The Life History of Selected Coastal Foredune Species of South Africa
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The Life History Of Selected Coastal Foredune Species Of South Africa Irma Cornelia Knevel THE LIFE HISTORY OF SELECTED COASTAL FOREDUNE SPECIES OF SOUTH AFRICA THESIS Submitted in fulfilment of the requirements for the degree of DOCTOR OF PHILOSOPHY at Rhodes University by IRMA CORNELIA KNEVEL Department of Botany Rhodes University December 2001 The beach is a stimulating place for humans, but a stressful one for plants that must remain for a lifetime… Barbour (1992) ABSTRACT South African dune fields are severely threatened by human expansion and in the long run the stabilisation of many dunes will be necessary. The alien grass Ammophila arenaria is the most important drift sand stabiliser at present in South Africa. Although not invasive, the current impact of A. arenaria on the dune systems of South Africa is considerable, and thus the stabilising benefit of the grass seemed to may be outweighed by its negative consequences. It is therefore preferable to use indigenous sand stabilising species. In order to define guidelines for the application of indigenous plants for stabilisation, their autecology should be studied first to enhance the chance of successful stabilisation results. The main aim of the present thesis was to gather information on the life history processes of selected indigenous, sand stabilising foredune species. To investigate the growth of foredune pioneer species, the common pioneer Scaevola plumieri was followed over a three-year period to determine the growth season and leaf phenology. Soil-borne pathogens are known to influence the growth and vegetation dynamics of foredune species. To examine this effect on the South African foredunes the rhizosphere soil and the roots of several species were studied. To test the effect of the nematode fauna on succeeding plant species a transplantation experiment was carried out. The seed stage is the only life-cycle stage that can survive unfavourable conditions. Therefore, the seed ecology of several foredune species was studied extensively to determine the reproductive season, the seed production, the fate of seeds after shedding (germination, seawater dispersal), germination requirements and seed bank strategy. Seeds of the species Arctotheca populifolia, Ipomoea pes-caprae, Myrica cordifolia, and Scaevola plumieri were subjected to germination trials, field observations on seedling survival, and scarification and stratification experiments. This was done to obtain information about the germination requirements and to determine the reproductive season and growth season. The seed bank strategy of the foredune species, as well as the seed bank density, was determined by extensive sampling along the Cape coast. The species S. plumieri thrived under sand accretion situations, which makes it a good candidate for stabilisation purposes. The growth of S. plumieri was seasonal, with the highest leaf production during spring and summer. The stem position on the foredune had a strong effect on the overall performance of S. plumieri, with the stems situated on the landward face of the foredune showing higher leaf and seed production. The ii nematode survey of soil and roots of several foredune species showed that all plant species featured a specific nematode fauna in the rhizosphere soil and the roots. The specific nematode fauna affected the growth of foreign plant species in the transplantation experiment, resulting in a lower root and/or shoot biomass production. Most of the foredune species produce seeds from spring to late summer. For S. plumieri the position of the stem on the dunes, as well as the predation of unripe seeds affected the number of seeds produced. The highest production was found for the landward faced stems. The S. plumieri seeds were able to float on seawater for at least three months without losing viability, as was observed for seeds of I. pes-caprae. The seeds of M. cordifolia, however, sank after a few days, but their viability was not affected. The rhizome fragments of A. arenaria and S. virginicus floated for 120 days, whereas the fragments of E. villosa sunk after one day. The viability of S. virginicus fragments was affected by the duration in seawater by an increase in sprouting time. The seeds of all species tested germinated readily under controlled conditions, except S. plumieri seeds which required a long lag-phase before germination. In the field the seeds of A. populifolia, I. pes-caprae and S. plumieri germinated, producing many seedlings. Only the seedlings of A. populifolia and S. plumieri survived. Of the species found in the foredunes 57% was represented in the soil seed bank. For most species, the seeds that were found in the seed bank showed viability of at least 40%. Many of the seeds found were older than one year, suggesting a short-term persistent seed bank. The present study is a start in filling the gap in information on dune pioneer and foredune species. The conclusion was that in general all species in the present study were easy to grow under controlled conditions, and thus could be used for stabilisation purposes. When the more rapidly growing pioneer species are planted in combination with succeeding foredune species, a functional and aesthetic ecosystem could be created. iii TABLE OF CONTENTS ABSTRACT ii FOREWORD ix LIST OF FIGURES xi LIST OF TABLES xiv LIST OF PLATES xvii CHAPTER 1: GENERAL INTRODUCTION 1 1.1 Dune stabilisation in South Africa 2 1.2 Immediate cause of study 3 1.3 Purpose of present study 4 1.4 General outline of thesis 8 CHAPTER 2: AN INTRODUCTION TO THE COASTAL ENVIRONMENT OF 10 SOUTH AFRICA 2.1 Geographical setting and ecological feature 11 2.2 Coastal sand dune characteristics 13 2.3 Dune formation and coastal vegetation 14 2.4 The Eastern Cape coastal dunes 17 2.5 Zonation, succession and plant communities of the Eastern Cape coastal 18 dunes 2.6 Plant functional types of coastal foredunes 23 2.7 Description of coastal foredune species 25 CHAPTER 3: REPRODUCTION AND GROWTH OF SCAEVOLA PLUMIERI 35 L. (VAHL.), A FOREDUNE PIONEER OF SOUTH AFRICA 3.1 Introduction 36 3.2 Material and methods 39 3.2.1 Species description 39 3.2.2 Study site 41 3.2.3 Sampling procedure Scaevola plumieri 42 3.2.4 Sand movement 45 3.2.5 Soil sampling 45 iv 3.2.6 Climate data 46 3.5.7 Data analysis 46 3.3 Results 48 3.3.1 Number of stems per plot 48 3.3.2 Leaf phenology adults and seedlings 49 3.3.3 Reproduction phenology 54 3.3.3.1 Number of peduncles per stem 54 3.3.3.2 Number of buds, unripe seeds and ripe seeds per stem 55 3.3.3.3 Unfertilised flowers and aborted seeds 59 3.3.3.4 Duration of the reproduction stages 62 3.3.4 Sand movement 65 3.3.5 Soil analysis 68 3.3.6 Climatic data 70 3.3.7 Summary 73 3.4 Discussion 75 3.5 Acknowledgements 83 CHAPTER 4: GERMINATION AND SURVIVAL OF SELECTED COASTAL 83 FOREDUNE PIONEERS OF THE EASTERN CAPE, SOUTH AFRICA 4.1 Introduction 84 4.2 Material and methods 86 4.2.1 Seed collecting and drying 86 4.2.2 Germination under controlled conditions 87 4.2.3 Seed scarification 89 4.2.4 Seed stratification 90 4.2.5 Seed germination, seedling establishment and survival in the field 91 4.2.6 Climate 91 4.2.7 Data analysis 91 4.3 Results 93 4.3.1 Germination under controlled conditions 93 4.3.2 Seed scarification 96 4.3.3 Seed stratification 99 4.3.4 Germination and seedlings establishment in field conditions 101 4.3.5 Climatic data 104 v 4.4 Discussion 106 CHAPTER 5: THE SOIL SEED BANK AND SEED CHARACTERISTICS OF 114 COASTAL FOREDUNE SPECIES IN SOUTH AFRICA 5.1 Introduction 115 5.2 The longevity of seeds: Soil seed bank classification and seed 117 characteristics 5.2.1 The soil seed bank 117 5.2.2 Ecological seed attributes 119 5.3 Material and methods 121 5.3.1 Seed bank sampling 121 5.3.2 Seed viability test 123 5.3.3 Soil seed bank diversity and vegetation 124 5.3.4 Seed characteristics 125 5.3.5 Seed longevity, species longevity and successional status 127 5.3.6 Data analysis 128 5.4 Results 130 5.4.1 Pilot seed bank sampling test 130 5.4.2 Vegetation versus soil seed bank 131 5.4.3 Soil seed bank sampling 135 5.4.4 Viability of seeds found in the soil 140 5.4.5 Seed characteristics 143 5.4.6 Summary 146 5.5 Discussion 149 CHAPTER 6: SEAWATER DISPERSAL OF SELECTED COASTAL 156 FOREDUNE PIONEERS IN THE EASTERN CAPE, SOUTH AFRICA 6.1 Introduction 157 6.2 Dispersal mechanisms and strategies 159 6.2.1 Strategies South African foredune species 160 6.3 Material and methods 163 6.3.1 Seed and rhizome buoyancy experiment 163 6.3.2 Post-buoyancy viability test 164 6.3.3 Data analysis 166 vi 6.4 Results 167 6.4.1 Seed buoyancy experiment 167 6.4.2 Post-buoyancy seed viability 168 6.4.3 Rhizome buoyancy experiment 170 6.4.4 Post-buoyancy rhizome viability 172 6.5 Discussion 176 CHAPTER 7: THE ROLE OF SOIL-BORNE PATHOGENS IN SOUTH 181 AFRICAN COASTAL FOREDUNE VEGETATION 7.1 Introduction 182 7.2 Coastal foredunes and soil pathogens 184 7.2.1 Soil-borne pathogens: The nematodes 184 7.2.2 The role of nematodes in vegetation dynamics 185 7.3 Material and methods 187 7.3.1 Field survey 187 7.3.2 Nematode extraction and identification 188 7.3.3 Soil analysis 190 7.3.4 Transplantation experiment 191 7.3.5 Data analysis 193 7.4 Results 194 7.4.1 Field survey 194 7.4.1.1 Nematodes associated with foredune species 194 7.4.1.2 Plant parasitic nematode feeding types in root and soil 196 7.4.1.3 Nematode species composition around each dune pioneer 198 7.4.1.4 Similarities nematode fauna alien and indigenous dune plants 198 7.4.1.5 Vegetation composition sampled plots 199 7.4.1.6 Soil analysis 200 7.4.2