FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 528

Roots from woody plants some distance from the mounds have been consistently
noted to enter the soil beneath the epigeal mounds and frequently to penetrate the mounds
themselves. In a humid West African savanna environment, Mordelet et al. (1996) found
that the roots of the palm Borassus aethiopum could extend as far as 20 m to exploit such
nutrient-enriched sites. Roots have also been reported to follow old infilled termite
galleries in subsoils beneath Kenyan coffee crops (Robinson, 1958), presumably because
of their lower material strengths and greater nutrient contents.
Kaiser (1953, in Araujo, 1970) described a specific root-feeding relationship with
Anoplotermes pacificus (Apicotermitinae) and Mill (1992) also referred to this species
as attacking the root tips of certain crop plants (Section III.4.3.2.1). Eggleton et al.
(1995) have suggested that more generalised feeding on fine roots by soil-feeding
termites may take place in certain environments. In some Amazonian forests, rapid
seedling growth in mounds follows their abandonment because the termites are no
longer present to control their growth (Salick et al., 1983). Further, the often extensive
proliferation of roots within unoccupied, degrading termitaria suggests that the termites
may play an active role in preventing the ingress of roots, while the colony remains active.
Recent Australian work has demonstrated the presence of infective propagules of both
ectomycorrhizal and arbuscular mycorrhizal fungi in the mounds of grass-harvesting,
litter-feeding and wood-feeding termites at concentrations similar to those in the
surrounding soils (Spain et al., in preparation). Also, ectomycorrhizal fungi (notably
Pisolithus sp.) have been noted to fruit on the surfaces of termitaria at some locations.
The presence of roots, fungal propagules and by implication hyphae, suggests that
termitaria are being explored either directly by the roots of ectomycorrhizal host species
(largely Eucalyptus and other sclerophyllous species that may be located some metres
distant from the mounds), or indirectly through their associated mycorrhizal fungi
(Spain et al., in preparation). Host plant species of vesicular-arbuscular mycorrhizal
fungi (grasses, forbs) occasionally grow on the surfaces of the epigeal termitaria of
grass harvesting and litter-feeding termites but occur more regularly and in greater
profusion on the erosional pediment surrounding the mounds. Thus, the inward
movement of nutrient elements to the mounds in transported termite food materials may
be at least partly balanced by a countervailing outward movement back to the adjacent
herbaceous vegetation and, at a larger radial scale, the surrounding trees.
A possibly quadripartite association between the widespread African wood-feeding
termite Sphaerotermes sphaerothorax (Macrotermitinae), cellulolytic and N-fixing
bacteria and the roots of the surrounding trees was described by Garnier-Sillam et al.
(1989). This termite has either lost or never developed a relationship with Termitomyces
and instead forms comb-like structures containing an active cellulolytic and N-fixing
microflora. Roots from the surrounding trees grow into the nest and the combs rest on
a mass of small roots and root hairs. Food materials in the form of small spheres
of masticated wood are broken down by some combination of the termites' own
enzymes, bacteria located in the hind gut or externally in the combs. Faeces in the lower
part of the nest are explored by root hairs and it seems likely that interchanges of
materials may occur between the roots, the termites and the two types of bacteria forming
the association.