Nursery and field response to ectomycorrhizal inoculation of forest trees in western Europe

François Le Tacon
Equipe de Microbiologie Forestiere, INRA-Nancy, 54280 Champenoux, France
It is essential for afforestation or reforestation to produce tree seedlings with efficient mycorrhizal associations enhancing tree growth and survival after transplantation. However, ectomycorrhizal deficiencies are frequent in forest nurseries, as different practices inhibit mycorrhizal development. Techniques of ectomycorrhizal inoculation of bare-root seedlings in classical nurseries after soil fumigation, or for the production of inoculated containerized seedlings on artificial substrate are now available. Sixty field experiments are presently available in western Europe. In almost all of them, several years after outplanting, there is an effect, positive or negative, of the mycorrhizal status inherited from the nursery phase. To improve field responses quantitatively and to enlarge the number of sites where a positive response can be obtained, efforts should be concentrated in three areas: the competitive ability, the efficiency and the stability of the artificially introduced fungi. The differences in effectiveness and competitivity between fungal species and strains is the basis of practical application. A major problem is the delineation and management of stable strains, and the recognition of these strains in field situations.
Even without sexual reproduction, ectomycorrhizal strains often present variations which could be of epigenetic or genetic origin. Moreover, most of the time, introduced and natural strains produce meiotic spores. The consequence is that introgression of the introduced genes in the local population is possible.
Competition and stability studies are now possible in long term experiments with new methods such as RFLP and PCR, gene sequencing and DNA probes.