Bushclump-grass interactions in a south-east African savanna : processes and responses to bush control.
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Date
1996
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Abstract
The objective of this study was to investigate woody-grass
interactions and the initial response of vegetation to bush
control in the mesic Eastern Cape bushclump savannas. The
occurrence of multi-species bushclumps, rather than single-trees,
presented an interesting variation to an otherwise well-studied
interaction. The effect of bushclumps on their local environment
was characterized. Since all woody-grass interactions involve
competition for irradiance, nutrients and moisture, a factorial
experiment was designed to discriminate these individual and
interactive effects. Mechanical and chemical bush control
measures were investigated in a formal, replicated experiment.
The herbaceous, woody and soil responses to bush control
treatments, for the first two seasons, are reported.
Bushclumps had a moderating effect on their microclimate when
compared with the open grassland. Lower maximum and higher
minimum temperatures, and higher humidity were the result of an
80-90% reduction in the irradiance regime. Soils beneath
bushclumps were more fertile than grassland soils. The
importance of bushclumps on sandier soils was discussed.
Bushclumps were characterized by a sparse shade-tolerant
herbaceous layer which contributed little to grazing capacity.
An aspect effect increased grass production in the grassland on
the south-facing side of bushclumps. Initial results suggest
that the lateral spread of woody roots could be as far as 25 m.
The factorial experiment tested the individual and interactive
effects of irradiance (normal sunlight, 40% and 80% shade),
nutrients (normal nutrient level, low and high nutrient addition
levels) and moisture (low, normal and additional moisture levels)
on the herbaceous layer. The interaction of 80% shade and high
nutrients had a detrimental effect on herbaceous production.
Deep shade did not affect herbaceous production, but Themeda
triandra showed etiolated growth, aerial tillering, an increase
in the number of leaves, and an increase in the proportion of
stem under deep shade. The root mass of the herbaceous layer
also decreased. This suggested that below-ground biomass
production was impaired at the expense of maintaining aboveground
biomass. The addition of nutrients significantly
increased herbaceous production and resulted in a change in sward
composition. Moisture was not an important factor in this
experiment.
Mechanical clearing in the bush control experiment resulted in a
significant increase in herbaceous production. Panicum maximum
colonized the ex-bushclump zone and contributed significantly to
the increased production. Oversowing with Chloris gayana
significantly increased grass yields. The two contrasting
seasons revealed the importance of rainfall in affecting
herbaceous production. The second season was characterized by
lower soil fertility and a decline in grass quality. This was
attributed to high grass production in the above-average rainfall
season. A four-fold increase in woody stem density after two
seasons demonstrated the coppicing ability of the woody layer
once mechanically cleared. Most of the coppice occurred within
the first season. Exceptional coppice growth characterized the
second season. Acacia karroo recruitment was mainly from seed.
Woody plants showed their susceptibility to chemical poisoning by
dropping their leaves within the first season. Many of these
individuals succumbed during the second season. Mortality was
greatest in woody plants with a smaller basal circumference.
Owing to the difficulty of accessing all woody stems in a
bushclump, mortality in bushclumps was lower than that in the
open grassland. Grass production in the bushclump and its
periphery were significantly increased in both seasons. This was
attributed to the increased productivity of mainly Panicum
maximum which took advantage of the increased irradiance regime.
Both the mechanical and chemical treatments displayed
significantly greater grass production in the open grassland
zone. This demonstrated the extent to which the woody layer had
competitively dominated the herbaceous layer.
Description
Thesis (M.Sc.Agric.)-University of Natal, Pietermaritzburg, 1996.
Keywords
Range management--Eastern Cape., Savanna ecology--Eastern Cape., Vegetation dynamics--Eastern Cape., Theses--Grassland science.