Influence of Drought Or Elephant on the Dynamics of Key Woodland Species in a Semiarid African Savanna
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Influence of drought or elephant on the dynamics of key woodland species in a semiarid African savanna Shaun Donovan MacGregor Bsc. Agric. (Hons) Submitted in partial fulfilment ofthe requirements for the Degree ofMaster ofScience in Agriculture in the Discipline ofRange and Forage Resources School ofApplied Environmental Sciences Faculty ofScience and Agriculture University ofNatal, Pietermaritzburg November 2000 11 Declaration I hereby declare that the contents of this dissertation comprises my own original work, except where otherwise acknowledged or stated. This work has not been submitted for degree purposes to any other university. -:?t#rr..... S. D. MacGregor ll1 Acknowledgements The compilation ofthis document was made possible by the assistance rendered from the following people and organizations. I am grateful to my parents, Kevin and Jacqueline MacGregor, for their self-denying financial support, their love and confidence throughout the compilation ofthis thesis, and my life. My supervisor, Professor Tim O'Connor, for his patience, professionalism and enthusiasm for my project. I thank De Beers Consolidated Mines Ltd for making the thesis possible and the assistance rendered by the management and staffofVenetia Limpopo Nature Reserve. I am especially grateful to Nigel and Merry Fairhead for their friendship, generosity and assistance throughout the practical component ofthis thesis. I am grateful to the Foundation for Research and Development for their much appreciated Grant Holder's bursary. To the staffand postgraduates ofthe Discipline ofRange and Forage Resources, especially Craig Morris for his consistently avid statistical and general aid. Katja Wiesner for her love, inspiration, and support. Thank-you for being there. IV Abstract Extensive drought- and elephant-related dieback ofColophospermum mopane and Acacia tortitis, respectively, offered an opportunity for increasing understanding ofthe causes of drought-related patch dieback, the factors influencing elephant utilization of woody plants, and the response of woody plants to both aforementioned determinants of savanna structure and function. The dendrochronological analysis of both species was undertaken to estimate potential rates of replacement, following extensive mortality. Areas ofdiscrete dieback were compared with adjacent paired areas of'healthy' vegetation, which revealed, on average, 87% and 13% loss ofbasal area by mortality, respectively. 'Live' and 'dead' plots did not differ in soil type, topography or mean slope, but differed in vegetation structure, soil surface condition, and soil chemistry. Although there was evidence of self-thinning, neither inter- nor intra-specific competition explained dieback. 'Dead', by comparison with 'live' plots, had changed from functioning as sinks ofsediment and water to sources, were less likely to retain water because ofa poorsoil surface condition, and were predisposed to drought effects because ofa greater proportion offines, and Na concentration. Dieback resulted from insufficient soil water for survival during a drought owing to the development ofa dysfunctional landscape during 50 years oflivestock ranching.Spatial heterogeneity within a landscape was suggested to enhance woodland resilience to severe droughts by ensuring the survival ofplants in run-on sinks or 'drought refuge' sites. Stem sections were removed from 40 multi-stemmed C. mopane trees and prepared for examination under a dissecting microscope. It was impossible to age C. mopane, owing to a hollow and/or dark heartwood. Nevertheless, the distribution of stem diameters suggested a single recruitment event. Fire scars attributed to the last recorded fire in 1948 could explain the trees' multi stemmed growth form and indicate that most trees ofVLNR were> 50 years of age. Growth rings were identified in 29 A. torti/is trees ofunknown age, but were not correlated with annual rainfall records. Growth rates varied between trees; mean ring width ranged from 1.4 to 3.5 mm (overall mean 2.4 ± 0.1 mm). A technique was proposed for predicting growth rate from annual rainfall, using selected data, and several factors potentially influencing ring width in semi arid environs were identified. Permanent ground-based transects were located within riparian (n = 16) and Acacia (n = 5) woodlands to monitor elephant utilization. Elephant had not changed the population structure ofthe woodlands by 2000, but had reduced stem density from 215.6 stems ha- 1 (1996) to 84.4 stems ha-I (2000). Acacia tortitis trees in the woodlands had branches removed, were debarked, uprooted and broken. Acacia tortitis trees in the riverine had lower levels of utilization, whilst Acacia ni/otica trees were mostly debarked. The method of elephant feeding varies within and between woody species, provided it is within the mechanical constraints of a certain size or species. Elephant behaviour is concluded to depend on spatiotemporal variation of forage abundance/quality, abundance of a preferred species, and species response (coppice or mortality). Elephant can cause a change ofvegetation state, and increase spatial homogeneity ofa plant population. The remnant population of woodland trees should provide the potential for recolonization, in which case the system would reflect the stable limit cycle. However, ifbrowsing inhibits seedling recruitment, the system could reflect either a multiple stable state system or an artificial equilibrium imposed on a stable limit cycle. v Table of contents ~ Declaration 11 Acknowledgements III Abstract IV Table of contents V Chapter 1. Introduction 1 Chapter 2. Literature review: Drought and elephant as determinants of savanna structure and function, and Dendrochronology 5 Introduction 5 Drought 5 Herbivory, mostly by elephant 6 Spatial dimensions of plant-herbivore interactions 6 A stable limit cycle versus multiple stable states 8 Factors influencing elephant response 10 Vegetation structure 10 Plant species and abundance 11 Season and nutrition 12 Plant vigour 13 Dendrochronology 14 Chapter 3. Study area 17 Ecosystem: definition 17 Physical Environment 17 Location 17 Clima~ 17 Geology 22 Hydrology 22 Vegetation 22 Animal populations 24 History 25 PART A: Colophospermum mopane 27 Chapter 4. Colophospermum mopane: a brief review of facts both general and specific to the study area 27 Distribution 27 VI Physiology 27 Value 28 Chapter 5. Patch dieback of Colophospermum mopane in a dysfunctional semi- arid African savanna 30 Introduction 30 Mclho~ 32 Data collection 32 Data analysis 37 Results 38 Patterns of dieback 38 Abiotic variables influencing dieback 41 Role of competition 45 Population structure and dieback 47 Influence of soil type 47 Regression model of dieback 50 Discussion 50 Preconditions and causes of patch dieback 50 Landscape variation in dieback 53 Dieback: manifestation of a dysfunctional landscape 54 Chapter 6. Coppicing (stem recruitment) of drought-stressed Colophospermum mopane trees 56 Introduction 56 Mefuo~ 57 Data collection 57 Data analysis 57 Results 58 Discussion 60 PART B: Acacia tortilis 62 Chapter 7. Acacia tortilis: life-history stages reviewed 62 Flowering 62 Dispersal 63 Germination 65 Growth and critical heights 66 Browse value 67 Chapter 8. Determination of annual diameter and height increments for trees in a semi-arid African savanna 68 Introduction 68 vu Mclho~ 69 Data collection 69 Data analysis 69 Annual diameter increment 69 Proposed technique for predicting annual diameter increment 69 Annual height increment 70 Results 71 Annual diameter increment 71 Proposed technique for predicting annual diameter increment 74 Annual height increment 77 Discussion 80 Annual diameter increment 80 Annual height increment 81 Proposed technique for predicting annual diameter increment 82 Anomalies associated with ring identification 82 Potential factors influencing ring width 83 Chapter 9. The functional response of elephant to Acacia tortiUs in the Northern Province, South Africa 85 Introduction 85 Methods 88 Data collection 88 Data analysis 90 Population structure ofAcacia tortilis 90 Elephant response to Acacia tortilis 90 Canopy removal 91 Broken and uprooted stems 93 Debarbng 94 Factors influencing the different methods ofutilization 95 Results 96 Population structure ofAcacia tortiUs woodlands 96 Elephant response to Acacia tortilis woodlands 98 Population structure 98 Canopy removal 99 Broken and uprooted stems 104 Debarking 115 Factors influencing the different methods offeeding 123 Elephant utilization of riverine Acacia tortiUs 125 Population structure 125 Canopy removal 126 Broken and uprooted stems 128 Debarking 128 Elephant utilization ofAcacia nilotica woodlands 128 Discussion 129 The functional response of elephant to Acacia tortiUs 129 Vlll The biological response ofAcacia tortilis to elephant feeding 131 The functional response ofAcacia tortilis to elephant feeding 133 Spatial heterogeneity of herbivory and its implications 135 A speculative interpretation of the past and future population processes of the Acacia tortilis woodlands 136 Chapter 10. General Discussion 138 Is patch dieback and woodland destruction cause for immediate concern regarding loss of biodiversity, with possible implications on the structure and function of the Colophospermum mopane and Acacia tortilis populations, respectively? 138 Patch dieback of Colophospermum mopane woodlands 140 Foraging behaviour ofelephant 141 Spatial heterogeneity 143 Future implications of current trends 144 References 148 Appendix 166 1 Chapter 1 Introduction