The role of mycorrhizal associations in plant nutrition increases with vegetation succession. As ecosystems develop and mineral soils give way to soils with increasing organic matter so does the importance of mycorrhizal associations increase for plant nutrition. There are three main types of mycorrhizae: (1) endo-mycorrhizae, (2) ectomycorrhizae and (3) vesicular-arbuscular mycorrhizae (VAM). In endomycorrhizae, the fungus penetrates and lives within the cells of the root cortex and external growth is limited to finely branched haustoria (arbuscules). These are the most widely distributed of mycorrhizae and are found on herbaceous species, especially orchids, as well as woody ericaceous plants such as heathers and rhododendrons. This type of mycorrhizal association is particularly beneficial to flowering plants that grow on nutrient-deficient soils. In the ectomycorrhizae a mycelium mantle is developed on the outside of the root. The hyphae penetrate the cortex of the root an form an intercellular meshwork termed a Hartig net, the outer mantle ofwhich replaces the piliferous (outer) layer of the root. This type of association is very common in temperate trees and appears to be essential for the proper growth of the trees. Vesicular-arbuscular mycorrhizae (VAM) are a form of ectomycorrhizae where the fungus lives between the cells of the cortex, forms finely intertwined, dendroid hyphae and penetrates the cells with temporary hyphal projections which may be swollen vesicles or finely branched hyphae called arbuscules. These are probably the most ancient forms of fungal association with higher plants and are widespread throughout the world.
It has been a long-standing assumption that fungal associations are less common in cold climates. Arctic soils lie under snow and ice for the greater part of the year and even in the growing season have a tendency to remain cold and often wet. A combination of anaerobic conditions and low temperatures have therefore been considered as presenting obstacles to the development of mycorrhizal associations particularly in plant communities in the High Arctic (Kytoviita, 2005). However, it is always dangerous to make generalizations about an area that is as heterogeneous as the Arctic especially now that many regions are showing clear signs of climatic warming.
In the subarctic soils ofnorthern Sweden it has been shown that on the drier fellfield prolonged shading, warming, and fertilization treatments of hybrid willow stands (Salix herbacea X Salixpolaris) more than doubled the above-ground biomass and shoot growth, but decreased the number of root tips per unit root mass with no long-term changes in total ectomycorrhizal colonization. The net result was therefore an increase in the density of the potential host plant tissues, which increased the intensity of fungal colonization in this ecosystem. An analysis of the fungal species also suggested that there was a shift from drought-stress-tolerant fungi towards a dominance of minerogenic fungi, which may take place if nutrient availability increases substantially because of anthropogenic disturbances (Clemmensen & Michelsen, 2006).
In the High Arctic on Axel Heiberg Island (approximately 80° N) VAM have been found in three genera of the Asteraceae (Arnica, Erigeron and Taraxacum). The Axel Heiberg Island is an example of a thermal oasis and is therefore ideal for functional fungal root endophyte development (Allen et al., 2006).
These examples are taken from favoured well-drained and warm sites. Climatic warming at high latitudes may make these sites more favourable for the development of mycorrhizal associations but in other regions where the climate is more oceanic, and where prolonged ice encasement and bog growth may be enhanced by increased nitrogen input from anthropogenic sources and higher temperatures, the conditions may remain adverse for the development of active mycorrhizal associations.
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