Many plants require the symbiosis to prosper while for others, such as maples and birches, it is not essential. Others, notably members of the Proteaceae, including the proteas, banksias and grevilleas of the southern hemisphere, rarely if ever form mycorrhizas. The fungi vary from obligate symbionts that can obtain carbon only from the host plant (as in endotrophic mycorrhizas) to facultative symbionts that can be free-living (as in some ectotrophic mycorrhizas (ECM) fungi). Endotrophic mycorrhizas are typically more common where phosphorus is limiting but nitrogen is not. Ectotrophic mycorrhizas become most important with increasing latitude as nitrogen becomes more limiting (see Chapter 8).
Ectomycorrhizas (ECM) Fungi: usually Basidiomycetes, rarely Ascomycetes. The fungal partner forms a pale mantle around the outside of the root with hyphae penetrating between the cells of the root. Found in 3% of the flowering plants but including 90% of temperate and boreal trees of the northern hemisphere including most conifers. Also found in southern hemisphere trees such as southern beeches Nothofagus and eucalypts Eucalyptus, and some tropical families including the dipterocarps (Dipterocarpaceae).
Three main groups are recognized. In every case the fungal hyphae penetrate into the cells of the root.
Arbuscular (AM) Fungi: usually Glomales (formerly included in the Zygomycetes but now assigned to a new group, the Glomeromycetes).
Found in a wide variety of families including some trees (Aceraceae: maples; Juglandaceae: walnuts, hickories; Ulmaceae: elms; Oleaceae: olives, ashes; Magnoliaceae: magnolias, tulip trees; Hamamelidaceae: sweet gums, witch hazels; Cupressaceae: cypresses, junipers; Araucariaceae: monkey puzzle; Ginkgoaceae: ginkgo; Taxodiaceae: redwoods, swamp cypress. Ericaceous Fungi: Ascomycetes.
Three types exist, all growing on members of the Erica family or close relatives, such as the heathers, rhododendrons and wintergreens Pyrola on very acid soils but also includes Indian pipes/yellow bird's-nest, Monotropa which are non-green and entirely myco-heterotrophic (living off fungi, see Box 3.1). Orchidaceous Fungi: Basidiomycetes.
Orchid seeds have almost no food reserves and so are totally dependent upon the fungus until the orchid becomes photosynthetic which may be a number of years after germinating. Some are non-green and permanently dependent upon the fungus. Based on information from Calow (1998), Thomas (2000) and Deacon (2006)
Minnesota, USA. Seedlings of the bur oak Quercus macrocarpa were planted in the fields at different distances from a northern pin oak Q. ellipsoidalis (focal tree) growing at the edge of the forest. Use of a focal tree of the same genus but different species reduces the possibility of root grafting which is common in oaks, while host specificity of fungi tends to be at the genus level. When seedlings were harvested after a year they found that mycorrhizal abundance and species richness were high near the focal tree, declined rapidly around 15m away (probably where the roots of the focal tree ended) and were uniformly low at 20 m. All seedlings up to 8 m from the tree were ectomycorrhizal but many at 16-20 m were uninfected. This demonstrates that ECM infection of seedlings is spatially complex and dependent on existing trees for infection.
The discovery that the mycorrhizal agaric fungus Laccaria bicolor traps springtails and supplies nitrogen to the trees with which it is symbiotic is important; in some cases 25% of the nitrogen used by the eastern white pine Pinus strobus is derived from these soil animals (Klironomos and Hart, 2001).
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