Abstract:
Mokala National Park (MoNP) has a history of arboricide use through South African National
Parks (SANPs) having bought commercial game farmland for its establishment in 2007.
Tebuthiuron arboricide is known to have been applied for controlling bush densification
during the period 1996 to 2004. Persistent negative impacts on MoNP vegetation, which are
ascribed to the historical arboricide use, have prompted this investigation from 2016 to 2017.
Bioassay experiments employing as test plants the tree species Vachellia erioloba and Vachellia
tortilis, the shrub species Senegalia mellifera and the grass Tragus berteronianus were conducted
in a glasshouse. Growth responses of these species were assessed upon their exposure to a
tebuthiuron concentration range that simulated expected levels in MoNP soil soon and long
after application. Chemical analysis as well as bioassay with the test species Lycopersicon
esculentum (tomato) were performed on soil samples collected from three depths (0−30, 30−60
and 60−90 cm) of the soil profile at two sites in MoNP where tebuthiuron was applied in
the past. The three woody test species showed differential, negative growth response to
tebuthiuron, and even growth of the grass species (T. berteronianus) was affected at the higher
concentrations. Evidence provided by the tomato bioassay and analysis performed on soil
samples collected in situ points to the putative presence of tebuthiuron, more than a decade
after the last use of arboricides for controlling bush densification.
CONSERVATION IMPLICATIONS : If the reported evidence of the presence of phytotoxic residue of
tebuthiuron in soil of MoNP would be substantiated through further research, such findings
could at least partly explain the failure of natural recruitment of vegetation in those areas
where the woody component was degraded because of arboricide application more than a
decade ago.
