The role of fire in preventing transitions from a grass dominated state to a bush thickened state in arid savannas

We investigated the role of fire in controlling bush thickening in arid savannas. Three controlled fires were initiated to investigate the responses of planted seedlings, saplings and mature shrubs. The stem diameters of seedlings, saplings and mature shrubs prior to the fire were measured. Other dimensions of the saplings and mature shrubs such as leaf mass were also estimated. The intensity of each fire was estimated. Fire temperatures adjacent to seedlings saplings and mature shrubs were measured using fire paints on steel plates. All three fires were moderately hot. One year after each fire, the mortality of seedlings in the fire treatments was very high (97.1-99.3%) and significantly greater than in adjacent controls (16.1-51.6%). Mortality and topkill of saplings and mature shrubs were negatively related to stem diameter. Fire temperature did not significantly affect mortality and topkill in most cases. Surviving saplings showed a much greater resprout response, relative to pre fire size, than larger shrubs and trees. Our results suggest that fire is crucial in interrupting the transition from open grassy savanna to thicket in arid savannas. Managers who prevent fires at this stage are likely to experience bush thickening in the future.

► We studied the effects of fire on Acacia mellifera in an arid savanna. ► Almost all seedlings were killed by the fire. ► Survived saplings resprouted and regrew rapidly after the fire. ► Most mature shrubs resprouted and almost all survived. ► Fires coinciding with establishment events maintain arid savannas in an open state.