EFFECTS OF THE 1990 91 DROUGHT ON RANGELAND IN THE STEYTLERVILLE KAROO/

During 1990/91, rainfall was 70% below the long-term annual mean of 27 1 mm in the Steytlerville Karoo, a semiarid, small-stock ranching dist rict of the south-eastern Cape. A drought severity index, that incorpo rated monthly rainfall and monthly averages of maximum daily temperatu re, showed that the deficit in plant-available moisture increased betw een January 1990 and September 1991, when drought was broken by 99 mm of rain. Following this unusual (P < 0.015) drought, perennial vegetat ion cover decreased from 45% to 21%. Mortality was greater among Karoo bushes (65%) than succulents (42%). Post-drought vegetation was domin ated by grasses, forbs and the alien biennial Atriplex lindleyi, all o f which emerged from soil-stored seed. Surviving shrub populations rec overed slowly by post-drought seeding or rooting from branches. A chan ge from Pentzia incana shrubland to grass and succulent dominance occu rred within 30 months. Currently acceptable methods of estimating carr ying capacity indicated that 30 months after the drought the capacity of the rangeland to carry domestic livestock was still 60% below pre-d rought levels for the region. Rodents, inconspicuous when the drought ended, were very numerous in 1994 following summer rain. Carrying capa city of ecotonal Karoo shrublands for herbivores and granivores appear s to be dynamic. Quantifying and tracking such variability is a challe nge for pastoralists and scientists alike.