Limitations of savanna trees in the highveld grasslands of South Africa

 

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dc.contributor.advisor Bond, William J en_ZA
dc.contributor.advisor Cramer, Michael D en_ZA
dc.contributor.author Wakeling, Julia en_ZA
dc.date.accessioned 2014-10-31T18:05:50Z
dc.date.available 2014-10-31T18:05:50Z
dc.date.issued 2009 en_ZA
dc.identifier.citation Wakeling, J. 2009. Limitations of savanna trees in the highveld grasslands of South Africa. University of Cape Town. en_ZA
dc.identifier.uri http://hdl.handle.net/11427/9019
dc.description Includes bibliographical references (leaves 85-91). en_ZA
dc.description.abstract 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. en_ZA
dc.language.iso eng en_ZA
dc.subject.other Botany en_ZA
dc.title Limitations of savanna trees in the highveld grasslands of South Africa en_ZA
dc.type Master Thesis
uct.type.publication Research en_ZA
uct.type.resource Thesis en_ZA
dc.publisher.institution University of Cape Town
dc.publisher.faculty Faculty of Science en_ZA
dc.publisher.department Department of Biological Sciences en_ZA
dc.type.qualificationlevel Masters
dc.type.qualificationname MSc en_ZA
uct.type.filetype Text
uct.type.filetype Image
dc.identifier.apacitation Wakeling, J. (2009). <i>Limitations of savanna trees in the highveld grasslands of South Africa</i>. (Thesis). University of Cape Town ,Faculty of Science ,Department of Biological Sciences. Retrieved from http://hdl.handle.net/11427/9019 en_ZA
dc.identifier.chicagocitation Wakeling, Julia. <i>"Limitations of savanna trees in the highveld grasslands of South Africa."</i> Thesis., University of Cape Town ,Faculty of Science ,Department of Biological Sciences, 2009. http://hdl.handle.net/11427/9019 en_ZA
dc.identifier.vancouvercitation Wakeling J. Limitations of savanna trees in the highveld grasslands of South Africa. [Thesis]. University of Cape Town ,Faculty of Science ,Department of Biological Sciences, 2009 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/9019 en_ZA
dc.identifier.ris TY - Thesis / Dissertation AU - Wakeling, Julia AB - 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. DA - 2009 DB - OpenUCT DP - University of Cape Town LK - https://open.uct.ac.za PB - University of Cape Town PY - 2009 T1 - Limitations of savanna trees in the highveld grasslands of South Africa TI - Limitations of savanna trees in the highveld grasslands of South Africa UR - http://hdl.handle.net/11427/9019 ER - en_ZA


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