Has the Karoo spread? A test for desertification using carbon isotopes from soils

Abstract

Karoo shrublands are widely believed to have spread at the expense of grasslands as a result of livestock farming since European settlement. We tested this view by analysing carbon isotope composition (?13C) of soil organic malter (SOM) at 11 sites spanning the area thought to have undergone the greatest changes. Results confirm a substantial shift of SOM ?13C from values characteristic of C4 grasses to C3 shrublands in surface soil layers. There is also evidence for a much earlier cycle of change, from deeper soil layers. Summer rainfall was strongly correlated with grass cover, as measured by SOM ?13C. Our study supports the view that grass cover has declined under grazing pressure, but not that grasslands covered most of the central Karoo before European settlement.