52. EASTERN MIXED NAMA KAROO

Synonyms:

False Upper Karoo (A36), False Karroid Broken Veld (A37).

Statistics:

77 784 km²; area transformed unknown; 1.08% conserved.

Locality & Physical Geography:

Eastern Mixed Nama Karoo reflects an extensive ecotone between the Nama Karoo Biome in the west and the Grassland Biome to the east. Altitude varies from about 1 400 m for areas north of the escarpment to around 700 m for sites below the escarpment. This is the typical Karoo landscape of mesas and buttes.

Climate:

Rainfall is between 300 and 500 mm per year, occurring mostly in late summer and autumn.

Geology & Soil:

Beaufort Group sandstones and shales dominate the landscape, with the flat-topped landscape shaped by many dolerite dykes and sills.

Vegetation:

A complex mix of grass- and shrub-dominated vegetation types, which are subject to dynamic changes in species composition dependent on seasonal rainfall events, occurs within this vegetation type. Common shrubs include Bitterkaroo Pentzia incana, Kapokbush Eriocephalus ericoides, Thornkapok E. spinescens and Hermannia spp., while grasses, such as Aristida spp., Eragrostis spp. and Redgrass Themeda triandra, may dominate the landscape after good summer rains, especially in the north-east. Trees are not abundant, except along the dry river beds where Sweet Thorn Acacia karroo is a common element. This type has the highest cover of herbs of all the Nama Karoo types, as well as numerous geophytes.

Key Environmental Parameters:

The north-east region of Eastern Mixed Nama Karoo is the only Karoo type in which fire is important in shaping the communities. This type has the highest rainfall of all the Karoo types and is thus ecotonal to grassland. As a result, it is relatively sensitive to grazing pressure and, depending on stocking density and rainfall conditions, may resemble either grassland or Karoo.

Economic Uses:

Although the wettest Karoo type, it is still too dry for crop production. However, this is the prime sheep and goat grazing area, producing much wool and meat. Irrigation along the Orange River is important - some of the biggest dams on the Orange River occur in this vegetation type. This vegetation type contains the agricultural research station at Carnarvon and is the best-researched Karoo type with regard to small-stock farming potential.

Conservation Status:

Poorly conserved - most of the conservation areas are immediately around the major dams. Acocks considered this the most degraded of all the vegetation types in South Africa. This is where all the classical research on desertification and Karoo encroachment has been undertaken. Although there is much support for Acocks's theory on Karoo encroachment due to overgrazing, some scientists maintain that this "invasion" is determined by rainfall events and that drought periods result in an increase in Karoo elements. This process is reversed during wet cycles in which the grasses again dominate. However, overgrazing does encourage Karoo encroachment and it is important that stocking densities should be maintained at the carrying capacity of the vegetation as determined by its rainfall at the time.

Key Reference:

Acocks (1988).

Author:

Timm Hoffman.