FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 433

Plant families may be predominantly ectomycorrhizal or predominantly endomycor-
rhizal although both forms occur in families such as the Salicaceae (Newman and
Reddell, 1987). Even in families considered largely mycorrhizal, species occur that
may not form these organs and, conversely, mycorrhizal associations may also occur
within families that are generally considered non-mycorrhizal (Tester et al., 1987). Of 200
families of vascular plants from all over the world, 23 have been identified within which
mycorrhizal colonisation is generally absent or poorly developed. This last group includes
the important families Chenopodiaceae, Brassicaceae and the Proteaceae, the last widely
represented in Australia and South Africa (Tester et al., 1987). Trappe (1987) stated
that approximately 12 % of the angiosperm species examined so far are facultatively
mycorrhizal, 18 % are typically non-mycorrhizal leaving 70 % as probably obligatorily
mycorrhizal. Effective mycorrhizal associations are often absent from or much reduced
in highly disturbed, aquatic and poorly drained environments.
A large number of fungi are known to enter into mycorrhizal associations with
higher plants although by far the greatest diversity occurs in those forming the external
or ectomycorrhizal type. Molina et al. (1992) estimated the number of fungi forming
ectomycorrhizal and ectendomycorrhizal associations at more than 5400 species, of
which ca. 25 % fruit underground. Most of the fungi entering into these associations
belong to the Basidtomycota ( ca. 92 %) but a number of Ascomycota ( ca. 8 %) and
a few Zygomycota of the genus Endogone (Haselwandter and Bowen, 1996) are also
involved. These fungi are rarely obligate symbionts since they can grow in the absence