The extent of the alien Tamarix invasion in South Africa

The extent of the alien Tamarix invasion in South Africa
Solomon W. NEWETE1, Samalesu G. Mayonde1, Marcus J. Byrne1,2
1School of Animal, Plant and Environmental Sciences, University of the Witwatersrand
2Centre for Invasion Biology, School of AP&ES, University of the Witwatersrand

Tamarix L. is one of four genera from the Old world family of Tamaricaceae. In South Africa it is represented by one indigenous (Tamarix usneoides) and two exotic (T. chinesis and T. ramosissima) species. The first record of the exotic species in this country is from the early 1900s, when they were introduced for erosion control on mine dumps. These plants have however escaped their original sites and invaded several riparian zones across South Africa. This study investigates the extent and identity of the exotic Tamarix species and their hybrids in the Eastern, Western and Northern Cape provinces, where they are most prevalent. Twelve different riparian zones were surveyed by recording plant density/quadrat, stem density/plant, plant canopy area and height. Leaf specimens were randomly collected from eight different plants for molecular identification of the Tamarix in each site. Only one of the 12 sites had a pure population of the native T. usneoides (Kakamas River, Northern Cape) and the insect abundance and diversity was greater on the indigenous species. Hybrids of the two exotic Tamarix species were the most widespread taxon found, followed by hybrids of the native T. usneoides with the exotic T. chinesis, which accounted for 80% of the total sites with exotic/native hybrids. Tamarix canopy covers 15-90% of the river bank, with the highest being in the Eastern and Western Cape provinces. This study therefore, gives evidence that these two provinces require an urgent management intervention to contain the spread of the weed and its hybrids.