and Basidiomycota. So, the ectomycorrhizal fungi
seem to be ecologically adapted to grow as symbionts.
They seldom show a high degree of host-specificity, so
it is quite common to find mycorrhizas of 10 or more
different fungi on a single mature tree. However, in
broad terms the ectomycorrhizal fungi can be grouped
into two types. Some are “generalists” with wide host
ranges, especially on young trees in newly afforested
sites (e.g. Laccaria spp., Hebelomaspp., Thelephora
terrestris, Paxillus involutus); others are more host-
restricted and tend to predominate on mature trees
(e.g. Suillus luteuson pines, Suillus grevilleion larch,
and the truffles, Tuberspp., on oak or beech). These
“mature” types have been shown to produce proteolytic
enzymes in culture, and this is thought to be significant
for their nitrogen nutrition. A few mycorrhizal fungi,
such as Laccariaand Hebeloma, can take up nitrate
from soil, but nitrate is readily leached out of soil by
rainwater. Most mycorrhizal fungi can take up ammo-
nium or amino acids from soil, but in the cool, wet,
acidic conditions of many northern regions of the
globe the rates of decomposition of organic matter are
very slow, and this can significantly limit the rates of
plant growth. The release of proteases by these fungi
can therefore be important in providing ectomycorrhizal
plants with nitrogen in the form of amino acids.
Fungal networking
The ectomycorrhizal fungi play major roles in ecosys-
tem functioning (Allen 1992). An extensive network of
hyphae and mycelial cords ramifies through the soil
from the mycorrhizal sheath (Fig. 13.8), and this net-
work can link many different plants within a habitat- even plants of different species because of the general
lack of host specificity of these fungi. For example, it
is known that young tree seedlings can be linked to a
“mother” tree or “nurse” tree by a common mycorrhizal
network, such that^14 CO 2 fed to the leaves of a larger
tree can be found as label in the roots and shoot tis-
sues of nearby seedlings. The amounts of nutrients trans-
ferred in this way may not be large, but a seedling
that has tapped into an existing mycorrhizal network
might benefit from this (Smith & Read 1997).
There are at least two other potentially significant
functions of this subterranean network. An estimated
70 –90% of ectomycorrhizal rootlets die and are
replaced each year. If these rootlets were not inter-
connected they would decompose and at least some
of the nutrients would be leached from the soil. The
mycelial connections could help to retain mineral
nutrients by withdrawing them from the degenerating
mycorrhizas to others that are still functioning.
Furthermore, as seen in Fig. 13.8, the system of
mycelial cords and fans of hyphae extends far beyond
the root zone. In observation chambers the peat sub-
strate can be allowed to dry to the point where non-
mycorrhizal seedlings die, but the mycorrhizal plants
remain healthy because the mycelial cords can trans-
port water from deeper in the container, beyond the
reach of the roots themselves. This role can be particu-
larly important in soils with poor water retention, such
264 CHAPTER 13
Fig. 13.7(a) Scanning electron micrograph of a cross-section of part of a mycorrhizal root, showing the fungal sheath
that surrounds the root. (b) Thin section of part of an ectomycorrhizal root. The arrowheads show hyphae invading
between the root cortical cells, forming the Hartig net. Nutrient exchange between the fungus and the root is thought
to occur in this region.
(a) (b)