nvestigating the influences of tidal inundation and surface elevation on the establishment and early development of mangroves for application in understanding mangrove rehabilitation techniques 1

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 1 – Introduction

1

1.1 Mangroves are highly threatened

1

1.2 Past attempts at mangrove rehabilitation

2

1.2.1 Defining mangrove rehabilitation

2

1.2.2 A lack of mangrove rehabilitation success

3

1.3 Ecological Mangrove Rehabilitation (EMR)

5

1.4 Aims and objectives

5

Chapter 2 – Literature Review

8

2.1 The roots of restoration science

8

2.2 The restoration of coastal wetlands

9

2.2.1 The evolution of coastal wetlands restoration

9

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 3 – 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 4 – 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 5 – 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 5 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 1 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 2 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 2 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 3 and 4 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
2.1: Watson inundation classification and the related Southeast Asian 17
mangrove species. Source: Watson, 1928.

CHAPTER 3
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

4.1: Performance matrix of the GLM models fitted.

55

4.2: Performance matrix of the mixed-effects models fitted.

57

CHAPTER 5
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
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
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
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, 7 and 9 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 1
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