Browsing by Author "Wakeling, Julia"

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    Limitations of savanna trees in the highveld grasslands of South Africa
    (2009) Wakeling, Julia; Bond, William J; Cramer, Michael D
    Many grassland areas throughout the world may support a woody biomass if fire is suppressed. It is puzzling that fire-tolerant savanna trees do not grow in these grasslands. The Highveld grasslands of South Africa are one such grassland. Hypotheses including fire, human intervention, grass competition and various attributes of soil have been proposed to explain the tree-less nature of the Highveld grasslands, but they have mostly been discounted. In this study it was hypothesised that cool temperatures or low nutrient availability would result in slow growth of saplings in grassland areas that would subsequently not be able to escape frequent fires. Alternatively, frost may exclude trees from grasslands. A seedling transplant experiment of savanna tree species of the Acacia genus, into grassland and savanna areas arranged across an altitudinal gradient, was used to compare growth in these varying climates over one growing season, and the influence of frost on seedlings in the following winter. Soils were collected from grassland and savanna regions to establish if nutrients varied between these areas, and seedling growth was measured in a pot experiment including these different soils. To minimise the effect of other variables, seedlings were watered and grass was excluded. Higher altitude grassland areas were cooler, and the grassland soils that were collected were nutrient-poor, relative to the savanna equivalents, with the exception of one nutrient-poor low altitude soil. Growth was well correlated to both temperature and nutrient availability, and in general there was slower growth in grassland climates and grassland soils compared to in savannas. These seedling growth rates were extrapolated to the growth rates of saplings in natural environments and the time it would take saplings to reach a height above flame height was calculated. This showed that although there were significant differences between growth rates in grassland and savanna soils, the magnitude of these differences was not large enough to prevent saplings from growing into adults in grassland soils. Differences in growth due to temperature variations, however, were large enough to suggest that saplings in grassland climates would grow too slowly to ever reach escape height between frequent fires. Frost caused damage to seedlings and decreased seedling survival at the highest elevation sites, but trees were absent well below the altitudinal limit of frost damage. There has been much discussion about the tree-less nature of the Highveld grasslands, but very little experimental work to back it. Neither temperature nor fire alone can explain the lack of trees. This study provides empirical evidence that slow growth due to low temperatures in combination with frequent fire could exclude savanna trees from the grasslands. The effect of grass competition still needs investigation. The incorporation of fire is useful as it is a fundamental part of C4 ecosystems; this, on top of a base of variable growth due to changes in resource availability. The barely recognised savanna tree-line deserves attention, as savannas continue to invade grasslands in a warming world.
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    Root suckering in Dichrostachys cinerea and a comparison of subspecies africana and nyassana
    (2004) Wakeling, Julia; Bond, William J
    Clonal growth has been studied little in Africa, and models of savanna tree-grass interactions have not included vegetatively reproducing trees or shrubs. Root suckering has been observed in both subspecies of Dichrostachys cinerea that occur in Southern Africa. This study, performed in Hluhluwe-iMfolozi Game Reserve, KwaZulu Natal South Africa, examines variability in subspecies distribution and the extent of root suckering under different fire regimes, and levels of herbivory. Variations in subspecies distribution are clear. Subspecies africana, with greater structural defenses, survives in areas of high herbivory but is rarely seen under high fire disturbance. The opposite is true for subspecies nyassana, which is poorly defended against herbivory. There was no difference in vegetative propogation by root suckering in areas of different fire regime, rainfall, soil type or degree of herbivory. The implication is that the management of the species will be a formidable task. A combination of burning during summer, when below ground reserves are depleted, and heavy browsing may aid in the control of this species.