Aspects of the invasion of southern tall grassveld by Aristida junciformis subsp. junciformis Trin. et Rupr.
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Date
1998
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Abstract
Aristida junciformis subsp. junciformis Trin. et Rupr. is an indigenous, densely tufted,
evergreen perennial grass associated with the degeneration of grasslands over large
areas of South Africa. More than two hundred thousand square kilometres of veld, c.
17% of the total land area of South Africa, contains A. junciformis. The aim of this study
was to improve our understanding of the mechanisms by which this species invades and
dominates grassland, specifically in this study, Southern Tall Grassveld.
Aristida junciformis also has a low nutritional value resulting in a very low grazing value.
The unpalatability of the plant is due to the high tensile strength of the leaves, whose
hard, fibrous laminas are very difficult for a grazing animal to crop once their length
attains 30 cm or more. This species lack of response to conventional grazing practices
has often resulted in a grassland in which the carrying capacity has diminished to such
an extent as to be virtually useless for grazing in a relatively short time.
The extent of encroachment of veld by A. junciformis appears dependant on the
frequency of disturbance and rest afforded to the veld. Annual burning and mowing
maintained the species composition of A. junciformis at levels <10% whilst protection
from fire, burning or grazing allows this species to dominate the herbaceous layer at
levels approaching 90%. Aristida junciformis does not become moribund and is unlikely
to die if left undefoliated. As the abundance of A. junciformis increases, veld
deterioration gradually accelerates through increased selective grazing on remaining
palatable species. Competition from adult A. junciformis plants increases the mortality of seedlings and
constrains growth of surviving seedlings. Tiller production of Aristida junciformis
seedlings declined from an average of 5.2 tillers per seedling in a no competition
situation to 2.2 tillers per seedling when subjected to full competition. Tiller production
of T. triandra seedlings decreased from 9.6 tillers per seedling free from competition to
3.3 tillers per seedling subject to full competition for resources.
Once these seedlings have become adult plants they are avoided by grazing animals
and grazing pressure on the remaining palatable species consequently increases. This
allows the A. junciformis plants to increase their size and density in the sward. Once this
density is sufficiently high, grass seedlings of either A. junciformis or more desirable
grass species such as T. triandra are unable to establish, eventually resulting in a monospecific stand of A. junciformis if left undisturbed.
Large amounts of caryopses are produced by A. junciformis - up to a 19 000 caryopses from a large mature plant (c. 38 000 caryopses/m²). Of these c. 40% is likely to be infertile but the remaining c. 60% viable caryopses are dispersed in a typically
leptokurtic distribution, the number of caryopses dispersed rapidly declining within a 10
metre radius. A large proportion of the caryopses was trapped in surrounding foliage but
in open swards caryopses had greater opportunity to be blown further distances than in
a closed sward. The density of A. junciformis caryopses on the soil surface was
positively correlated with the density of flowering adult plants in the area and varied from
400 caryopses/m² (density of parent plants c. 0.6 plants/m²) in less effected areas to 11000 caryopses/m² (density of adult plants c. 6 plants/m²) in severely encroached areas. The primary function of the three awns appears to be orientating the caryopsis correctly
for in its descent from the parent plant to expedite germination. Caryopses orientated
vertically with the awns uppermost exhibited the highest and most rapid germination
(67%) compared to caryopses lying horizontally (35%) whilst only 1% of inverted
caryopses germinated. Caryopses trapped in litter and effectively held off the soil surface
failed to germinate. Removal of the glumes from A junciformis seed greatly enhanced
the rate and overall germination of the seed except for inverted seed of which <1%
germinated.
The highest numbers of A. junciformis seedlings (32 seedlings/m²) were found in those
areas with the highest density of caryopses on the soil surface. Despite the large
amounts of caryopses produced, dispersed and landing in apparently suitable micro-sites
for germination, comparatively few A. junciformis seedlings (n=992) were found and
overall germination ranged between 1 % and 4% of the initial caryopses density on the
soil surface. Seedling survival through winter was low with only 13% surviving to the
following spring. The basal areas of A. junciformis increased overall by 66% whilst that
of other grass species increased overall by only 3% in the time monitored. These results
suggest that the primary method of encroachment of A. junciformis in the grass sward
appears to be through vegetative expansion and not seedling recruitment.
Frequent defoliation of the sward and avoidance of overgrazing to enhance the
competitive abilities of palatable species and provide as high a fuel load as possible
appear to be the most economically and logistically feasible ways to remove or at least inhibit A. junciformis veld encroachment at present.
Description
Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1998.
Keywords
Aristida junciformis junciformis., Invasive plants--KwaZulu-Natal., Grassland ecology--KwaZulu-Natal., Range management--KwaZulu-Natal., Vegetation dynamics--KwaZulu-Natal., Theses--Botany.