6 Wood Discoloration

84 3 Physiology
1989). The soil quality (ventilation, water permeability, stabilization of soil
particles) is increased. The trees are more resistant to drying stress. In addition,
mycorrhizal fungi play a role in tree defense against fungal pathogens
(Strobel and Sinclair 1992). The fungi benefit from the supply of photosynthates
(carbohydrates) from the trees and from supplements, e.g., thiamine.
As much as 30–35% of the photosynthate by a beech forest is metabolized by
the mycorrhizal fungi (Jennings and Lysek 1999).
About one-third (2,000 species) of the “higher fungi” which grow in forests
are mycorrhizal fungi (Egli and Brunner 2002). Among them there are many
edible mushrooms (e.g., Boletus edulis, Cantharellus cibarius), but also poisonous
species (e.g., Amanita species). Endotrophic mycorrhizal fungi are
usually Ascomycetes. Ectotrophic fungi are usually Basidiomycetes such as
Amanita species, B. edulis or the truffles (Ascomycetes). The about 150 VAM
symbionts belong to the Zygomycetes, often to the genus Glomus.
Many trees, like beech, oak, spruce, chestnut, pine, larch and willow, become
stunted in sterile culture and previous mycorrhizal inoculation of seedlings
improved tree growth (Ortega et al. 2004). Several obligatory mycorrhizal fungi,
like B. edulis, only fruit in association with roots, partly host-specifically or
with a narrow host spectrum, like Amanita caesarea predominantly associated
with oaks, usually however hardly host-specifically, like A. muscaria at birch,
eucalypts, spruce and Douglas fir (Werner 1987). The trees are usually less
specific: Pinus sylvestris forms mycorrhizas with at least 155 fungal species and
Picea abies with 118 fungi (Korotaev 1991).
Artificial mycorrhization may de done in the tree nursery or during planting
or by injection in the root area of old trees (Egli 2004; Evers and Pampe
2005). About 500,000 l mycorrhizal inoculum was produced worldwide in 2003
(Grotkass et al. 2004).
With regard to the significance of the mycorrhizas in view of the forest
dieback by pollution (Flick and Lelley 1985), there is a trend that young trees
already show a fungal community, which is typical for old trees. The changed
mycorrhiza was rated as signal for tree damage: “The fungi disappear before the
trees” (Cherfas 1991). A negative correlation was found between the frequency
of fungal occurrence and the content of nitrogen and sulfur compounds as
well as ozone in the atmosphere: 71 species of fungi were observed in a certain
area of the Netherlands from 1912–1954 and only 38 species between 1973 and
1982. Also, the size of the fruit bodies decreased (Cherfas 1991). According
to Schönhar (1989), the change of the mycorrhiza is particularly based on
nitrogen imissions by fertilization. The possible role of mycorrhiza in forest
ecosystems under CO2-enriched atmosphere in view of the global atmospheric
change was discussed (Quoreshi et al. 2003). Experimental drought investigated
in view of the expected reduction in water in Mediterranean regions showed
that drought treatment did not delay mushroom appearance, but reduced
mushroom production by 62% (Ogoya and Peñuelas 2005).
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