Woody species increase and development on the southern end of Disaneng Village, Ratlou Municipality, North West Province, South Africa

Abstract

Woody species were quantified at a selected study area near Disaneng (Ratlou municipality) in the North West Province (NWP) of South Africa. Woody plant development was also assessed, using both land surveys and remote sensing techniques. This was compared to woody plants in a selected benchmark (reference) site. The encroaching woody species (Grewia flava, Senegalia mellifera, Vachellia tortilis and Ziziphus mucronata) were divided according to height classes so that specific densities could be determined. The total woody species density in the benchmark was 421 TE ha-1, where one tree equivalent (TE) represents a tree of 1.5 m high. The three selected study sites in the same ecological zone as the benchmark recorded woody species densities of 9 640 -, 6 461 - and 7 820 TE ha-1 respectively with an average of 7 974 TE ha-1. Woody plants were the densest within the 0.5 - , 1 - and 2 m height classes. Senegalia mellifera (50%) was the dominant woody encroacher and it was found in all the study sites. Vachellia totilis contributed 18% of the total woody plant density whereas Grewia flava and Ziziphus mucronata were recorded at 16% of total woody plant density. This thickening is termed 'bush thickening' and was conspicuous in the communally managed rangelands where the study area was located. Remote sensing imagery was used where Spot 5 images was degraded to 20 m in order to make it homogeneous with Spot 2 and 4. The area encroached with woody species in 2006, 2010 and 2014 were 293 -, 307- and 433 ha respectively, a clear indication that the problem of bush thickening is worsening. The woody species densities increased by 4.72% and 41.22% from 2006-2010 and 2010-2014. The overall accuracy assessment (the measure of how accurate the assessed values from ground truthing are in relation to the aerial photographs or satellite images) values for 2006, 2010 and 2014 were 85, 91 and 94%. The classification method used was texture analysis (Mean Euclidean Value) and the correlation coefficient between land survey data and remote sensing was 0.99. Woody encroachers are 'threatening' communally managed rangelands in the study area as it reduces the herbaceous layer and thus lowering the grazing capacity of the rangelands.