Mangrove vegetation structure dynamics and regeneration

 

Thesis Philosophiae Doctor Scientiarum

 

Farid Dahdouh-GuebaS


 

General Introduction

 

Mangroves as a formation with their typical features have been reviewed extensively (Field, 1995, 2000; Stafford-Deitsch, 1996; Spalding et al., 1997; Dahdouh-Guebas & Koedam, 2001; Dahdouh-Guebas et al., 2001).  In this introduction only a selected number of highlights of the mangrove ecosystem, which are important in the framework of the present study, will be given, followed by some of the threats faced by mangrove forests.  Then, the objectives of the present study are formulated and shown as a scheme representing the separate units of this work.  These separate units or case-studies are then extended one by one, each comprising its own introduction, objectives, study sites and methodology, all of which are specific to the respective case-study.  The general discussion and epilogue link all of the case-studies, highlight the applicability of aspects of the study and suggest tracks of application in the framework of development -cooperation.

 

 

Mangrove ecosystems   

 

Mangrove forests, which are thriving in the intertidal zones of tropical countries flanking the equator on all continents, are ecotones between the marine and the terrestrial environment.  Because of this particular location at the edge of both land and sea, much of the fauna and flora in the mangrove consists of specialized species.  The mangrove ecosystem has a high primary production and locally (e.g. within a lagoon) supports an increased secondary and detrivorous production.  Together with members of the Protista and Cyanobacteria, mangroves support virtually the entire primary production.  Plant litter forms an important part of nutrients and energy necessary for breakdown by macrobenthos and micro-organisms in mangrove lagoons.  The plant material is first transformed into particulate organic material by macro-invertebrates such as crabs and other macrobenthos, and is then enzymatically reduced to dissolved organic material.  The full trophic dependence of the secondary producers and higher level organisms on the production of the mangrove, or alternatively, the prime contribution to food sources of the mangrove within the coastal ecosystems, is almost a paradigm.  The issue of outwelling of mangrove material is increasingly revisited and some studies find outwelling is not always the case (Hemminga et al., 1994; Marguillier et al., 1997; Bouillon & Dehairs, in press; Bouillon et al., in press, subm.a, subm.b) and sometimes even the opposite ‘inwelling’ may be true, i.e.the deposition of estuarine material in mangrove sediments (Bouillon et al., subm.b).

 

Mangroves also have a high rate of recycling within the ecosystem.  In addition, the high productivity of the mangrove forest is supported by an increased supply of nutrients and by an availability of a large diversity in niches that are suited for the breeding, spawning, hatching and nursing of both sedentary and migratory marine species (e.g. Macintosh, 1988; Baran & Hambrey, 1998; Baran, 1999; Rönnbäck et al., 1999; Blaber et al., 2000; Nagelkerken et al., 2000; Naylor et al., 2000).  Together with its importance with respect to the productivity of the mangrove trees themselves such as for wood (e.g. Dahdouh-Guebas et al., 2000b; Chapter II), tannins or medicines (e.g. Premanathan et al., 1999) at a local or global level, this can explain the economic importance of this ecosystem.  However, despite the invaluable role played by the mangrove, this ecosystem has become very threatened.  Rönnbäck (1999) states that the undervaluation is partly due to the difficulty involved in placing a monetary value on all relevant factors, but lack of ecological knowledge and a holistic approach among those performing the evaluation may be even more important determinants.

 

Mangrove trees have developed a number of morphological and physiological adaptations that make them very suited for the intertidal habitat.  The most obvious feature is the development  of aerial roots, under the form of pencil roots (e.g. Avicennia), peg roots (e.g. Sonneratia), knee roots (e.g. Bruguiera), plank roots (e.g. Xylocarpus) or prop roots (e.g. Rhizophora), associated with gas exchange.  The fruits or propagules of many mangrove trees are viviparous, which means that a young individual starts growing leaves and a root while still attached to the parental tree, another morphological adaptation.  Considering that the mangrove soil is relatively muddy, this means that when dropping at low tide, these propagules establish by the planting strategy (Van Speybroeck, 1992), whereby they actually plant themselves under the parental tree.  During high tide on the other hand the propagule drops in the water and is transported by it (unless entangled), after which it strands elsewhere (stranding strategy of Van Speybroeck, 1992).  This is an important factor when considering the spatio-temporal changes in stand structure  and composition or ‘vegetation structure dynamics’ of the mangrove, as in the present study.  The physiological adaptation necessary to cope with the salt water from the ocean is a mechanism to exclude salt (e.g. Bruguiera, Lumnitzera, Rhizophora, Sonneratia) or to secrete it (e.g. Avicennia) (Tomlinson, 1986).  For a long time it was however a paradigm that mangroves need salt for their survival : they are facultative halophytes that can perfectly well thrive in freshwater, but they are often out competed by terrestrial vegetation under these conditions and are forced into habitats where the conditions are too harsh for terrestrial or freshwater plants (i.e. where the salinity is too high).

 

 

Mangrove services and mangrove threats

 

People increasingly realise the important position that mangrove forests occupy along tropical and subtropical coasts.  Apart from the above mentioned functions and services the benefits are emphasised, merely when negative consequences related to mangrove destruction become evident in a short time frame.  One of the best examples is the function of mangroves as ‘living dykes’ preventing coastal erosion, a function which is unfortunately often neglected until the first intense storm after mangrove deforestation (Pearce, 1996, 1999).  Removal of mangroves over the last few decades has dislocated the sedimentation of suspended matter supplied by rivers, which normally occurs within the mangrove, to the immediate surroundings of seagrass beds and coral reefs, which are often closely associated with the mangrove, but that will be adversely affected by such sedimentation processes.

 

The enormous ecological, social and economic importance of the mangrove (Clough, 1993; Pernetta, 1993a,b,c,d,e,f; Lefebvre & Poulin, 2000, United Nations University, 2000) as well as the ecological importance (biodiversity) of the coral reef demands that the mangrove ecosystem be managed in a sustainable way.  Therefore regeneration programmes are playing an increasingly important role.  The latter goes hand in hand with scientific research on the distribution strategies (Rabinowitz, 1978a,b; Van Speybroeck, 1992; Clarke & Myerscough, 1993; McGuinness, 1997a) and the restoration of mangroves (Teas, 1977; Lewis, 1982,1999; Field, 1996, 1998a, 1998b, 1999, 2000; Turner & Lewis, 1997; Lewis et al., 1999; Stevenson et al., 1999; Ellison, 2000; Erftemeijer & Lewis III, 2000; Lewis III, 2000; McKee & Faulkner, 2000).  However, since mangrove management beyond the scale of community-based management types was non existent until the recent past, the scientific documentation of the evolution and the use of mangroves are very restricted.  Besides a wider biogeographical analysis (universality of conclusions within a certain type of study) also a temporal analysis (historic evolution of the mangrove in a certain site) is required.

 

The possibilities and constraints of both natural and artificial mangrove regeneration  hold a central position in mangrove research.  Next to various physico-chemical factors required to allow the development  of mangrove juveniles, the juveniles are supposed to establish in a certain area in the first place.  This is often countered by propagule predation (Smith, 1987a,b; Smith et al., 1989; Osborne & Smith, 1990; McKee, 1995a; McGuinness, 1997b).  The influence of the adult mangrove vegetation on the young and juvenile individuals has generally little been studied although it might generate relevant information for the evaluation of existing natural mangrove forests, particularly those under increasing anthropogenic  pressure.  At the same time it largely contributes to setup, protection and management of plantations in the framework of mangrove regeneration (e.g. Lee et al., 1996).  The role of propagule predation is important in all cases.

 

 

Objectives and layout

 

Human interference in mangrove areas has caused substantial changes in both physiognomy and species composition, and world-wide this ecosystem is subjected to an increasing anthropogenic stress.  Nevertheless the ecological and derived socio-economical importance of mangroves was clearly established.  A will to protect and manage the mangrove forests for their ecological services, as wildlife sanctuaries or for sustainable exploitation can be clearly discerned.  This is however not matched by our understanding of factors governing their establishment, dynamics and regeneration, which impedes rational management plans.

 

The present research deals with the investigation of the regeneration potential of mangroves, and it is based on the study of the spatio-temporal vegetation structure of adult, young and juvenile mangrove individuals.  It includes aspects of natural regenerative constraints.

 

The working hypothesis adopted states that young and juvenile trees predict future adult trees and indicate temporal and spatial dynamics.  Since this hypothesis can only be tested on a decadal time scale (in the future), explorative and descriptive approaches have been used to understand their dynamics and to pinpoint a number of its causes.

 

 

Research framework

 

The research framework in which the present PhD study fits, includes different researches, each of which constitutes a piece of the entire puzzle (Fig. 1).  Underneath a general overview of this research framework will be given, followed by the specific questions (alternatively read : hypotheses) addressed in this PhD study (numbers refer to the research framework in figure 1).  

 

 

This study starts with the mapping of the present vegetation structure through airborne remote sensing t.  Fieldwork in which we assess the horizontal and vertical distribution of adult, young and juvenile trees completes the analysis of the vegetation structure at present u.  Retrospective investigation of the same area is done through sequential aerial photography from the past v.  These three first steps constitute the basis of the present PhD research.  The juvenile vegetation layer is subjected to regenerative constraints, amongst which propagule predation is investigated in-depth w.  In combination with the evolution in the recent past of the mangrove and the present distribution of adult trees, both the young and the juvenile vegetation layer can be used to make a prediction for the future, which can be either positive or negative with respect to the extent or composition of the mangrove x.  In case of a positive prediction we may still be interested in the assessment of the natural regeneration potential and make a comparative study between mangrove forests with various degrees of disturbance y.  This is part of the research framework is only in part covered here and is predominantly investigated by Kairo (in prep.).  In case of a negative prediction artificial regeneration and restoration may be necessary z.  Two questions must be addressed then : first, which forest areas need rehabilitation, and second, which tree species are going to be used to rehabilitate those areas ? {  The study on propagule predation contributes to the answer on both questions.  The steps introduced so far are part of the present research on mangrove vegetation structure dynamics and regeneration (footnote on original title : Dynamics and regeneration potential of mangroves : horizontal vegetation structure analysis of juvenile and adult individuals and research into regenerative constraints).  The links with other research frameworks is given in grey in figure 1.  The study of the genetic differentiation in the adult trees of various mangrove populations in part provides elements to answer the question of desirability of propagules from other populations for restoration (Abeysinghe, 1999) |.  The investigation of hydrology in general or as a regenerative constraint in particular provides information to the rehabilitation of mangroves (Verheyden, in prep.) }.  The questions that have often been forgotten by mangrove researchers and for which I designed a separate framework (carried out by Bosire, 1999, in prep.) are “What happens with the artificial regeneration plots 10 years from now ?”, “How functional (sensu oecologiae) do these plantations become ?” and “Is there faunal and floral recruitment into the often monospecific plots ?”.  Therefore we compare the artificial forest with a natural mangrove and a naked area and investigate the environmental factors, the floristic succession and the faunistic recruitment ~.

 

Central for the future in the restoration of mangroves is the possibility of executing a map-based regeneration programme that integrates the results of the different studies.

 

Concentrating on the present research, we can separate the objectives in two series.  In a first series of objectives the research addresses the following questions for two study areas in both Kenya (Gazi Bay and Mida Creek) and Sri Lanka (Unawatuna-Galle and Pambala-Chilaw) :

 

·    What is the spatial vegetation structure of the mangroves tu ?

Answering this question gives a basic description of the study areas in terms of physiognomy and floristic composition on a spatial scale.  Using phytosociological approaches, completed with various measurements such as tree diameter, a fairly accurate description of the mangrove is generated.  This is done for adult, young and juvenile individuals in several parallel respectively orthogonal transects.  Through the use of geographic information systems (GIS) the vegetation structure  is clearly visualised, and transect data are overlayed.  Combining the qualitative GIS-output with correlation analysis and the use of ordination methods such as Detrended Correspondence Analysis (DCA) and Canonical Correspondence Analysis (CCA), the link between the adult, the young and the juvenile vegetation structure may be recognized.  (Chapter VI, VII, VIII).

 

·    How has the mangrove area evolved v ?

Sequential aerial photography, once more through use of GIS  technology, yields information on the disturbance of the study areas in the first place.  One of the objectives is to compare the vegetation structure and its dynamics  in an undisturbed and in a disturbed mangrove forest.  (Chapter VI, VII).

 

·    What is the mangrove regeneration potential uxyz ?

Investigation of the juvenile abundance both on the parental tree and on the mangrove floor, for instance by monitoring juvenile rooting , produces insight in the reproductive phenology and the successful establishment, hence in the regeneration potential.  (Chapter VIII, XII).

 

·    To which extent propagule predators constrain regeneration w{  ?

Counts of burrows and/or individuals of propagule predators outlines the distribution of the predators and allows comparison with the different vegetation structures.  Through plantations and preference experiments the predation intensity is examined as one of the regenerative constraints.  (Chapter IX, X, XI).

 

This research outline has the overall objective to purvey an approach to assess the mangrove ecosystem in its regeneration potential in a reliable way, and conclude whether human interference through artificial regeneration is necessary to guarantee the survival of a particular mangrove forest.  The study of mangrove juvenile predators, as they often constitute a regeneration problem, is therefore inherent to this research.  The realisation of this overall objective requires a full integration of the questions addressed above, which subsequently require intensive field work in an area relevant in general to the understanding of human interference in mangroves and specifically to the research objectives of this thesis.

 

A second series of objectives deals with subject areas that originated during the research tasks of the first series of objectives, either as interesting additional cases or as problem-solving for complex situations encountered :

 

·    What is the floral composition and distribution of mangroves in Sri Lanka ? (in support of tuv)

Doubt arose as to which species were present in the Sri Lankan lagoons in which the above research was carried out, as well as in neighbouring lagoons, and at the same time publications of mangrove species lists for Sri Lanka reported data conflicting with the field reality.  An in-depth field inventory for the South-West coast was executed for that purpose.  (Chapter III).

 

·    Which are the features that can be recognised from airborne remote sensing imagery of mangrove areas ?  (in support of tv)

In the framework of the identification of mangroves on aerial photographs, the question arose as to whether a ‘compromise interpretation key’ could be made, a key that resulted from a compromise in image attributes at species or genus level over the various study sites.  (Chapter IV, V).

 

·    What is the vegetation structure of a mangrove at its biogeographical limit (comparative to tu) ?

Having seen the diversity in vegetation structures in countries located centrally (from a latitudinal point of view) in the distribution range, the vegetation structure at a biogeographical limit was questioned.  A preliminary analysis of the northernmost mangrove vegetation of the West-African coast was done (Parc National du Banc d’Arguin, Mauritania).  (Chapter XIII).

 

·    Which are the problems associated to the use of the Point-Centred Quarter Method (PCQM) for the study of mangrove vegetation (in support of future research on uy}~) ?

Because of field experience the estimates of the tree density and basal area of mangrove vegetation using the established PCQ-Method were revisited and a number of problems to be solved where highlighted.  (Chapter XIV).

 

 

The results of all the above objectives have been valorised as manuscripts engaged in a peer-review process, and are presented in the present dissertation in a logical sequence together with additional annexed material.  It includes an introductory block, a case-study block and a recapitulate block as well as annexed material (numbers again refer to the research framework) :

 

Introductory block

 

Chapter I  :      General Introduction

Chapter II :      Utilization of mangrove word products around Mida Creek (Kenya) amongst subsistence and commercial users.

 

Case-studies

 

Chapter III :    A revision of the floral composition and distribution of mangroves in Sri Lanka (tuv).

Chapter IV :     High resolution vegetation data for mangrove research as obtained from aerial photography (tv) .

Chapter V :      A note on the identification of mangroves from aerial photography  in Kenya and Sri Lanka (tv).

Chapter VI :     Four decade vegetation dynamics in Sri Lankan mangroves as detected from sequential aerial photography : a case study in Galle  (tuv).

Chapter VII : Two decade vegetation dynamics in Kenyan mangroves as detected from sequential aerial photography : a case study in Gazi Bay (tuv).

Chapter VIII : An ordination study to view past, present and future vegetation structure dynamics in disturbed and undisturbed mangroves forests in Kenya and Sri Lanka (tuvxyz).

Appendix I :      Field keys for Kenyan Mangrove Crabs (w).

Chapter IX :     An explorative study on grapsid crab zonation in mangrove forests in Kenya (w).

Chapter X :      Propagule predators in Kenyan mangroves and their possible effect on regeneration (w{).

Chapter XI :     Propagule predation in Sri Lankan mangroves and its effect on vegetation structure (w{).

Chapter XII : Regeneration status of mangroves under natural and nursery conditions in Galle and Pambala, Sri Lanka (yz).

Chapter XIII : Are the northernmost mangroves of West Africa viable ? - a case study in the Parc National du Banc d’Arguin, Mauritania - (comparative to uv)

Chapter XIV : Empirical estimate of the reliability of the use of the Point-Centred Quarter Method (PCQM) in mangrove forests (in support of future research on uy}~).

 

Recapitulate block

 

Chapter XV :    General discussion

Epilogue

 

References

 

See Bibliography

 


PhD Table of Contents


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