Investigating the vulnerability of an African savanna tree (Sclerocarya birrea ssp. caffra) to fire and herbivory

Abstract

Sclerocarya birrea ssp. caffra (marula), a typical savanna tree, is vulnerable to the effects of fire, herbivory and their combination. This paper investigated the relative importance of these agents of disturbance, at the level of the individual stem, by specifically focusing on the following questions: (i) What is the greatest cause of mortality in adult marula stems in conservation areas with both elephants and fire? (ii) Does fire interact with bark stripping to cause adult stem mortality and if so what is the dominant mechanism? (iii) At what stem diameter are marulas resistant to fire? Field surveys quantified the extent of damage in marula individuals in the Kruger National Park, South Africa, highlighting the high levels of extreme herbivory such as toppling (7%) and pollarding (8%), relative to bark stripping (only 6% with more than 50% of the circumference stripped). In addition to extreme herbivory, the progression from bark stripping through to invasion of the soft, exposed heartwood by wood borers, often facilitated by fire, through to toppling of the weakened stem after successive fires, appears to be the dominant mechanism by which fire interacts with herbivory to cause adult stem death. Bark stripping and fire manipulation experiments indicated that bark stripping failed to increase the vulnerability of stems to fire directly through transport tissue damage. However, the combination of bark stripping and fire reduced the ability of the stem to regrow bark, increasing the vulnerability of the exposed stem to boring insects and future fires. Fire manipulation experiments were used to identify the minimum stem diameter of resistance to fire. Marula resisted stem death when greater than 3.4 cm in basal diameter.This paper emphasizes the importance of both fire and herbivory in the development of woody plant population structure and by extension, the relative proportion of trees and grasses in savanna landscapes.

Characterising wide spatial variation in population size structure of a keystone African savanna tree

“…The remaining populations in the adult dominated group coexist with elephants. Long-term recruitment failure has resulted in a cohort of individuals, the density of which is decreasing with time due to a combination of elephant utilisation and frequent fire (Helm et al, 2011b). The lack of seedlings also indicates that recruitment in these populations is highly episodic.…”

“…Indeed the individuals that are missing are between 14 and 20 cm in diameter for GT north , MKS and MBW, but for L5 and L17 the missing individuals are between 5 and 20 cm. Evidence suggests that marula is able to escape the fire trap from 2.75 to 3 m in height (Jacobs and Biggs, 2001), which is equivalent to approximately 7 cm in basal diameter (Helm et al, 2011b). Hence, the missing individuals at L5 and L17 could be due to fire suppression.…”

“…Recent attempts at explaining the marula ''missing size class'' in the KNP have drawn on hypotheses that elephants (Jacobs and Biggs, 2002a) or small browsers such as impala (Haig, 1999;Helm et al, 2009) are preventing seedling recruitment; or alternatively that excessive fire prevents seedlings/saplings from recruiting into the upper canopy (Jacobs and Biggs, 2001). Disentangling the effects of fire and elephant is difficult since both in combination appear to be responsible for mortality (Helm et al, 2011b). Some sources however, rather attribute the ''missing size class'' to variable growth through the size classes, combined with the recruitment bottleneck caused by the fire/herbivore trap (Midgley and Bond, 2001).…”

“…Once established the plant is highly resistant to fire and 0378-1127/$ -see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2011.09.024 herbivory (Helm et al, 2011b). However, the combination of heavy elephant utilisation and frequent fire is lethal in established trees (Helm et al, 2011b).…”

“…doi:10.1016/j.foreco.2011.09.024 herbivory (Helm et al, 2011b). However, the combination of heavy elephant utilisation and frequent fire is lethal in established trees (Helm et al, 2011b). In its natural habitat, unstable marula population structures (i.e.…”