This thesis investigates the significance of different mycorrhizal fungi, belonging to different functional types (arbuscular mycorrhiza-AM and ectomycorrhiza-EcM), in Salix repens . A comparison between above-ground and below-ground observations on ectomycorrhizal fungi (EcMF) indicated that neither diversity nor abundance above-ground can be used to estimate below-ground diversity or abundance. In all habitats S. repens was highly EcM and slightly AM. Low colonization by arbuscular mycorrhizal fungi (AMF) reflects plant control over mycorrhizal colonization. Arbuscular mycorrhizal colonization was higher in spring than in other seasons, coinciding with higher above-ground P concentration. In sequential inoculations, in the short term AMF interfered with EcMF but the reverse was not observed, whereas in simultaneous inoculations no mutual suppression was observed. Mobilization of plant defence reactions by AMF was transient and led to co-existence between AMF and EcMF. Plant response to AMF, but not of EcMF, depended on internal shoot phosphorus concentration. None of the eleven EcMF investigated exerted the full range of mycorrhizal benefits. Two strategies of EcM were recognized, viz. root manipulation and root replacement. Magnitude of plant response and amount of mycorrhizal colonization were not correlated. Mycorrhizal benefits showed conditionality with regard to pH and nitrogen-to-phosphorus ratio.Genetic variation of S. repens had a large effect on the symbiosis; there was also a large plant origin x soil type interaction. Fungal species origin had only a minor effect on symbiotic effectiveness. After waterlogging intensity of AM colonization was increased and EcMF colonization was decreased. Both changes reflect the differential expression of survival strategies. High mycorrhizal fungal diversity, both in taxonomic and in functional terms is both determining and determined by the wide ecological amplitude of S. repens .