Spatial and competitive responses of tree and grass roots to increased nutrients and water in semi-arid African savanna
Bachelor Thesis
2012
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University of Cape Town
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Abstract
Savannas show highly significant structural diversity under different bioclimatic conditions and previous studies have attempted to explain the tree-grass coexistence through a combination of resource and disturbance based controls. The competitive strengths between herbaceous C₄ grasses and C₃ woody tree seedlings and saplings in the rooting zone are significantly impacted by increased resource availability. The lack of adequate understanding of the semi-arid savanna edaphic environment and associated relationships between tree-grass roots prevents predictions of future responses of these ecosystems to rainfall and nutrient changes. Here I show that grasses are strongly favoured with increased resource availability in a semi-arid African savanna. I used δ¹³C values of the sampled fine roots and the end member values of the target tree and grass species to determine the relative proportion of tree versus grass roots at each soil depth. Soil water content was determined gravimetrically and total soil and root nitrogen was delivered from isotopic analysis. Bray II and dry-ash methods were used to determine total phosphorous in the soil and roots respectively. Both tree and grass roots were shown to be significantly concentrated in the top 15 cm of the soil and present to a depth of 40cm. This study not only describes the distribution of the soil nutrients and water but also the root nutrient concentrations and the associated responses in fine root biomass, which indicates that the future herbaceous grassy layer will be significantly increased and hold a competitive advantage over the woody tree cover. The African savanna environment is projected to undergo concurrent changes in the climate and anthropogenic nutrient deposition, which will have impacts on these economically and socially significant ecosystems. A decrease in the tree : grass ratio will have highly significant biogeochemical feedback loops to the regional climate and greatly impact our ability to adequately manage these ecosystems. This study significantly contributes to the understanding of the tree-grass coexistence in savannas and has great potential for directing further research.
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Wallington, C. 2012. Spatial and competitive responses of tree and grass roots to increased nutrients and water in semi-arid African savanna. University of Cape Town.