An assessment of the effect of season of grazing, stocking rate and rainfall on the dynamics of an arid rangeland on the west coast of South Africa.
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Date
2005
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Abstract
A grazing trial investigating the effect of season of grazing and stocking rate initiated
at the Nortier Experimental Farm in 1988 provided an opportunity to assess the
response of the veld to both grazing and environmental influences in an arid
environment. The trial allowed an assessment of the relative influence of internal
(equilibrium) and external (non-equilibrium) forces on the dynamics of an arid
rangeland. This study involved the analysis of a nine year data set stretching from
1988 to 1996 and served to provide evidence supporting the existence of an
equilibrium/non-equilibrium continuum in rangeland dynamics. The most significant
implication of this result is that rangeland systems should not be classified as either
equilibrial or non-equilibrial, but rather according to a continuum extending between
equilibrium and non-equilibrium poles. The exact position of any system on this
continuum is a function of the relative influence of internal and external forces on its
species dynamics.
The dynamics of the veld at the Nortier Experimental Farm showed significant
response to both grazing and environmental variables suggesting conformity to both
equilibrial and non-equilibrial paradigms. Both ordination and analysis of variance
highlighted the importance of rainfall particularly in the fluctuations of the
predominant grass species, Ehrharta calycina, which increased in abundance with
rainfall. Partial ordination enabled the assessment of species variation following the
removal of variation associated with rainfalL Partial ordinations revealed the gradual,
directional movement of samples through multivariate space in response to grazing
treatments. Individual plant species were also shown to be responding to grazing, the
extent of which was influenced by season of grazing and stocking rate.
Both the partial ordinations and the ANOVA showed Melothria sp., Tetragonia
fruiticosa and Hermannia scordifolia as increasing and Ruschia caroli as decreasing
in absolute abundance in response to grazing. Season of grazing was shown to
significantly influence the abundance of H. scordifolia over time.
The 'shrublherb complex', which constitutes the 'key resource' at the Nortier
Experimental Farm displayed an increase in absolute abundance over the duration of
the trial. This increase in absolute abundance was accompanied by an increase in the
relative abundance of the palatable component of this resource. The application of
medium to heavy stocking rates during spring, summer and autumn and low stocking
rates during winter resulted in elevated absolute abundances of palatable plants.
Furthermore, low stocking rates, when averaged across all season of grazing
treatments, resulted in a significantly higher absolute abundance of unpalatable plants.
These findings provide the basis for the development of management principles for
the Strandveld Vegetation Type.
The application of medium to heavy stocking rates within a rotational grazing system,
as recommended by the literature dealing with grazing systems in the Karoo, is
supported by the results of the Nortier grazing trial. Medium to heavy stocking rates
should be applied during spring, summer and autumn and low stocking rates during
the winter months. Furthermore, it is recommended that rests of between 12 and 14
months should be afforded to portions of the veld periodically due to the variability in
growth, flowering and fruiting times ofdifferent plants in the Karoo.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2005.
Keywords
Range management--Western Cape--Nortier Experimental Farm., Karoo vegetation--Management., Vegetation dynamics--Karoo., Rotational grazing--Western Cape--Nortier Experimental Farm., Sustainable agriculture--Karoo., Strandveld (Western Cape)., Theses--Grassland science.