Published online by Cambridge University Press: 10 December 2009
Introduction
Ectomycorrhizal fungi (EMF) form symbioses with forest trees. These fungi are mainly basidiomycetes and ascomycetes and they probably evolved from saprotrophic fungi when organic matter began to accumulate in certain soils 200 million years ago (Cairney, 2000). The tree host benefits from the symbiosis through improved nutrient acquisition since the fungus explores the soil efficiently for nutrients (especially N and P) in return for host carbon (Smith & Read, 1997). Forest trees with ectomycorrhiza usually dominate in acidic soils with thick litter layers, with the associated EMF forming extensive external mycelia with a high capacity to take up nutrients from the soil. The fungi form mantles around the root tips with large storage capacities (Read, 1991). The trees invest large amounts of carbon in the ectomycorrhizal (EM) symbionts, especially under nutrient-poor conditions, with up to 20% of photosynthetic assimilates allocated to the mycorrhizal symbionts (Finlay & Söderström, 1992). This large carbon source for EMF gives them an advantage over non-symbiotic microorganisms in the soil ecosystem since these are usually carbon-limited (Aldén et al., 2001). The importance of current photosynthate for soil processes was recently demonstrated by Högberg et al. (2001), who girdled trees in northern Sweden and found a rapid reduction in soil respiration of more than 50% within 1–2 months. More precise calculations the second year after girdling demonstrated that 65% of total respiration was contributed by EM roots, their associated EMF and other rhizosphere microorganisms (Bhupinderpal-Singh et al., 2003).
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