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grown birch seedlings were planted in these soils.
Only the pioneer fungi formed mycorrhizas in these
conditions, and this was related to basidiospore ger-
mination, because only the pioneer fungi have basidio-
spores that germinate readily – the spores of the “later”
fungi germinate extremely poorly (often less than
0.1% germination in any conditions that have been
tested). Second, birch seedlings were raised aseptically
(without mycorrhizas) then planted beneath older
trees in a field site so that they would be infected by
the fungi already established in the site. Some of these
seedlings were planted directly into the undisturbed soil,
but others were planted where the soil had first been
removed with a corer (like the corers used for making
holes in putting greens) and then replaced immediately
(Fig. 10.5). All the seedlings in the cored positions
became infected by pioneer fungi, presumably from
spores in the soil. By contrast, all the seedlings planted
in undisturbed positions were infected by the “later”
fungi, presumably from hyphal networks which radi-
ated through the soil.
So, the pattern of mycorrhizal establishment on
birch in previously treeless sites can be summarized as
follows. The pioneer fungi infect young seedlings in
nurseries or in the field, from basidiospores that land
on the soil surface and are washed into the root zone.
They probably have annual cycles of infection from

basidiospores as the root system grows and expands

into new soil zones. The “later” fungi cannot establish

initially because their spores germinate poorly. But

they germinate eventually, especially in older parts of

the root zone, and become dominant by spreading as

mycelial networks to infect further root tips.

In natural woodlands and forestry plantations,

seedlings are likely to be infected directly by the

“later” fungi, but in new sites they will initially be

infected by pioneer fungi. This has practical con-

sequences, because only the pioneer fungi are suitable

for mycorrhizal inoculation programs, commonly

used in land-reclamation sites. The fungi most often

used for this are the puffball, Pisolithus tinctorius, and

the toadstool-forming fungus Paxillus involutus. Both are

pioneer colonizers that tolerate relatively high levels of

toxic minerals and the low water-retention properties

of mine-spoil and other land-reclamation sites.

Exogenously imposed dormancy

In laboratory conditions, most asexual spores germinate

readily at suitable temperature, moisture, pH and

oxygen levels. Some germinate even in distilled water,

although most require at least a sugar source, and a

few have multiple nutrient requirements. However, in

188 CHAPTER 10

Fig. 10.5Experimental study of mycorrhizal successions in field sites. (See Fox 1986.)