Ectotrophic mycorrhizas
Ectotrophic mycorrhizas, or ectomycorrhizas, are
found mainly on woody plants, including many
species of coniferous and broad-leaved trees outside of
the tropics. For example, ectomycorrhizas are typically
found on trees such as pine, spruce, larch, oak, beech,
birch, and eucalypts. But tropical trees and even some
temperate trees (sycamore, ash, poplars) have arbuscular
mycorrhizas, and some trees (e.g. willows) can have both
types. The fungi involved in ectomycorrhizal associ-
ations are principally members of the Basidiomycota
that produce many of the common toadstools of the for-
est floor (e.g. Amanita, Boletus, Cortinarius, Hebeloma,
Lactariusspp.; Fig. 13.6) but ectomycorrhizas also are
formed by some Ascomycota, including the truffle
fungi. Given the range of plants and fungi involved in
this type of symbiosis, it is thought that ectomycor-
rhizal associations evolved independently on several
occasions in the last 130 –180 million years.
The characteristic feature of ectomycorrhizas is the
presence of a substantial sheath of fungal tissue that
encases the terminal, nutrient-absorbing rootlets (see
Fig. 7.10), and the rootlets themselves are often shortFUNGAL SYMBIOSIS 261
0.8
0.2
0
0.4
0
0.6
2 4 8 101214
Simpson’s diversity index6130
90
70
100
0
120
2 4 8 101214
Shoot biomass (g m–2)
6
80
110
160
80
Number of mycorrhizal fungal species100
0
120
2 4 8 101214
Root biomass (g m–2)
6
140
7
3
2
4
0
6
24 81012
Hyphal length (m g–1soil)6
5
20
10
0
0
15
2 4 8 101214
Soil P (mg kg–1soil)6
5
2,500
500
Number of mycorrhizal fungal species1,000
0
1,500
2 4 8 101214
Plant P (mg m–2)
6
2,000
14
Fig. 13.4An increase in the number of different arbuscular mycorrhizal fungi in a soil leads to an increase in plant pro-
ductivity and plant biodiversity. (From van der Heijden et al. 1998, with permission from the publisher.)