干旱区生态环境知识资源中心

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 (delta(13)C) of soil organic matter (SOM) at 11 sites spanning the area thought to have undergone the greatest changes. Results confirm a substantial shift of SOM delta(13)C from values characteristic of C-4 grasses to C-3 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 delta(13)C. Changes in the relationship with increasing soil depth suggest that land use, and not climate, has driven the decline of grasses. We used the grass/rainfall relationship to map changes in shrub cover from the past (subsoil layers) to the present (surface layers). Past vegetation included a stable shrubland zone in the south and west, a broad transition zone with grass/shrub mixtures in the central Karoo and a grassland zone north and east of the Orange River. These zones are still identifiable in the more recent SOM record bur the grass zone hers been extensively invaded by shrubs. Our study supports the view that grass cover has declined under grazing pressure, but not that grass lands covered most of the central Karoo before European settlement.