FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 452

between species and strains and may be reversible; they may depend on complex
equilibria between auxin oxidases produced by roots and inhibitors of auxin oxidases
produced by fungi which allow fungal hormones to act on the roots. In consequence,
the hormonal equilibrium and its effect on the association, may vary along the root system
and change over time (Moser and Haselwandter, 1983). Roots also produce non-specific
materials in their exudates which stimulate spore germination on contact (Smith and
Read, 1997). Finally, soil fertility and particularly phosphate levels, may greatly influ-
ence the relationship.
Spread of propagules . While the majority of ectomycorrhizal fungi fruit above ground
and their spores are spread by air currents, arbuscular mycorrhizal fungi and many
ectomycorrhizal fungi fruit underground or in the litter layers. As indicated above,
spores may largely survive passage through the intestinal tract of the animals that ingest
them, either purposely or unintentionally with their food (Section IV.3.1.2.2). Smaller
organisms may be important for more localised spore spreading and these and other
propagules are commonly spread by earthworms of different ecological strategies
(Allen, 1991; Reddell and Spain, 1991a). The propagules of both arbuscular and
ectomycorrhizal fungi are also spread by wind and water, notably through the activities
of organisms or other agencies that expose the surface soil (Allen, 1991, Allen et al ., 1989).
Colonisation. Within the soil, the hyphae of arbuscular mycorrhizal fungi grow along
the root and enter it at several sites from where the fungal network ramifies internally
throughout the root. The extent to which colonisation extends and the time over which
it occurs vary greatly although colonisation never affects more than 50-80 % of the root
(Sutton, 1973). Observations suggest that mycorrhizal roots have longer life spans than
non-mycorrhizal roots, and that infectivity in excised mycorrhizal roots may persist for
several months after connection with the parent plant has been severed (Persson, 1982;
Ferrier and Alexander, 1985). Mycorrhizae will only provide nutrients to the root as long
as the root is able to provide carbohydrates.
The hyphae of both arbuscular and ectomycorrhizal fungi can also ramify from one
root to another and establish networks between plants of the same and different species
(Chiariello et al., 1982; Read et al., 1985). While the ecological significance of these
networks is not yet clear, inter-plant transfer of C (Graves et al., 1997; Simard et al.,
1997), N, water and other nutrients has been noted. Successions have been observed
in the colonisation of roots by mycorrhizal fungi and 'early-' and 'late-stage' species
have been reported from mycorrhizal sequences, notably in plantations and in rehabilitated
minesites (see, for example, Gardner and Malajczuk, 1988). However, Molina
et al. (1992) consider such models inadequate for predicting the species distribution of
fungi in natural forests, especially those subject to periodic disturbance.
Water uptake. Mycorrhizal colonisation alters the water relationships of both arbuscu-
lar and ectomycorrhizal host plants although the mechanisms of this are not completely
clear. Interactions with nutrient status are also involved (Smith and Read, 1997).
The greater soil volume explored following mycorrhizal colonisation may also
increase water uptake, since hyphae and ectomycorrhizal rhizomorphs are able to absorb