A demographic perspective on bush encroachment by Acacia karroo in Hluhluwe-Imfolozi Park, South Africa

Abstract

Acacia karroo has invaded large areas of the Hluhluwe part of the Hluhluwe-Imfolozi Park, despite the relatively frequent fire regime (mean and median fire frequency of 2.9 and 1.3 years, respectively) of this area. We surveyed A. karroo demography and post-fire responses to determine what is facilitating this encroachment. We found no seedlings and few individuals with small lignotubers. This suggests that encroachment is due to release of already established individuals, rather than recent or continuous recruitment of new individuals. Post-fire height growth rates of basal resprouts is about 0.5 m y^–1, which is too slow for escape from damage by fire, given the present frequent fire regime. Analysis of post-fire responses suggests that individuals can resprout from the stem at relatively small sizes (1.2–2.2 m tall). We suggest that stems can incrementally grow beyond the zone of fire influence because post-fire resprouting does not only take place from the base, after individuals are above 1.2 m in height. Bush encroachment is suggested to be due to low fire intensity, which allows relatively small plants to resprout from the stem and thus incrementally grow in size until escape size.

Keywords: Acacia karroo, bush encroachment, fire

African Journal of Range & Forage Science 2008, 25(3): 147–151