi in forest nurseries has generated renewed interest in the ecology of these symbiotic fungi. However, information on the dissemination and persistence of introduced symbionts is scarce due to the limitation of the current identification methods. To identify ectomycorrhizal fungi on single root tips, we investigated the polymorphism of the PCR‐amplified ribosomal DNA intergenic spacer (IGS) from a wide range of ectomycorrhizal fungi. To investigate the reliability of this molecular approach in large‐scale surveys, the dissemination and persistence on Douglas fir seedlings of the introduced Laccaria bicolor S238N were assessed in a forest nursery in the Massif Central (France). Several hundred ectomycorrhizas and fruiting bodies were sampled from plots where control and L. bicolor inoculated‐Douglas fir seedlings were grown for 1.5 years. PCR typing of mycorrhizas indicated that trees inoculated with L. bicolor S238N remained exclusively colonized by that isolate (or sexually derived isolates) for the entire test period. In contrast, control seedlings were infected by indigenous isolates of Laccaria laccata and Thelephora terrestris. The molecular evidence for the persistence of the introduced mycobiont despite the competition from indigenous isolates of the same species provides further illustration of the potential of exotic species for large‐scale microbial application.

The fungus-specificity of mycorrhization helper bacteria (MHBs) used as an alternative to soil fumigation for ectomycorrhizal inoculation of bare-root Douglas-fir planting stocks with Laccaria laccata

Mycorrhization helper bacteria (MHBs) isolated and selected from the Douglas fir-Laccaria laccata symbiotic system have previously been shown to be fungus-specific: they promote ectomycorrhizal establishment of Laccaria laccata but inhibit mycorrhiza formation by other fungi. In this paper, two experiments in a nursery producing two years-old bare-root Douglas-fir planting stocks confirm the specificity of MHBs under field conditions. They also show that, by selectively helping the introduced L. laccata against the resident symbionts, MHBs are an interesting alternative (safer and easier) to soil fumigation for the success of routine controlled mycorrhization of planting stocks in forest nurseries.

Mycorhizes, pépinières et plantations forestières en France.

La première démonstration de la nécessité de l’association des arbres forestiers avec des champignons mycorhiziens a été faite il y a déjà plus d’un siècle (Frank, 1894). L’objectif de la mycorhization contrôlée des arbres forestiers est d’associer, au stade de la pépinière, les jeunes plants avec des souches fongiques mycorhiziennes performantes. Après transfert de ces plants en forêt, les souches fongiques inoculées peuvent survivre et assurer ainsi une meilleure reprise ou une meilleure croissance des arbres.

Nutritional and microbiological aspects of decline in the Vosges forest area (France)

Abstract The aim of this study was to experimentally determine a possible influence of microbial or chemical properties of soil in Norway-spruce decline. Soils collected from declining and healthy forest sites of the Vosges were subjected to a range of soil treatments. Three glasshouse experiments were conducted. Soil is directly involved in the spruce decline by inducing nutrient deficiency, which can be partially corrected by fertilization. The addition of calcium carbonate increases mycorrhizal status and restores normal mineral nutrition (especially phosphorus nutrition) and growth. The low pH of the soils and the presence of a noxious microbial factor are involved in the poor mycorrhizal infection and nutrients status of Norway spruce. Fungi rather than other microorganisms could be involved; they cause no root necrosis, and might be some species of Penicillium, Trichoderma, Acremonium or Cylindrocarpon regularly recovered from the roots. This microbial component involved in the reduced mycorrhizal development of seedlings could be suppressed by soil pasteurization and fungicide application. Possible remedial treatment of declining spruce stands in the Vosges should include liming and fertilization.