6 Wood Discoloration

3.8 Competition and Interactions Between Organisms 83
Fig.3.5. Major forms of mycorrhizas.
Ek ectotrophic, En endotrophic, VA
vesicular-arbuscular (modified from
Hock and Bartunek 1984)
The ectotrophic mycorrhiza (ectomycorrhiza) occurs predominantly on
conifers and hardwoods of the boreal and temperate zone, particularly associated
with Pinales, Fagales and Salicales. In many conifers and in beech
and oak, the association is obligatory, and in other trees like elms it is facultative
(Müller and Löffler 1992). The predominant part of the mycelium grows
at the surface of side roots and forms a dense mycelial coat at the root tips.
The hyphae penetrate between the cells of the outer root tissue by dissolving
the middle lamellae, and coat the cells completely as “Hartig net” (Kottke
and Oberwinkler 1986). The colonized roots do no longer possess root hairs;
instead hyphae or rhizomorphs radiate into the soil.
In the endotrophic mycorrhiza (endomycorrhiza) of the orchids, only a loose
hyphal net is formed around the root, and the hyphae settle inside the cells
in the root bark area. As an intermediate, the ectendotrophic mycorrhiza is
particularly present on roots of 1 to 3-year-old conifers, whereby the hyphae
penetrating into the bark cells degenerate with ageing.
The most frequent form, the vesicular arbuscular mycorrhiza (VAM) occurs
associated with over 200,000 wild and cultivated angiosperms, in addition,
with Ginkgo biloba, Taxus baccata and Sequoia gigantea and S. sempervirens
(Werner 1987), as well as predominant form in tropical forests. In the VAM,
the unseptate hyphae extend inside the root cells bubble-shaped (vesicles)
or branch out tree-shaped (arbuscules). The arbuscules develop by hyphal
branching and become enclosed by the peri-arbuscular membranes from the
plant (Bothe and Hildebrandt 2003).
The benefit for the trees is the improved nutrient (amino acids) and mineral
(N, P, K, Mg, Cu, Zn, Fe) support and the better water supply (Smith and
Read 1997) due to the larger absorption area. Soils with frequently occurring
ectomycorrhiza are commonly characterized by a lower nutrient content, the
trees growing there would not be competitive without mycorrhizas (Schönhar
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