INVESTIGATING THE INFLUENCES OF TIDAL INUNDATION AND SURFACE ELEVATION ON THE ESTABLISHMENT AND EARLY DEVELOPMENT OF MANGROVES: FOR APPLICATION IN UNDERSTANDING MANGROVE REHABILITATION TECHNIQUES OH RUI YING, RACHEL (B Sc (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF SOCIAL SCIENCES DEPARTMENT OF GEOGRAPHY NATIONAL UNIVERSITY OF SINGAPORE 2015 Declaration I hereby declare that this thesis is my original work and it has been written by me in its entirety I have duly acknowledged all the sources of information which have been used in the thesis This thesis has also not been submitted for any degree in any university previously Oh Rui Ying, Rachel 23 January 2015 i Acknowledgements To my supervisor, Dr Daniel Friess – I would like to express my deepest gratitude for the guidance received and for sharing the countless opportunities that allowed me to learn from and communicate with other academics and like-minded people The academic and personal growth gleaned has been invaluable Deepest appreciation to my academic inspirations – Dr Edward L Webb for strengthening my natural inclinations towards ecological research and Dr Roman Carrasco, for incepting a love and appreciation for statistical methods Many thanks also, to Benjamin Brown, Dominic Wodehouse and a third who wish to remain anonymous, for the precious time and effort for academic discussions with this greenhorn Your dedication, passion and in-depth knowledge have been most inspiring Working in the mangroves is not for the faint-hearted I am genuinely thankful for those who have mucked around in the mud with me – Akhzan Nur Iman, Andi Darmawansyah, Ikhsan Ismail, Regista Rapa, Rio Ahmad, Sardis Andis, Suwardi, Teguh Nagir, Yusran Nurdin, Benjamin Lee Chengfa, Lee Bee Yan, Leong Mun Kidd and Tan Bo Hui I am humbled also, through working with my team of Bengali bhai (Bengali: brother) and Indian ahney (Tamil: brothers) To Derek Yap (Camphora Pte Ltd) and Salad Dressing Architecture Company – deepest appreciations for sharing the collaborative opportunity in setting up the mesocosm experiment I am forever grateful to the Mangrove Lab team and my friends for being my pillars of support Special mention goes to Serene Ng, my comrade-in-arms, for being a constant companion on this academic journey with her encouragement and sweet nature, over our many cups of juice and chai Finally, I am eternally indebted to my family, for their unwavering faith, patience and unconditional support ii Table of Contents Declaration i Acknowledgements ii Summary vi List of Tables vii List of Figures ix Chapter – Introduction 1.1 Mangroves are highly threatened 1.2 Past attempts at mangrove rehabilitation 1.2.1 Defining mangrove rehabilitation 1.2.2 A lack of mangrove rehabilitation success 1.3 Ecological Mangrove Rehabilitation (EMR) 1.4 Aims and objectives Chapter – Literature Review 2.1 The roots of restoration science 2.2 The restoration of coastal wetlands 2.2.1 The evolution of coastal wetlands restoration 2.2.2 Approaches in coastal wetland restoration 11 2.3 Adopting the Ecological Mangrove Rehabilitation (EMR) 11 approach 2.3.1 The 6-step EMR approach 2.4 The importance of surface elevation and inundation hydroperiod 12 15 for mangrove rehabilitation success 2.4.1 Field studies relating surface elevation and mangrove 16 distributions 2.4.2 Experimental studies relating seedling responses to 18 inundation 2.4.3 Experimental studies relating propagule establishment to 20 inundation iii Chapter – Surface elevation is an important factor in achieving mangrove 24 rehabilitation success 3.1 Introduction 24 3.2 Materials and methods 25 3.2.1 Study area 25 3.2.2 Field data collection 27 3.2.3 Post-rehabilitation vegetation survey in aquaculture 29 ponds and reference mangrove forests 3.2.4 Genera-specific surface elevation envelopes and 30 prediction maps 3.3 Results 31 3.3.1 Vegetation established in aquaculture ponds and 31 reference mangrove forests 3.3.2 Genera-specific surface elevation envelopes and 34 prediction maps of mature mangrove trees in aquaculture ponds 3.4 Discussion 40 3.4.1 Surface elevation affects propagule establishment and 40 seedling development 3.4.2 Low surface elevations hinder propagule establishment: 43 mid-corrective actions are required in approaching successful re-vegetation at rehabilitation site 3.4.3 Using surface elevation data to predict future 45 establishment 3.5 Summary 46 Chapter – Interspecific variations in survival and growth responses of 47 mangrove seedlings to three contrasting inundation durations 4.1 Introduction 47 4.2 Materials and methods 48 4.3 Results 53 4.3.1 Seedling survival 53 4.3.2 Seedling growth responses 55 iv 4.3.3 Root length of Rhizophora seedlings 4.4 Discussion 57 58 4.4.1 Impacts of prolonged inundation on seedling survival 58 4.4.2 Impacts of prolonged inundation on seedling growth 61 4.4.3 Interactions between inundation and other physical 63 factors that affect seedling growth 4.5 Summary 64 Chapter – General Discussion 66 5.1 Principal findings 66 5.1.1 Effects of surface elevation on mangrove establishment 66 5.1.2 Effects of prolonged inundation on seedling survival and 67 development 5.2 A synthesis: Reconciling a field study and a mesocosm 68 experiment 5.3 Implications for mangrove rehabilitation 69 5.4 Recommendations 5.4.1 Identifying disturbance-free periods that favour 73 establishment and colonisation 5.4.2 Implement mid-course corrections 74 5.4.3 Long-term monitoring of recovery trajectory 74 5.5 Conclusions 75 References 76 v Summary Human exploitation and conversion of natural mangrove ecosystems is causing widespread ecosystem degradation and loss Yet, mangroves have the potential to recover functionality through secondary succession Human-mediated mangrove rehabilitation projects have been implemented in response to mitigating such losses However, projects vary in success rates, most of which could be attributed to the lack of identifying site-specific cause(s) of mangrove degradation, and/or the barriers and stressors that have prevented natural recovery via secondary succession The aim of this thesis is to contribute to the understanding of hydrologic management in the success of mangrove rehabilitation projects The focus is on how tidal inundation and surface elevations influences the establishment, survival and development of mangroves in early developmental stages This was achieved via a field study and a mesocosm experiment Overall, analyses suggested that (a) establishment of mangroves were restricted to specific surface elevations in rehabilitation site (-1.511 m ≤ x ≤ 0.228 m WGS 84) , and (b) Avicennia seedlings were incapable of tolerating prolonged inundation durations beyond hours (diurnal regime) compared to Rhizophora seedlings This highlight that there are species-specific thresholds beyond which mangrove establishment is impeded, and where early development is constraint Taken together, the results suggest that surface elevation is potentially a key influence on the successful establishment and development of mangroves as it controls inundation hydroperiod It highlights also, the need for rehabilitation planners and practitioners to include consideration of altering surface elevation in degraded sites to favour natural colonisation and establishment in achieving rehabilitation success Chapter provides an introduction to the issues facing mangrove rehabilitation It outlines some of the potential reasons why certain mangrove rehabilitation projects vi aimed at reversing mangrove losses have ended in failure It introduces the EMR approach, which has been applied in achieving reforestation success in coastal wetlands, and the aims and objectives of this thesis in contributing to this set of rehabilitation knowledge Chapter starts with an introduction and review of literature regarding coastal wetland ecosystems restoration The chapter progresses with how this knowledge has been applied to the rehabilitation of mangroves, as encapsulated in the Ecological Mangrove Rehabilitation (EMR) approach To create a foundation and link to subsequent empirical chapters, Chapter concludes with an in-depth summary of studies that investigated the effect of prolonged inundation on seedling survival and growth, and a discussion on the contributing physiological processes that were proposed to explain the observed effects Chapters and will discuss the two objectives outlined above: the large scale rehabilitation project and the controlled mesocosm experiment Each chapter is self-contained and consists of a short introduction, methods, results and a discussion of the observed results The final chapter, Chapter 5, provides a synthesis of the findings and the implications of this study for future mangrove rehabilitation efforts, recommendations for further research 466 Words vii List of Tables CHAPTER 2.1: Watson inundation classification and the related Southeast Asian 17 mangrove species Source: Watson, 1928 CHAPTER 3.1: Number of seedlings/saplings and trees surveyed across aquaculture 32 ponds and reference forests 3.2: Minimum, interquartile range and maximum surface elevation of 35 established trees surveyed in reference forest sites, based on WGS 84 datum CHAPTER 4.1: Performance matrix of the GLM models fitted 55 4.2: Performance matrix of the mixed-effects models fitted 57 CHAPTER 5.1: A summary of the EMR rehabilitation techniques employed across eight 71 mangrove rehabilitation projects in achieving successful reforestation (Lewis & Brown, 2014) viii List of Figures CHAPTER 2.1: A traditional view of restoration options for a degraded system, 10 illustrating the idea that the system may proceed along different trajectories and that the goal of restoration is to guide the trajectory towards some desired state Source: Hobbs & Norton, 1996 2.2 The 6-step EMR approach (Lewis & Brown, 2014) 13 2.3: Schematic representation using an Avicennia alba propagule to illustrate 23 the three thresholds to be surpassed before establishment (1) The propagule first has to acquire a minimum root length during an inundation-free period to resistant against floating up during tidal inundation (2) Then, roots have to be long enough to resist hydrodynamics by waves and currents (3) Rooted seedling may still be dislodged via mixing or erosion of surface sediments Source: Balke et al., 2011 CHAPTER 3.1: (a) Regional setting of Makassar (black box), South Sulawesi, 27 Indonesia; (b) former aquaculture ponds at Kuri Caddi and reference forests at both Kuri Caddi and Kuri Lompo; and (c) broken lines delineate the disused aquaculture ponds in Kuri Caddi, extracted from Google Earth (dated February 2014) 3.2: (a) Dike walls that have undergone strategic breaching, and (b) 29 regrading of selective dike walls to produce substrate at lower surface elevations (foreground) Red arrows point to existing dike walls 3.3: The interquartile range represents surface elevation envelopes per species of seedling/saplings surveyed in (a) aquaculture ponds and (b) reference forests Whiskers indicate maximum and minimum values and empty circles indicate outliers ix 34 3.4: The interquartile range represents surface elevation envelopes per genus 35 (i.e Avicennia spp., Excoecaria spp., Rhizophora spp and Sonneratia spp., surveyed in reference forest sites Whiskers indicate maximum and minimum values and empty circles indicate outliers 3.5: Map of aquaculture ponds showing surface elevation changes (i.e grade 36 down, grade up), location of pile of broken branches and established vegetation where each green triangle represents an established individual (surveyed in June 2014) 3.6: (a) Each green triangle represents one established vegetation individual 39 (surveyed in June 2014) Predicted elevation ranges where (b) Avicennia spp., (c) Rhizophora spp and (d) Sonneratia spp might establish in the future as trees are represented as green areas CHAPTER 4.1: (a) Aerial view of experimental set-up, (b) side and (c) aerial view of 49 each pair of reservoir and experimental tank 4.2: Photographs of (a) the actual mesocosm set-up and the experimental 50 pots with (b) Rhizophora and (c) Avicennia seedlings 4.3: Proportion of seedlings alive per inundation treatment (A = Avicennia 54 spp.; R = Rhizophora spp.; 5, and represent the number of inundation hours) 4.4: Cumulative stem height of both Rhizophora (top row) and Avicennia 56 seedlings (bottom row), segregated by inundation treatment, across weeks to 11 Standard errors are indicated by whiskers 4.5: Barplot of (a) mean stem height and (b) mean root length of Rhizophora seedlings across three Inundation Treatments (R5, R7 and R9) Standard errors are indicated by whiskers x 58 ... management in the success of mangrove rehabilitation projects The focus is on how tidal inundation and surface elevations influences the establishment, survival and development of mangroves in early developmental... Discussion 66 5 .1 Principal findings 66 5 .1. 1 Effects of surface elevation on mangrove establishment 66 5 .1. 2 Effects of prolonged inundation on seedling survival and 67 development 5.2 A synthesis:... Chapter – Introduction 1. 1 Mangroves are highly threatened 1. 2 Past attempts at mangrove rehabilitation 1. 2 .1 Defining mangrove rehabilitation 1. 2.2 A lack of mangrove rehabilitation success 1. 3 Ecological