FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 526

mosses and lichens on their surfaces.
Mound surfaces are subject to erosional attrition through the action of water and
wind and are therefore unstable surfaces, at least during the seasons when rainfall is high.
The products of this erosion may form a chemically-fertile pediment (Okello-Oloya
et al., 1985), slightly elevated above the surrounding soils and without the physical
conditions inimical to plant growth possessed by the termitaria. If, as shown above, the
materials built into the mounds are easily dispersed, surface crusts may develop around
the bases of the mounds.
The moisture contents of the soils surrounding termite mounds and other pavement-
like structures may be elevated by runoff (Eldridge, 1994) leading to a higher moisture
status in the materials forming the bases of termite mounds, and their surrounding
erosional pediments. Some combination of the more favourable moisture status and
the higher nutrient contents of the materials available to the root systems of the surround-
ing plants often leads to enhanced plant growth on and close to the mound base. In sites
subject to seasonal flooding, the slightly-increased elevation of the pediment may permit
a better aeration of these soils during the wetter part of the year. Figure IV.66 illustrates
the substantial decline in vegetation biomass that occurs with increasing distance from
the bases of the mounds of Amitermes laurensis in savanna environments in northeastern
Australia (Spain and McIvor, 1988). Similar situations pertain in the soils surrounding
certain African termite mounds, although the spatial scales may be different (see, for
example, Arshad, 1982).
The growth strategies of the plant species that occur on and close to the mounds
of grass and litter-feeding termites differ from those unaffected by the dynamics of
mound erosion and water runoff. The annual grasses that grow on the pediment and close
to the mounds compete well under the higher nutrient conditions present and grow and
set seed rapidly. Similarly, many of the herbaceous dicotyledonous plants growing in
this situation are biennial species but have the capacity to complete their life cycles
within a single season and consequently behave functionally as annuals. In contrast,
herbaceous vegetation not influenced by the mounds is normally dominated by slow-
growing perennial grasses and sedges and the few annual grass species present are small.
These effects are illustrated by the changes that occur, as a percentage of plant biomass,
in four categories of plants with increasing distance from the mounds at a site in
northeastern Australia (Figure IV.67) (Spain and McIvor, 1988).

5.4.2.3 Termitaria, roots and mycorrhizal fungi

In both savanna (Okello-Oloya et al., 1985) and rainforest (Salick et al., 1983) environ-
ments, the termitaria of grass-harvesting and litter-feeding termites may form 'islands'
of enhanced fertility within the generally nutrient-poor soils of the surrounding
landscape. While the mound exteriors are normally hard and impenetrable, the interiors
are not. A network of galleries radiates from the bases of most termitaria and a proportion
of these and the voids within the mounds may be backfilled with organic materials.
Such openings into the mounds provide access for the roots of surrounding plants which
frequently exploit the often elevated nutrient resources contained within and beneath
the mounds.