structure and biomass accumulation of natural mangrove forest at gazi bay, kenya

83 Pages·2014·1.44 MB·English

STRUCTURE AND BIOMASS ACCUMULATION OF NATURAL
MANGROVE FOREST AT GAZI BAY, KENYA

GITHAIGA, MICHAEL NJOROGE

I56/CE/15321/08

A thesis submitted in partial fulfillment of the requirements for the award of the degree of
Master of Science in the School of Pure and Applied Sciences of Kenyatta University

AUGUST 2013
ii

DECLARATION
This thesis is my original work and has not been presented for a degree in any other University
or any other award
Githaiga, Michael Njoroge

Signed ________________ Date ______________

Supervisors:
We confirm that the work reported in this thesis was carried out by the candidate under our
supervision.
Prof. Kiplagat Kotut
Department of Plant and Microbial Sciences
Kenyatta University
Signed ________________ Date _________________

Dr. Francis Kariuki
Department of Plant and Microbial Sciences
Kenyatta University
Signed ________________ Date ________________

Dr. James G. Kairo
Marine Ecology and Environmental Programme
Kenya Marine and Fisheries Research Institute
Signed ________________ Date ________________
iii

DEDICATION
To my lovely wife Marion, my three daughters, Vanessa, Nelius and Diana, my son, Eric and to
all who care for posterity.
iv

ACKNOWLEDGEMENT
My sincere thanks go to the entire Department of Plant and Microbial Sciences of Kenyatta
University. My deep gratitude goes to my supervisors, Prof. Kiplagat Kotut and Dr. Francis
Kariuki of Kenyatta University; and Dr. James Kairo of Kenya Marine and Fisheries Research
Institute (KMFRI), Mombasa, for their guidance throughout the study.
I also wish to extend my gratitude to the mangrove team in Kenya and abroad. I particularly
single out Prof. Mark Huxham of Edinburgh Napier University, Dr. Martin Skov (Bangor
University), and Mr. Joseph Langat for their guidance and technical advice throughout the course
of the study. Messier Alfred Obinga; Laitani Suleimani and Tom Peter are thanked for their
support in the field work.
Lastly, I wish to extend my gratitude to my lovely wife Marion Wambui, my three daughters,
Vanessa, Nelius, and Diana and my son Eric for their perseverance during the entire period I was
away for this study. Thanks also to my mother, Miriam Wairimu for teaching me how to walk
and for her nurture, sacrifice and prayers. The contributions of many others not mentioned here
are not forgotten but all your kind support will be forever engrained in my memory.
This study was supported by Rufford Small Grant Project through Earthwatch Institute of UK.
Additional support was made through John Ellermans Foundation, International Ocean Institute
(IOI), and Aviva PLC to whom I am very grateful.
v

TABLE OF CONTENTS

DECLARATION .................................................................................................... ii
DEDICATION ...................................................................................................... iii
ACKNOWLEDGEMENTS .................................................................................. iv
TABLE OF CONTENTS ....................................................................................... v
LIST OF TABLES ................................................................................................. x
LIST OF FIGURES .............................................................................................. xi
LIST OF APPENDICES ...................................................................................... xi
ACRONYMS AND ABBREVIATIONS .......................................................... xiii
ABSTRACT ......................................................................................................... xiv
CHAPTER ONE ..................................................................................................... 1
1.0 INTRODUCTION ....................................................................................... 1
1.1 Background information................................................................................................1
1.2 Statement of the problem ..............................................................................................2
1.3 Justification ...................................................................................................................3
1.4 Objectives ......................................................................................................................4
1.4.1 General objective .....................................................................................................4
1.4.2 Specific objectives ...................................................................................................4
1.5 Hypotheses ....................................................................................................................4
CHAPTER TWO ................................................................................................... 5
2.0 LITERATURE REVIEW ..............................................................................................5
vi

2.1 Mangrove environment .................................................................................................5
2.2 Mangrove forest structure .............................................................................................6
2.3 Productivity of mangroves ............................................................................................6
2.4 Mangrove phenology .....................................................................................................9
2.5 Threats to mangroves ..................................................................................................10
2.6 Mangroves and climate change ...................................................................................11
2.7 Mangrove forest at Gazi Bay.......................................................................................12
2.7.1 Climate at Gazi Bay ...............................................................................................12
2.7.2 Mangrove species at Gazi Bay ...............................................................................13
2.7.3 Mangrove exploitation and restoration at Gazi Bay ..............................................14
CHAPTER THREE ............................................................................................. 16
3.0 MATERIALS AND METHODS .............................................................. 16
3.1 Study site .....................................................................................................................16
3.2 Study design ................................................................................................................17
3.3 Physico-chemical factors .............................................................................................18
3.3.1 Measurement of height above datum .....................................................................18
3.3.2 Measurement of redox potential and pH ................................................................19
3.3.3 Sediment surface water salinity and temperature ..................................................19
3.3.4 Sediment grain size analysis ..................................................................................19
3.4 Assessment of mangrove forest structure ....................................................................20
3.4.1 Structural attributes of mangroves .........................................................................20
3.4.2 Quality of the mangrove poles ...............................................................................21
3.4.3 Natural regeneration...............................................................................................21
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3.5 Biomass accumulation estimates .................................................................................21
3.5.1 Above ground biomass accumulation ....................................................................21
3.5.2 Below ground biomass increment ..........................................................................22
3.5.3 Total biomass and ratio of BGB: ABG: accumulation ..........................................22
3.5.4 Leaf phenology ......................................................................................................23
3.6 Data analysis................................................................................................................23
CHAPTER FOUR ................................................................................................ 24
4.0 RESULTS ................................................................................................... 24
4.1 Soil physico-chemical factors .....................................................................................24
4.1.1 Height above datum of the four mangrove zones ..................................................24
4.1.2 Sediment physico-chemical factors .......................................................................25
4.1.3 Sediment grain sizes ..............................................................................................27
4.2 Mangrove forest structure ...........................................................................................28
4.2.1 Importance value ....................................................................................................28
4.2.2 Complexity index ...................................................................................................29
4.2.3 Tree height and diameter at breast height (DBH) variation across the zones ........30
4.2.4 Stem density ...........................................................................................................33
4.2.5 Quality classes of the stems in the mangrove forest ..............................................33
4.2.6 Natural regeneration...............................................................................................34
4.3 Biomass accumulation estimates .................................................................................36
4.3.1 Above ground biomass accumulation ....................................................................36
4.3.2 Below ground biomass accumulation ....................................................................37
4.3 3 Total biomass accumulation and ratio of AGB:BGB ............................................40
viii

4.3.4 Leaf phenology ......................................................................................................42
4.3.5 Relationships ..........................................................................................................43
CHAPTER FIVE .................................................................................................. 45
5.0 DISCUSSION ............................................................................................. 45
5.1 Mangrove forest structure ...........................................................................................45
5.1.1 Importance value (IV) of mangrove species ..........................................................45
5.1.2 Complexity index ...................................................................................................46
5.1.3 Tree height and DBH relationship across zones ....................................................46
5.1.4 Stem density ...........................................................................................................47
5.1.5 Natural regeneration...............................................................................................47
5.1.6 Quality of the stems in the mangrove forest of Gazi .............................................48
5.2 Biomass accumulation estimates .................................................................................49
5.2.1 Above ground biomass accumulation ....................................................................49
5.2.2 Below ground biomass accumulation ....................................................................50
5.2.3 Total biomass and ratio of AGB:BGB ...................................................................52
5.4 Leaf phenology ............................................................................................................54
5.3 Regression equation on biomass accumulation ...........................................................55
CHAPTER SIX ..................................................................................................... 56
6.0 CONCLUSIONS AND RECOMMENDATIONS .................................. 56
6.1 Conclusions .................................................................................................................56
6.2 Recommendations .......................................................................................................57
6.2.1 Recommended areas for further research ..............................................................57
6.2.2 Recommendation for policy intervention ..............................................................57
ix

REFERENCES ..................................................................................................... 59
x

LIST OF TABLES

Table 1: Mangrove species found in Kenya ................................................................................. 14
Table 2: Structural attributes of western, middle and eastern forest blocks ................................. 29
Table 3: Summary of structural characteristics of Gazi mangroves ............................................. 30
Table 4: Quality classes of the poles in the three forest blocks of Gazi mangrove forest ............ 34
Table 5: Juvenile densities (saplings ha-1) in the three forest blocks of Gazi mangrove forest .... 35
Table 6: Pearson correlation coefficient between above ground biomass (AGB) accumulation
and selected soil environment properties during the dry season (DS) and rainy season (WS) at
Gazi mangrove forest .................................................................................................................... 44