Ectomycorrhiza (ECM) is crucial for the survival of ecologically important arctic scrubs such as Betula, Salix and Dryas, and the widespread herbaceous Bistorta vivipara. Ectomycorrhizal roots are characterised by three structural components:

• a sheath or mantle of densely packed hyphae, which encloses the root;
• a labyrinthine inward growth of hyphae, called the Hartig net, between the epidermal and cortical cells; and
• an outwardly growing system of hyphae, the extraradical or external mycelium, which form essential connections both with the soil and the sporocarps.

Mycorrhizal roots are easy to recognise since they are swollen, shortened and more branched than non-mycorrhizal roots. Depending on the higher plant or the fungal species, the mycorrhizal roots have different shapes, sizes, textures or colours. However, for a confident identification of the fungus on an ECM root, molecular methods are essential. The exchange of nutrients takes place between the hyphae of the Hartig net and the neighbouring root cells. Especially in areas with poor soils and/or cold climate, ECM significantly facilitates the uptake of water, nitrogen, phosphorus, calcium, magnesium, potassium and trace elements.

Only 3 % of seed plants, most of them woody, are ECM. However, their global importance is far greater than what one might expect, because of their disproportionate occupancy of large terrestrial land surface habitats such as the boreal taiga and the arctic or alpine tundra.

The fungi forming ECM do not belong to a phylogenetic entity, but are found in many Basidiomycota, some Ascomycota, and even Mucoromycota. This means that the ability to form EMC has developed several times during the evolution of the fungal kingdom. Several ECM fungi produce prominent sporocarps and belong to the conspicuous “mushrooms” found on arctic tundra, where the ECM Basidiomycota comprises genera as Lactarius (Russulales), Leccinum (Boletales), Laccaria, Cortinarius, Inocybe, Hebeloma, Naucoria (Agaricales). Ectomycorrhizal Ascomycota are encountered in the genus Helvella (which also comprises saprotrophic species). Moreover, many fungi with rare and/or inconspicuous sporocarps have recently been detected by molecular analysis of ECM roots from arctic sites. Important are the Basidiomycota Sebacina (Sebacinales), Thelephora and Tomentella (Thelephorales), and the Ascomycota Cenococcum geophilum. They have been vastly underrepresented in sporocarp investigations from the Arctic but seem to have a high biomass in the soil. On the other hand, some groups with abundant fruiting, e.g. Lactarius and Russula, have been found in low numbers on ECM roots.

Fungi with dark septated hyphae have melanised cell walls and belong to the Ascomycota (e.g. Phialocephala, Cadophora and Leptodontidium). They are found on roots of plants with ectomycorrhizal fungi (see below), such as Dryas or Bistorta vivipara, but also on roots of many non-ectomycorrhizal plants. Ecologically, they constitute a heterogeneous group that functionally overlaps with saprotrophic soil fungi occurring on the root surface (rhizoplane-inhabiting fungi), pathogenic fungi, and mycorrhizal fungi. Melanins may protect the dark septate hyphae from extreme temperatures and drought, and so broaden the ecological niche of these fungi. This may explain why fungi with dark septated hyphae are so frequent in arctic areas.

The diversity of ECM fungi in the Arctic is surprisingly large compared to the low diversity of higher ECM host plants in this environment; Salix, Dryas, Betula and Bistorta vivipara. Nonetheless, these plants are among the more thermophilous species in Arctic climate. For instance, on Svalbard Betula nana is restricted to the climatically most favourable places along the south side of Isfjorden, whereas Salix polaris, Dryas octopetala and Bistorta vivipara are more or less frequent in areas with a summer warmth index above 6 ºC. Molecular data suggest that ECM host plants have re-colonised the Arctic post-glacially. The same is also the case for the ECM fungi. They may have been present during many previous interglacial periods, but were probably wiped out repeatedly by the glaciations and then re-colonised the Arctic areas several times.

The host specificity of ECM fungi is low, shown by molecular investigations of root systems of Salix, Dryas and Bistorta vivipara. No significant relationship was observed between the number of ECM fungi obtained per root system and host plant species. The most abundant order on the roots, irrespective of host species, was Agaricales followed by Thelephorales and Sebacinales.

Studies of ECM fungi on roots of Dryas on Svalbard have demonstrated an average of 7.5 taxa per plant root system in Ny-Ålesund and 6.5 in Longyearbyen. This is rather similar to the average amount of taxa found in mountains in mainland Norway (Tromsø and Finse). The number of root-associated fungal taxa on ECM plants does thus not seem to decline with increasing latitude, at least for Dryas. Moreover, even at a local scale there was little overlap in fungal taxa across root systems, indicating a lack of small-scale spatial structuring. A possible explanation is that plant root systems, together with the fungal mycelia, are three-dimensional, displaying a large fractal-like geometric structure. Both associated biotic factors such as soil microbes, as well as abiotic factors like minerals and water supply vary at micro scales in the soil surrounding the ECM-structure, both spatially and temporally. A high number of micro-niches are thus expected, explaining the high fungal diversity and the high degree of spatial turnover in fungal communities in arctic environments. A corresponding study of ECM on roots of Bistorta vivipara on Svalbard likewise found a lack of small-scale spatial structuring between individual root systems. However, there was a strong spatial component to the fungal community composition, with community similarity being highest within sites and regions, and with a weak structure along the latitudinal gradient. However, there was no spatial autocorrelation between sites, indicating that environmental factors are more important to community composition than spatial location.

The lack of small-scale spatial structuring suggests that ECM fungi in the Arctic do not usually form common mycelial networks across plant roots. Under marginal arctic conditions, the organismal energy budget is under pressure. Resources are primarily allocated to survival and reproduction rather than competition and therefore, host specialisation and the production of common mycelial networks may be too risky.