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.

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.