Big is not better: small Acacia mellifera shrubs are more vital after fire

Abstract

Fire and acacias are vital components in savanna dynamics but little is known about the relationship between postfire mortality and size of Acacia species. We determined mortality, height, and height of resprouts of the encroaching shrub species Acacia mellifera in a semi‐arid South African savanna 2 years after fire. As expected, resprouting ability after topkill was high, only 9% of the studied shrubs died completely. Surprisingly, shrubs that died in the fire were significantly taller than their resprouting conspecifics. Results from quantile regression show that the height of regrowth relative to the total height of taller shrubs is less than in smaller shrubs, despite taller shrubs having more access to below‐ground resources. We offer two possible explanations for these unexpected results: in taller shrubs, the maximum longitudinal growth rate of resprouts may be reached and therefore, resources may be invested in a greater number of resprouts or stored as reserves. Alternatively, resprouting ability may be impaired in old age by a senescence effect caused by the accumulation of physiological dysfunctions.