Agriculture Reference
In-Depth Information
concentrate nutrients that are later released with the erosion of termitaria that may
be deserted or contain active colonies;
increase water infiltration (up to eight-fold in comparison with surfaces not affected
by termites) especially in the vegetated strips where termitaria with active colonies are
most common;
increase water runoff (up to a maximum of 98 %) on unvegetated surfaces between
strips where fine-textured soil eroded from termitaria tends to form an impermeable
surface crust;
enhance plant species richness and density of woody species such as
Combretum
micranthum
(increased three fold) and
Boscia senegalensis
(increased ten fold).
The combination of the above effects may enhance or even determine, the dynamics
of this vegetation system by favouring germination and growth of trees on the uphill
part while mortality of trees on the downhill part is followed by death of termitaria and
accelerated erosion of their structures.
5.4.2.2
The vegetation associated with epigeal termite mounds
While they may strongly influence the nature and productivity of the vegetation surround-
ing them, few termitaria support an extensive growth of vegetation on their surfaces.
A notable exception to this generalisation is the distinctive and specialised vegetation
of African 'high termitaria' which are considered separately.
Despite the general lack of actively-growing vegetation on the surfaces of most epigeal
termitaria, the materials comprising certain of these termitaria are well provided with plant
nutrient elements. Table IV.23 illustrates selected properties of the mound materials of the
tropical Australian litter-feeding species
Amitermes laurensis
(Okello-Oloya
et al.,
1985).
As shown in the Table, organic C, total N and an index of 'plant-available' P were all
substantially elevated compared with equivalent values for the surface soils of the
inter-mound spaces. A feature of these mound materials is the substantial excess of the
sum of the 'exchangeable' Ca, Mg, Na and K ions present over the cation exchange
capacity indicating the large quantity of soluble bases present.
If the mound materials of many grass-harvesting and litter-feeding termites are
broken up and supplied with water, commensurate with their enhanced nutrient status
they may support substantially greater plant growth than their adjacent, unaffected
surface soils (Okello-Oloya and Spain, 1986) (Table IV.23, p. 517). This may not be true
of the wood-feeding species whose mound materials often differ little from the
surrounding soils in their concentrations of plant nutrient elements (Lee and Wood, 1971b).
It appears that the densely-packed arrangement of soil particles in most mounds
creates a compact surface structure whose physical condition is unfavourable to plant
establishment and growth. Most mound materials, except during the brief seasonal
periods when they are highly hydrated, may be too strong and their water held too firmly
to constitute a satisfactory physical growth medium for most plants. However, despite
their normally compact surfaces, the mounds of some grass-harvesting and litter-feeding
termites may support a sparse growth of plants for periods during the wet season. In such
humid environments as rainforests, termite mounds often support the growth of algae.
