The relative impacts of grazing, fire and invasion by buffel grass (Cenchrus ciliaris) on the floristic composition of a rangeland savanna ecosystem

Abstract

The relative impacts of grazing, fire and invasion by buffel grass (Cenchrus ciliaris) on floristic diversity and composition were evaluated for a Eucalyptus savanna ecosystem used for rangeland pastoralism in north-eastern Australia. Floristic data from a 10-year fire and livestock grazing experiment spanning periods of drought and above-average rainfall and multi-scaled data from varying levels of invasion by buffel grass were analysed. These original data were supplemented by the results of other studies. Light grazing by cattle had no discernible impacts on plant diversity and composition and burning resulted in a short-term pulse of annual forb abundance. Minor variations in edaphic properties had a substantial influence on floristic composition. Large perennial grasses increased in abundance with rainfall and annual forbs were abundant after rainfall events during drought. Species richness was diminished substantially at scales up to 1000 m2 by buffel grass and native perennial grasses showed the greatest declines. Land clearing, which facilitates invasion by buffel grass, has been greatly reduced by legislative controls but after nearly half of the savanna ecosystem has been cleared. The study suggests that livestock grazing with light to moderate stocking rates and burning are compatible with the conservation of floristic diversity in savannas but the invasion of buffel grass will continue to diminish diversity and represents a threat for the future. Land clearing exacerbates the spread of buffel grass and the control of this practice is an important contribution to the conservation of savannas.