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Accueil du site → Doctorat → Afrique du Sud → 2021 → The causes and consequences of Seriphium plumosum L. encroachment in semi-arid grassland communities of Gauteng province, South Africa.

University of KwaZulu-Natal (2021)

The causes and consequences of Seriphium plumosum L. encroachment in semi-arid grassland communities of Gauteng province, South Africa.

Pule, Hosia Turupa

Titre : The causes and consequences of Seriphium plumosum L. encroachment in semi-arid grassland communities of Gauteng province, South Africa.

Auteur : Pule, Hosia Turupa.

Université de soutenance : University of KwaZulu-Natal

Grade : Doctor of Philosophy (PhD) Botany 2021

Résumé partiel
Seriphium plumosum L encroachment in the semi-arid grassland communities of South Africa results from a complex interaction of environmental factors, which pose a major challenge to managing the rate and extent of its encroachment. Woody plant encroachment effects on ecosystem functioning are non-universal, with both positive and negative effects equally probable. Although the causes and consequences of woody plant encroachment are documented worldwide, this research on S. plumosum is lacking, thus hindering the development of control measures for its encroachment. In chapter 2, the effect of abiotic (aspect, rockiness, sand and soil fertility (soil organic carbon, phosphorus, magnesium and pH) and biotic (S. plumosum density, basal cover and veld condition) factors on S. plumosum encroachment susceptibility in grassland communities was explored using logistic regression. A decrease in basal cover had 0.926 times likelihood estimate of increasing S. plumosum encroachment susceptibility. An increase in S. plumosum susceptibility was also observed with decrease in sand and magnesium concentrations. These results have showed that, overgrazed and/or burned areas with well-drained, nutrient poor soils in the semi-arid grassland communities of Gauteng Province are likely to become encroached by S. plumosum. In chapter 3, we used two grassland communities, Carletonville Dolomite Grassland (CDG) and Rand Highveld Grassland (RHG), and abiotic factors (Soil Organic Carbon (SOC), Total nitrogen (TN), Phosphorus (P), Potassium (K), Sodium (Na), Calcium (Ca), Magnesium (Mg) and pH) to investigate habitat preferences of Seriphium plumosum L. in the semi-arid grassland communities of Gauteng, South Africa. Bottom slope regions in RHG had significantly higher S. plumosum density (59.00 ± 8.62 plants/50 m2 ) than bottom slope regions in CDG (19.75 ± 3.01plants/50 m2 ). Soil sodium content was similar in CDG slope positions and higher than in RHG. Furthermore, 9 Seriphium plumosum density was positively related to components of soil fertility ; P (r 2 = 0.1270 ; P = 0.0036), K (r 2 = 0.0786 ; P = 0.0237), Na (r 2 = 0.0686 ; P = 0.0350), Ca (r 2 = 0.0681 ; P = 0.0358), and SOC content (r 2 = 0.0669 ; P = 0.0374). The results revealed that S. plumosum density in grassland communities increases with increasing soil sand content and decreasing soil fertility. Chapter 4 was aimed at exploring the interactive effects of S. plumosum canopy cover, fire and soil depth on components of soil fertility ; K, P, Mg, TN, Na, Ca, SOC and pH. All the components of soil fertility measured showed a significant decrease with increasing soil depth and post-fire treatment (P< 0.05). The results have shown that S. plumosum encroachment contributes to accelerate rather than reverse rangeland degradation. Furthermore, fire alone is not sufficient to control S. plumosum encroachment in grassland, since it may reduce grass competitive ability, which in turn improves the conditions for S. plumosum seed germination and recruitment

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