Seed ecology and growth comparisons of native (Virgilia divaricata) and invasive alien (Acacia mearnsii and A. melanoxylon) plants: implications for conservation By Stefan Goets Submitted in fulfilment of the requirements for the degree Magister Scientae in the Faculty of Science at the Nelson Mandela Metropolitan University. April 2017 Supervisor: Dr T. Kraaij Co-supervisor: Dr K.M. Little DECLARATION I, Stefan Albert Goets (210033134), hereby declare that the thesis for the degree Magister Scientae is my own work and that it has not previously been submitted for assessment or completion of any postgraduate qualification to another University or for another qualification. ………………………………. Stefan Albert Goets Official use: In accordance with Rule G5.6.3, 5.6.3. A treatise/dissertation/thesis must be accompanied by a written declaration on the part of the candidate to the effect that it is his/her own work and that it has not previously been submitted for assessment to another University or for another qualification. However, material from publications by the candidate may be embodied in a treatise/dissertation/thesis. i TABLE OF CONTENTS ABSTRACT .................................................................................................................................. iv ACKNOWLEDGEMENTS ............................................................................................................. vi LIST OF FIGURES ...................................................................................................................... viii LIST OF TABLES .......................................................................................................................... ix GENERAL INTRODUCTION .......................................................................................................... 1 Rationale ............................................................................................................................................. 1 Study area ........................................................................................................................................... 3 Study species ....................................................................................................................................... 4 References .......................................................................................................................................... 8 CHAPTER 1 - Literature review: The dynamics and effects of plant invasions, and the management thereof, also considering seed ecology and plant growth competition ........... 15 Introduction ...................................................................................................................................... 15 The dynamics of plant invasions ....................................................................................................... 16 Effects of invasive alien plants .......................................................................................................... 25 Management of invasive alien plants ............................................................................................... 29 Conclusion ......................................................................................................................................... 36 References ........................................................................................................................................ 36 CHAPTER 2 - Can the application of different germination stimuli aid the management of invasive alien plants? ............................................................................................................... 54 Abstract ............................................................................................................................................. 54 Introduction ...................................................................................................................................... 54 Methods ............................................................................................................................................ 56 Results ............................................................................................................................................... 61 Discussion.......................................................................................................................................... 64 Acknowledgements ........................................................................................................................... 66 References ........................................................................................................................................ 66 CHAPTER 3 - Comparing soil-stored seed banks as a measure of performance between two invasive alien species (Acacia mearnsii and A. melanoxylon) and an indigenous analogue (Virgilia divaricata) ................................................................................................................... 72 Abstract ............................................................................................................................................. 72 Introduction ...................................................................................................................................... 73 Methods ............................................................................................................................................ 74 Results ............................................................................................................................................... 77 ii Discussion.......................................................................................................................................... 79 Acknowledgements ........................................................................................................................... 83 References ........................................................................................................................................ 81 CHAPTER 4 - Comparing growth performance between two invasive alien species (Acacia mearnsii and A. melanoxylon) and an indigenous analogue (Virgilia divaricata) ................... 89 Abstract ............................................................................................................................................. 89 Introduction ...................................................................................................................................... 89 Methods ............................................................................................................................................ 92 Results ............................................................................................................................................... 95 Discussion.......................................................................................................................................... 97 Acknowledgements ......................................................................................................................... 101 References ...................................................................................................................................... 101 Appendix 1: GLM output tables ...................................................................................................... 106 CHAPTER 5 - Synthesis and conclusions ................................................................................ 107 Summary of major findings ............................................................................................................. 107 Implications for management ......................................................................................................... 109 Study contributions and limitations ................................................................................................ 110 Recommendations for future study ................................................................................................ 111 References ...................................................................................................................................... 112 iii ABSTRACT The abundance of invasive alien plants, with their numerous associated negative effects, has resulted in challenging and costly management requirements in South Africa. Management of invasives is further complicated when species have rapid inherent growth rates, atmospheric nitrogen fixing capabilities and large, persistent seed banks. Management strategies other than the usual mechanical, chemical, and biological (and combinations thereof) methods may be more effective. An alternative strategy is ’habitat management’, whereby management may be aimed at favouring native species. If the native species could compete favourably with, and possibly exclude, the invasive species, which is more likely when the species are ecologically similar, management may be more effective. This study investigated the competitive performance of two invasive species, Acacia mearnsii De Wild. and Acacia melanoxylon R.Br., and an indigenous ecological analogue, Virgilia divaricata Adamson, on the southern Cape Coast of South Africa. Both acacias are included among the five worst invasive species in the native forest and fynbos shrubland ecosystems. Performance was compared among the three species in terms of germination success (responses