Encroaching woody plant species and boreholes: is cattle density the main driving factor in the Olifants Drift communal grazing lands, south-eastern Botswana?

Fifteen environmental variables, among them cattle density (hereafter 'pres sure index'), were measured along grazing gradients of eight boreholes at O lifants Drift in south-eastern Botswana. The aim was to investigate whether cattle density, among other variables, had any direct relationship with th e occurrence of bush-encroaching woody plant species. This is important bec ause high cattle densities in the communal rangelands of Botswana have been held responsible for the increasing dominance of woody encroachers. On the basis of the t-values and the variance inflation factors (VIF) obtained fr om CANOCO (canonical community ordination) programme, four environmental va riables were retained as significantly influencing woody plant species comp osition along the grazing gradients: cattle density, soil nitrogen, distanc e from borehole and tree cover. The retained variables were forward-selecte d in a regression and cattle density explained most (33%) of the variance o ut of the total 60% explained by the four variables. Woody plant species as sociated with high cattle density were also found on high nitrogen sites, a nd these were thorny plant species that have been previously classified as encroachers. Therefore, high cattle density around boreholes is associated with bush-encroaching woody plant species. High cattle density is also like ly to be found around natural water points (pans and wells), rivers and kra als, hence encroaching woody plant species are likely to dominate in such p laces. Shifting of the high cattle density foci (boreholes, wells, kraals, etc.) is likely to result in most of the grazing pasture being composed of the encroaching woody plant species at the expense of the non-encroaching w oody plant species. (C) 1998 Academic Press.