Browsing by Author "Kruger, Laurence M"

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    The demography of Balanites maughamii : an elephant-dispersed tree
    (2012) Bijl, Alison; Midgley, Jeremy J; Kruger, Laurence M
    Balanites maughamii is an ecologically and culturally valuable tree species, heavily impacted by elephants, which strip bark selectively off the largest trees, increasing their susceptibility to fire damage. Elephants also break intermediate sized trees extensively, keeping them trapped in non-reproductive stages. The trees can however survive breaking, stripping and · toppling by elephants, as well as top kill by fires, because they resprout vigorously in response to damage. They also produce root suckers. independently of disturbance. Vegetative reproduction buffers the populations from the infrequent recruitment of seedlings, and facilitates the maintenance of populations over the short term. Balanites maughamii trees are reliant on African elephants (Loxodonta africana) for seed dispersal and to provide a germination cue through mastication. In the absence of elephants, the population experiences a recruitment bottleneck, but root suckers functionally replace seedlings and fill the "recruitment gap", so over the short term, the population is resilient. In all populations, whether elephants are present or not, another hurdle affects recruitment, and it is seed limitation due to seed predation pre- and post- dispersal. Cafeteria experiments revealed that bushveld gerbils (Tatera leucogaster) were removing many seeds but do not scatter- or larder-hoard. They are simply seed predators.
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    Patterns and mechanisms of stem mortality in Acacia nigrescens induced by elephants and fire
    (2007) Moncrieff, Glenn; Midgley, Jeremy J; Kruger, Laurence M
    Increasing elephant populations have been implicated in the decline of woody vegetation throughout Africa. The problem is particularly relevant to the Kruger National Park in South Africa, where elephant populations have almost doubled in the last 10 years. One manner in which elephants utilize trees is by stripping their bark. The role of bark stripping in increasing stem vulnerability to fire and the mechanism through which fire damage is mediated were investigated by experimentally removing bark and burning Acacia nigrescens stems. Field surveys were conducted in order to investigate patterns of bark stripping in relation to mortality patterns of large trees occurring subsequent to natural fires. In the experimental study, an increasing probability of mortality was associated with increasing amount of bark removed when trees were burnt. However, when trees were stripped but not burnt, simulating damage to cambium and phloem, none died in the 4-month period over which the experiment ran. This was taken as evidence that fire-induced xylem damage causes stem mortality. However, fire did kill a greater proportion of the remaining stem cambium around the circumference when bark had been removed. The field surveys indicate that bark stripping by elephants is frequent on large stems (44%) and that larger trees are more heavily impacted. The only variable measured that explained mortality patterns well was the percent of bark removed around the stem circumference up to 3m (p = 0.0076). These results indicate that damage to xylem is important in determining post-fire survival and that bark stripping by elephants increases the vulnerability of stems to fire. This increased vulnerability is a result of both increased damage to cambium and damage to exposed xylem. The high proportion of trees stripped by elephants and the increase in vulnerability to mortality associated with bark stripping suggests that unless elephant population growth is curbed, large Acacia nigrescens trees will eventually be eliminated from this ecosystem.