While this system has not been applied to the arid grassland and karoo, there is unlikely to be any valid reason why it should not prove as successful here as it has in the humid grasslands. In such areas, the agro-ecological ideal, or the climax vegetation, is a climatic climax. The Decreasers are therefore members of the climatic climax, and the Increaser I category is not represented, and is omitted from the assessment. Much more than in the humid grasslands and bushveld, the basal cover and species composition in arid grassland and karoo vary according to the rainfall, which is erratic. The benchmark sites should therefore be analysed with each assessment of the sample site. Particularly in the Karoo, climatic climax may be difficult to locate because of extensive degradation of this type of veld. In such case, the benchmark should be located in the best veld available, this representing an immediate, practicable goal towards which the veld might be managed.
Relationship between the basal cover factor (BCF) and the ratio of the basal cover of the sample site (BCS) to that of the benchmark (BCB)
It is assumed that the basal cover factor contriburtes nothing for the current grazing capacity when the basal cover of the sample site is half of that of the benchmark, and that the contribution is negative at lower basal covers. The objective of defining a relationship of this type is to deliberately penalise veld which has a poor protective vegetal cover over the soil. An important weakness is the present unsatisfactory method of measuring basal cover.
For the topographic factor (TF), it is assumed that grazing capacity is maximal on flat land, and decreases as the run-off increases as a result of an increase in slope or the presence of drainage channels. Seasonal wetness and existing severe erosion are also assumed to be grounds for reducing the recommended stocking rate.
The soil erodability factor (SEF) varies linearly with the soil erodability rating. Where the erosion hazard is low (soil erodability rating of 7), SEF is 0.25 and where the hazard is high (soil erodability rating of 1), SEF is 0.00. Where soil series have not been determined so that it is not possible to use directly the soil erodability ratings, then the buide at the foot of tsble above may be used.
As a worked example, the current grazing capacity of the sample of veld described, which has an average slope between 5 and 15% and a moderately erodable soil, and is situated in Bioclimatic Group 6, is calculated in the following way:
