Browsing by Author "Botha, Susan"

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    The influence of fire severity on recruitment of fynbos with particular emphasis on seed size : a field study in the Cape Peninsula
    (2001) Botha, Susan; Bond, William J
    Fifty-two sites throughout the Cape Peninsula area burnt in the January 2000 fires were sampled to investigate the effects of fire severity on the recruitment of invasive and fynbos plants with particular emphasis on seed size. Four hundred fynbos species were divided into big- (>l0mg) and small-seeded (<10mg) species. Different measurements of burnt plant skeletons were examined to find the most appropriate indicator of the biological effects of a fire. It was found that the measurement of thinnest branch diameter of plant species with similar growth forms can be used as estimates of fire severity. Seedling density of fynbos species decline with an increase in fire severity, whereas recruitment of alien seedlings is not affected by fire severity. Big-seeded species and seedlings showed no significant correlation with fire severity, whereas the density of small-seeded species and seedlings correlated significantly with fire severity. It is likely that variations in fire severity that influences the depth to which seeds are killed will affect the ability of small seeds to emerge. High severity burns may thus have a different community composition by favouring the survival of big-seeded species.
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    The influence of rainfall variability on savanna tree seedling establishment
    (2006) Botha, Susan; Bond, William J; February, Edmund C
    My thesis investigates the rainfall requirements necessary for tree seedling survival of different savanna species in the first year of growth. Results from a field drought experiment showed that dry season survival for seedlings of Acacia nigrescens and Philenaptera vialacea was not related to wet season length. Instead mortality was highest immediately after germination despite irrigation to minimize dry spells. Furthermore there was no relationship between seedling size and the length of the dry spell seedlings of Acacia nigrescens and Philenaptera vialacea grown in basalt clay in the greenhouse, could survive. This was in contrast to seedlings of Scleracarya birrea and Terminalia sericea grown in granite sand, where number of dry days survived increased with number of growth days received. There was no significant differences in rainfall variability (t-test: tL III = 1.56, P = 0.121, F-ratio = 1.13) between a semi-arid (Satara) and semi-mesic (Pretoriuskop) site within the Kruger National Park despite significant differences in rainfall totals (t-test: tl, III = 4.40, P < 0.000, F-ratio 3.41). Results from a soil-water model show that seedling recruitment can occur at least twice a decade in the Satara area, which should be frequent enough to allow a transition to a woodland state over time. These results suggest that a mechanism other than the effect of rainfall variability on seedling establishment is required to explain the co-existence of trees and grasses at Satara. The lack of field germination, the absence of a seed bank and the germination of added seed at the field experiment site after rainfall emphasizes seed limitation as an important bottleneck at Satara. Higher herbivore biomass at Satara compared to Pretoriuskop may limit seedling establishment opportunities by destroying seed and seedlings of tree species.