Abstract
Seven sheathing mycorrhizas of nursery grown P. sitchensis were characterised using the consistent intrinsic morphological features of each mycorrhizal association, primarily sheath morphology and hyphal ornamentation. These features were observed by means of light microscopy together with transmission and scanning electron microscopy. The fungal symbiont of each mycorrhiza was isolated and then used to synthesise mycorrhizas aseptically. The morphology of the synthesised mycorrhizas was shown to be greatly influenced by the growth medium, thus care must be taken in extrapolating from results obtained in such experiments. Additional proof of the mycorrhizal nature of a fungal isolate was obtained by comparing the hyphal ornamentation of the isolate with that of the original mycorrhizal fungus. Four of the sheathing mycorrhizas were formed by basidiomycete fungi of which two have been identified as Thelephora terrestris (Ehrh.) Fr. and Laccaria laccata (Scop, ex Fr.) Cooke, two by isolates of the "E-strain" group of fungi and one by a dark sterile isolate. The majority of the sheathing mycorrhizas of P. sitchensis seedlings and transplants in the nursery studied were formed by "E-strain" fungi. Mycorrhizas formed by the other fungi were sporadically distributed. Replacement of the "E-strain" fungi by other mycorrhizal fungi from short roots was frequently observed. Under infertile conditions the mycorrhizal fungi were shown to have a differential effect upon the growth of P. sitchensis. The "E-strain" fungi were less effective than some of the other mycorrhizal fungi. Thus the majority of transplants leaving the nursery studied do so equipped with sheathing mycorrhizas which are unsuitable for infertile afforestation sites.