Fire and the demography of camelthorn (Acacia erioloba Meyer) in the southern Kalahari - evidence for a bonfire effect?

Fires in arid environments are rare, so are not deemed as important as in mesic savannas. We investigated mortality and resprouting amongst camelthorn (Acacia erioloba) after two fires (at Vaalbos National Park and Susanna farm) in semi-arid savanna near Kimberley, South Africa. Resprouting response 18 months after a fire was the greatest amongst < 6.5 m high trees; extent of foliage damage by fire and bark thickness were the next best predictors of resprouting vigour amongst that size class. The largest size class (8-12 m height) of A. erioloba suffered the greatest mortality rates (40% and 83% at Vaalbos and Susanna respectively), with damage either severe or minor. We hypothesize that large tree mortality rates are partly attributable to well-developed assemblages of flammable subcanopy plants producing a bonfire beneath trees. These mortality rates indicate that fire reduces both tree abundance and relative representation of large trees, and although able to resprout, A. erioloba is fire-sensitive, which may explain its restriction to Kalahari sands where rainfall is less than 900 mm year(-1). Therefore, although relatively infrequent, fires shape Kalahari woodland structure, particularly as A. erioloba is long lived and slow growing. Large trees have been shown to be important to biodiversity in the southern Kalahari, so frequent fires could impact biodiversity.