Plant-browser-soil interactions: nutrient cycling at the ecosystem level

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BACKGROUND AND JUSTIFICATION

The African savanna biome includes more large mammalian herbivores (> 5 Kg) than any other continent (McNaughton and Georgiadis 1986; Owen-Smith and Cumming 1993), as would be expected from the suitable combination of precipitation and soil fertility (see du Toit 1995; Olff et al. 2002). The exceptional species richness in African savannas likely depends on the high degree of specialization of ungulate species to particular habitats and the highly variable savanna ecosystem over temporal and spatial scales (du Toit and Cumming 1999; du Toit 2003).

UNGULATE BROWSING AND ACACIA TREE RESPONSES

Plants may reduce herbivore damage through resistance, tolerance and/or changes in phenological traits (see Agrawal 2000). Specifically, (1) plant resistance traits (i.e. thorns) reduce herbivore performance or damage, (2) tolerance traits (i.e. mass compensatory growth ability) reduce negative effects when herbivore damage has already occurred, and (3) phenological “escape” reduces plant availability when herbivores are most active. Plant responses to herbivory likely depend on levels of habitat resource availability (Coley et al. 1985; Bryant et al. 1983; Herms and Mattson 1992; Bardgett and Wardle 2003), as well as on interspecific competition and/or frequency and intensity of disturbance events.

BROWSING EFFECTS ON PLANT COMMUNITY COMPOSITION

Impacts of ungulate browsing on vegetation structure and dynamics have been recognized as significant in boreal forests (Kielland and Bryant 1998; Horsley et al. 2003) where historically, much emphasis had been given to abiotic factors as main responsible for vegetation structure and dynamics (see Bryant and Chapin 1986). Indeed selective browsing negatively affects species with nutrient rich tissues (Ritchie et al. 1998; Peinetti et al. 2001), and contributes to a shift in dominance towards unpalatable evergreen species within the vegetation community (Pastor et al. 1988; Kielland and Bryant 1998; Wardle et al. 2001; Horsley et al. 2003).

STUDY AREA: THE TSHOKWANE SECTION OF THE KRUGER PARK

The study was conducted in a central-eastern region of the Kruger National Park (Fig. 2.3), based at the Tshokwane ranger station (24° 47´ S, 31° 52´ E). The experiments were carried out in the Satara land system on basaltic soil, which consists mainly of fine-leaved tree savanna or bushveld, dominated by Acacia nigrescens, Sclerocarya birrea and Dichrostachys cinerea (Fig. 2.3; Venter et al. 2003). The soil is general high in clay and nutrients and dominated by Acacia trees that enhance nitrogen availability and therefore attract herbivores.

DECLARATION
ACKNOWLEDGEMENTS
CHAPTER 1 Introduction
1.1 Background and justification
1.2 Ungulate browsing and Acacia tree responses
1.3 Browsing effects on plant community composition
1.4 Plant-browser-soil interactions: nutrient cycling at the ecosystem level
1.5 References
CHAPTER 2 Study area
2.1 Kruger National Park at a glance
2.2 Study area: the Tshokwane section of the Kruger Park
2.3 Site description and research assumptions
2.4 References
CHAPTER 3 Responses of a woody plant community to long-term browsing by indigenous ungulates in a southern African savanna
3.1 Introduction
3.2 Methods
3.3 Data analysis
3.4 Results
3.4.1 Browsing intensity
3.4.2 Grazing intensity
3.4.3 Population structure of Acacia nigrescens
3.4.4 Effects of browsing on spinescence, palatability and evergreenness
3.4.5 Vegetation community composition and species
distribution
3.5 Discussion
3.5.1 Browsing-grazing gradient
3.5.2 Browsing effect on population structure of Acacia nigrescens
3.5.3 Browsing effect on vegetation community composition
3.6 Conclusion
3.7 References
CHAPTER 4 Ungulate browsing as an ecosystem process: plant-soil-browser interactions in a southern African savanna
4.1 Introduction
4.2 Methods
4.2.1 Browsing/grazing intensity
4.2.2 Litter decomposition: August placement
4.2.3 Litter decomposition across species and sites
4.2.4 Litter biomass, soil depth and soil nutrient pool
4.2.5 Termite activity
4.3 Data analysis
4.4 Results
4.4.1 Litter decomposition rates – August placement
4.4.2 June placement
4.4.3 Soil analysis and litter composition
4.4.4 Visitation of termites to litter bags
4.5 Discussion
4.6 References
CHAPTER 5 Plant tolerance, resistance and phenology: responses from Acacia nigrescens to ungulate browsing in an African savanna
5.1 Introduction
5.2 Methods
5.2.1 Mass compensation and morpho-functional traits.
5.2.2 Annual net shoot growth, leaf N and tree phenology
5.3 Data analysis
5.4 Results
5.4.1 Mass compensation and morpho-functional traits
5.4.2 Annual net shoot growth, leaf N and phenology
5.5 Discussion
5.6 Conclusion
5.7 References
CHAPTER 6 Ungulate browsing and its effects on suppressed juvenile forms of woody species in a eutrophic African savanna
6.1 Introduction
6.2 Methods
6.2.1 Compensatory growth and leaf N
6.2.2 Net annual height growth, density and gulliver distribution
6.3 Data analysis
6.4 Results
6.4.1 Compensatory growth ability
6.4.2 Morpho-functional traits
6.4.3 Gullivers demography and distribution
6.5 Discussion
6.5.1 Gulliver resprouting abilities
6.5.2 Morpho-functional traits
6.5.3 Gulliver distribution and abundance
6.6 Conclusion
6.7 References
CHAPTER 7 Ungulate browsing and plant defensive traits: modelling changes in plant productivity and soil nutrient availability in savanna
7.1 Introduction
7.1.1 Conceptual definition
7.1.2 Assumptions
7.2 Operational definition
7.2.1 Specific formulae of the ALLOCATE model
7.2.2 Aspects related to the plant-browser system in a semi-arid eutrophic savanna
7.3 Results
7.4 Discussion
7.4.1 Tolerance vs resistance: plant biomass and soil nutrient availability
7.4.2 Plant community composition and nutrient cycling
7.5 References
CHAPTER 8
8.1 Conclusion
8.2 References