Assessing effect of rainfall on rate of alien shrub expansion in a southern African savanna

Understanding the environmental factors governing the spread of alien shrubs is crucial for conserving biodiversity. In the semi-arid savannas of Africa, alien shrub invasion often occurs simultaneously with native shrub encroachment but climate-dependent differences in encroachments of native and alien shrubs have never been properly quantified. A combination of historical aerial photographs and field measurements was used to compare the spread of the invasive shrub Lantana camara L. with that of native encroaching shrubs over a 31-year period in a protected semi-arid savanna in Zimbabwe, southern Africa. We tested whether the response of this invasive alien shrub to rainfall differs from that of native shrub encroachers. Both the invasive shrub L. camara and native encroaching shrubs spread significantly faster during high rainfall years than in dry years. However, the response of L. camara to annual rainfall was stronger than the response of native encroaching shrubs. During years of above-average rainfall, the mean annual rate of spread of L. camara was at least twice that of native shrub encroachers, whereas in other years natives spread at the same rate as the alien shrub. This is a novel finding suggesting that in water-limited savannas, pulses in rainfall may accelerate the spread of some invasive alien species.