Expansion of the Grassland Biome in the eastern Karoo corresponds with changes in rainfall and livestock numbers

Abstract

The persistent spread of shrublands is a global phenomenon observed across semiarid grassland-shrubland boundaries. Observations in South Africa, however, have detected a contrasting trend of increasing grass cover across the transition between the Nama-Karoo and Grassland Biomes over the last few decades. A west-to-east gradient of increasing mean annual rainfall, and underlying geology, controls the natural transition of Karoo dwarf shrublands to semiarid grasslands. The availability of historical vegetation surveys and landscape photographs, weather, and livestock census records, made it possible to assess the nature, extent, and drivers of vegetation change across this biome transition. Rainfall has been generally higher over the last four decades compared to the years prior to the original surveys. This, together with a reduction in livestock numbers, is the main driver of the westward expansion by ∼100 km of perennial grasses, and a general increase in dwarf shrub and total vegetation cover. Rangeland condition, as indexed by estimates of grazing capacity, has improved significantly. Despite a structural shift towards grassland-dominance, the original species complement has persisted. The rainfall-driven increase in grass fuel loads in the region, however, places these rangelands at risk of becoming altered by increasing fire frequency.