Exotic plant species invasion in south west New South Wales: influence of a rare flooding event and grazing

S.K. Florentine and M.E. Westbrooke, School of Science and Engineering, Centre for Environmental Management, University of Ballarat, PO Box 663, Victoria 3350, Australia.


Arid land plant species rely on infrequent rainfall events for their recruitment, growth and reproduction. On rare occasions, well above average rainfall can create natural disturbances on this landscape. Such episodic flood events not only facilitate native species irruptions but also create conditions for exotic species to spread into new territory. Though studies have examined the effect of flooding events on native species recruitment, little is known about flooding facilitated weed invasion in arid landscapes in Australia. The main objectives of this study were to investigate: (i) the distribution of exotic plant species in flooded and control (unflooded) areas in relation to a 1997 episodic flooding event and (ii) the influence of grazing in flooded and unflooded open and fenced plots on weed colonization.

Across flooded fenced and open plots eleven exotic species from five families were recorded. Ten of these were herbs or annual grasses, but one was the South American shrub Nicotiana glauca that had invaded large areas. In the flooded area N. glauca stem density increased from zero (September 1998) to 554 ha-1 (September 1999). In October 2003 stem density was three times higher in the flooded open plots than previous years. In contrast N. glauca was not recorded from control (unflooded) fenced and unfenced plots. At October 2002 and 2003 sampling in the flooded unfenced plots a significant number of plants had resprouted. Though 11 exotic species were recorded in flooded fenced and control plots, N. glauca was the most aggressive invader. Future flood events may provide the opportunity for this aggressive species to further extend its territory. Control measures should be adopted to prevent further infestation of this species into fragile arid ecosystems.

Key words: arid-zone, flooding, Nicotiana glauca, tree tobacco, weed invasion.


Plant Protection Quarterly (2005) 20 (2) 42-45.