Thèses sur le sujet « Biology of mangrove »

like vivipary and the hydrochorous dispersal of propagules become an absolute

necessity. As propagule dispersal and early growth allow for the replenishment of

existing stands and colonization of new habitats, many authors recognize the

importance of these stages in structuring mangrove populations and communities.

However, when it comes to the actual propagule dispersal and recruitment

mechanisms, there is an apparent lacuna in the current understanding of

mangrove ecology. The period between the mature propagule falling from the

parental mangrove tree and the early growth of the established seedling, under

various possible circumstances, remains in the dark. In this study we focus on this

particular period by investigating both the places where these propagules end up

as the pathways their dispersal units follow. And we go one step further.

Mangrove forests are being destroyed worldwide at a threatening pace despite

their tremendous asset to coastal human communities and associated biological

species. The effect of human-induced (cutting and mangrove conversion to

aquaculture ponds) as well as indirectly and/or ‘naturally’ evolving disturbances

(sea level rise) on propagule hydrochory occupies an important place in this study.

Dispersal of water-buoyant propagules of the family Rhizophoraceae and

Acanthaceae (now including the Avicenniaceae) was studied in Gazi Bay (Kenya),

Galle and the Pambala-Chilaw Lagoon Complex (Sri Lanka). The study sites

differ both in tidal regime and vegetation structure, covering an interesting variety

of ecological settings to examine propagule dispersal. Field data and experiments

ranging from micro/ mesotopographical measurements and successive propagule

counts to hydrodynamic and propagule dispersal experiments were collected or

executed in situ.

Two main methodological approaches were employed. Firstly, the question on

mechanisms of propagule recruitment was addressed by statistically investigating

the effect of microtopography, top soil texture and above-ground-root complexes on

the stranding and self-planting of propagules (Chapter 2&3). Afterwards,

suitability maps were created using Geographical Information Systems (GIS) to

assess whether a particular mangrove stand has the ability to succesfully

rejuvenate. Furthermore, the effect of degradation (tree cutting) (Chapter 2&3),

sea level rise (Chapter 2&4) and microtopography-altering burrowing activities of

the mangrove mud lobster Thalassina anomala (Chapter 3), was incoporated in the

GIS-analyses. Secondly, the combined set-up of hydrodynamic modelling and

ecological dispersal modelling was developed to simulate propagule dispersal

pathways influenced by dispersal vectors (tidal flow, fresh water discharge, wind),

trapping agents (retention by vegetation or aerial root complexes) and seed

characteristics (buoyancy, obligated dispersal period) (Chapter 5&6). This type of

approach provided the possibility to explore propagule dispersal within its

ecological context, but was also applied to an implication of shrimp pond area

restoration (Pambala-Chilaw Lagoon Complex, Sri Lanka) (Chapter 5) and to

evaluate changes in propagule dispersal when sea level rises (Gazi Bay, Kenya)

(Chapter 6).

The main findings regarding propagule recruitment indicate that propagules are

not distributed equally or randomly within a mangrove stand, yet species-specific

distribution for anchorage occurs. Characteristics of the environment

(microtopography, top soil texture and above-ground root complex) influence

propagule recruitment in a way that complex root systems (e.g. pencil roots and

prop roots) facilitate the entanglement of dispersal units and a more compact soil

texture (like clay and silt) and a predominant flat topography creates suitable

areas for stranding and self-planting of propagules. This combines effects of

existing vegetation and abiotic factors on mangrove propagule establishment.

Since propagule dispersal is not solely determined by species-specific propagule

characteristics (e.g. buoyancy, longevity, etc.), I emphasize that propagule sorting

by hydrochory has to be viewed within its ecological context. Propagule retention

by vegetation and wind as a dispersal vector, deserve a prominent role in studies

on propagule dispersal. The significance of dense vegetation obstructing long

distance dispersal (LDD in its definition of this work), mainly in inner mangrove

zones, supports our main finding that propagule dispersal is largely a short

distance phenomenon. ‘Largely’ is here understood as quantitatively, not

excluding epic colonization events of rare but important nature.

In accordance with the Tidal Sorting Hypothesis (TSH) of Rabinowitz (1978a),

smaller, oval-shaped propagules were found to disperse over larger distances than

bigger, torpedo-shaped propagules. We can however not fully support the TSH

because (1) these differences are no longer valid when comparing between torpedoshaped

propagules of different sizes and (2) propagule dispersal is not always

directed towards areas more inland, but can be strongly concentrated towards the

edges of lagoons and channels

Anthropogenic pressure on mangrove ecosystems, more specifically clear-felling or

mangrove conversion to aquaculture ponds, imposes limitations on propagule

recruitment due to reduced propagule availability and a decrease in suitable

stranding areas where the architecture of certain root complexes, like prop roots

and pencil roots, function as propagule traps. These types of pressure appear to

have more severe consequences on propagule dispersal than the effect of sea level

rise on mangroves. Mangrove forests, which are not situated in an obviously

vulnerable setting, can be resilient to a relative rise in sea level if a landward shift

of vegetation assemblages and successful early colonization is not obstructed by

human-induced pressures. Also, and this renders mangrove forests vulnerable in

spite of their intrinsic resilience, when the ‘capital’ of forest is severely reduced or

impoverished as happens extensively worldwide, the ‘interest’ on this capital,

understood as propagule availability, delivery and trapping, will not allow them to

efficiently cope with sea level rise, putting sustainability of mangrove ecosystem

services and goods at risk.

In a larger framework of mangrove vegetation dynamics, knowledge on propagule

dispersal will benefit management strategies for the conservation of mangroves

worldwide, besides its fundamental interest to fully fathom the ecology of this

particular marine-terrestrial ecotone formation.
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Chapters 3 and 6 describe the structural attributes and regeneration status of the peri-urban mangroves of Tudor creek. Based on species importance values the dominant mangrove species were Rhizophora mucronata Lam. (Rhizophoraceae) and Avicennia marina (Forssk.) Vierh. (Acanthaceae)&61607; Lumnitzera racemosa Willd. (Combretaceae), reported in an earlier floristic survey, was not encountered. Tree density varied between 1,264 trees ha–1 and 1,301 trees ha–1, which are within the range of values reported for similar forests globally. However, the size-class structure showed the numerical dominance of small trees over larger trees. The spatial distribution pattern of adults and juveniles varied greatly between sites had a close to uniform pattern (Morisita‟s Index Iδ << 1) for adult trees, but a tendency for clustered distribution (Iδ >> 1) for juveniles. This pattern of distribution is not expected for a healthy forest. The distribution of regenerating seedlings was mainly impacted by canopy gap sizes. These chapters shows that unmanaged but exploited peri-urban mangroves are structurally degraded, having enlarged canopy gaps, characterised by spatial and temporal heterogeneity in edaphic conditions that influences regeneration. This enlarged gaps and edaphic heterogeneity imposes longer periods for canopy closure by lowering regeneration and promoting mortality of seedlings. Larger gaps (> 60m²) had lower regeneration levels and intermediate gaps (20-50m²) had adequate regeneration. The occurrence of R. mucronata seedlings and saplings in the understorey under all cover types and inundation confers advantages to this species under the current disturbance regime. Disturbances include sewage pollution, unregulated harvesting and siltation. The current status of the forest is reminiscent of a recovery phase, a multiphase species succession stage, after a major disturbance event, accompanied by recurrent anthropogenic pressure. This study shows that species composition and thus recovery of a mangrove forest after disturbance depends in part on the balance between subsequent large-scale natural and recurrent small-scale human disturbances.

Chapter 4 assesses the human dependence on the peri-urban mangrove. Through questionnaires and field surveys, the study demonstrates the challenges of questionnaire surveys targeting respondents involved in exploitation of sensitive resources. This potentially limits if not inhibit information flow. Firewood is the ubiquitous form of mangrove wood utilisation, exploited at both subsistence and commercial scales. Forest assessments indicate the lack of preferred or specific harvesting sites, with R. mucronata being the most harvested, probably due to its distribution range and ease of access. Sewage pollution was viewed with mixed feelings. Many appreciate the nutrient enrichment of the sewage rather than the filtration capacity of mangroves, resulting in the usage of sewage for irrigating small plots of subsistence farms. The study shows that resource exploitation is intense in an urban setting due to an economic drive and an increasing demand. In the rural setting on the other hand, utilisation included both subsistence and commercial charcoal production. This has promoted more efficient, destructive and unsustainable exploitation levels. Associated benefits of these activities grossly under-value the ecosystem goods and services in addition to degrading the ecosystem. Our observations, consistent with other studies, shows that management of mangroves for wood extraction in urban areas may not be a viable and/or sustainable option, as it conflicts with the essential „filtration‟ and „habitat provisioning‟ functions and services of mangrove ecosystems. These functions and services are increasingly important in a “diminishing” urban environment. This arises out of the lack of adequate alternatives and conflicting interests in growing urban areas. It is recommended that „adaptive‟ and „participatory management’ based on multiple uses and users, with specific legislative, education and institutional interventions. Integrating local ecological knowledge, may further expedite the formulation of sustainable management plans for peri-urban mangroves. Chapter 5 presents insights on the productivity of an under-valued, over-exploited and sewage polluted peri-urban mangrove through litter fall studies on three common mangrove species, R. mucronata, A. marina and S. alba. The study covers a period of two years. The mean annual litter fall was estimated at 12 ± 3 t ha-1yr-1 for the whole stand, which is within the range of values reported for similar forests occupying the same latitudinal range. Litter fall, in both content and quantity was highly seasonal, with high rates occurring in the dry North Easterly Monsoon (NEM) season, January-April (ca. 5 ± 1 g DW m-2 day-1) and lower rates in the cool and wet South Easterly Monsoon (SEM) season, June-October (ca. 3 ± 0.5 g DW m-2 day-1). R. mucronata recorded the highest annual rate of 15 ± 3 t ha-1yr-1. No significant differences in litter fall rates were observed between A. marina and S. alba, (11 ± 3 and 10 ± 5 t ha-1yr-1 respectively). Sewage exposure levels did not appear to influence litter production rates despite higher nutrient levels in completely exposed sites. δ15N varied with species; A. marina (6.48 ± 0.03‰) and S. alba (6.76 ± 0.24‰) having higher composition than R. mucronata (3.88 ± 0.64‰). The leaf C:N ratio strongly correlated with elevated leaf δ15N signature. Higher C:N ratio for R. mucronata corresponding with lower leaf δ15N (3.88 ± 0.64‰) signature, and lower C:N ratio for A. marina and S. alba (6.48 ± 0.03‰ and 6.76 ± 0.24‰ respectively) corresponding with higher δ15N signature. This reflects species specific response to raw sewage exposure. This implies the forest has a more open N cycle, favouring δ15N accumulation within the system. This study shows that sewage exposure does not necessarily translate into elevated productivity in mangroves, but may alter litter nitrate content depending on species, possibly altering the decay of litter and nutrient cycling.

Chapter 7 presents a synthesis linking the findings to possible implications on the general status of the mangrove ecosystem. The major disturbances observed for the peri-urban mangroves of Tudor creek include (i) domestic sewage pollution, (ii) recurrent unregulated harvesting; and (iii) recurrent annual siltation during the rainy season. Our observations indicate that

(i) Raw domestic sewage pollution may not be harmful to the mangrove vegetation, but may affect edaphic conditions through nutrients enrichment. Sewage pollution effects, though not qualitatively proven in our study, enhance growth of mangrove trees. This is due to increase in amounts of nutrients available for biomass formation, observed as leaf nitrates resorption efficiencies. However, the raw domestic sewage is reported to alter the general healthy decomposing aerobic-anaerobic mangrove system to a complete anaerobic system. This tends to lower efficiencies in nutrient cycling, and cause accumulation of nutrients in the sediments. Observations within the same site and other East African mangroves (under the PUMPSEA project) indicate negative effects of sewage on the sediment cyanobacterial diversity, with an increase in microalgal abundance. Furthermore, within Mikindani (a sewage impacted site in Tudor creek), high rates of sediment ΣCO2 production indicate a system under stress due to the presence of easily degradable organic matter. (ii) Un-regulated harvesting creates and enlarges canopy gaps, lowering availability of seed bearing trees, altering species composition and stem size distribution due to its selective nature, and lowers regeneration under the enlarged canopy gaps. This strongly lowers recovery rates after major disturbances. (iii) Siltation stands out as a major cause of degradation. Siltation is extrinsic in nature, a result of poor land use practices. This probably makes it a major issue of concern due to its impact on regeneration. A major siltation event, associated with the 1997-1998 ENSO, is widely identified as a cause of enlarged canopy gaps. Little recovery has occurred 10 years after the event due to recurrent anthropogenic pressure. The combined effects of these factors have important implications on growth, productivity and recovery of the mangrove ecosystem. The effects include shifts or changes in mangrove tree species distribution. This has lowered the system functional diversity and response diversity, and therefore ecosystem resilience - viability of the ecosystem. It is recommended that integrated adaptive management, based on sound knowledge of the system is the recommended approach. The participation of stakeholders (government institutions, the private sector and local communities) is crucial for managing peri-urban mangroves for sustainability. Not intervening may only result in a worst case scenario. Especially with the current global financial crisis, more locals will turn to „cheap‟ mangrove firewood.

En vue de l‟augmentation de l‟urbanisation dans les multiples écosystèmes du monde, les mangroves n'étant pas une exception, on observe des obstacles à la viabilité de ces écosystèmes, c.-à-d. leur capacité inhérente de se développer, de s‟étendre ou de récupérer après des perturbations sous des conditions urbaines. Nous avons adapté une approche systémique pour établir la viabilité des mangroves périurbaines de la baie de Tudor à Mombasa, Kenya. Trois aspects importants des mangroves périurbaines sont évalués. Cela inclus (i) les aspects structurels (structure de la végétation et la régénération), (ii) aspects fonctionnels (productivité) et (iii) aspects humains (socio-économiques). Les Chapitres 1 et 2 introduisent l'étude, en exposant brièvement les objectifs et le domaine d'étude. Les chapitres présentent un panorama des adaptations physiologiques des palétuviers et des attributs écologiques qui rendent l‟écosystème de la mangrove unique et hautement adaptable à un environnement intertidal rigoureux et dynamique. Les chapitres suivants exposent l‟étendue et le statut des mangroves au Kenya, leur importance socio-économique et la législation qui cadre leur gestion et conservation.

Les chapitres 3 et 6 décrivent les caractéristiques structurelles et les étapes de la régénération des palétuviers périurbains de la baie de Tudor. Par ordre d'importance des espèces, les palétuviers les plus souvent rencontrés étaient Rhizophora mucronata (Rhizophoraceae) et Avicennia marina (Forssk.) Vierh. (Acanthaceae)&61607; Lumnitzera racemosa Willd. (Combretaceae) n'a pas été rencontrée lors d‟un voyage floristique précédent. La densité d'arbre varie entre 1.264 arbres ha-1 et 1.301 arbres ha-1, ce qui correspond aux moyennes rapportées pour des forêts comparables dans le monde. Cependant, la structure des classes de taille montre une dominance numérique des palétuviers moins développés par rapport aux plus développés. Le pattern de distribution spatiale des adultes et des juvéniles varie considérablement entre les sites et montre presque un modèle uniforme (l'index de Morisita; Iδ << 1) pour les individus adultes. En revanche, pour les juvéniles on retrouve une tendance de distribution groupée (Iδ >> 1). Ce modèle de distribution n'était pas attendu pour une forêt non altérée. La distribution de la régénération des pousses est principalement dictée par la taille des gaps. Ce chapitre montre que les mangroves périurbaines, exploitées et non aménagées, sont structurellement dégradées, ayant des larges gaps caractérisés par une hétérogénéité des conditions édaphiques qui influencent la régénération. Quant aux gaps les plus larges, on retrouve un taux de régénération plus faible. Les gaps élargis et l‟hétérogénéité édaphique imposent de plus longues périodes de fermeture des canopées, en diminuant la régénération et promouvant la mortalité des pousses. Les gaps plus grands (> 60m2) ont des niveaux de régénération plus bas et les gaps intermédiaires (20-50m2) ont une régénération adéquate. L‟occurrence de pousses et de juvéniles de R. mucronata dominent le niveau inférieur de tous les types de couvertures végétales et d‟inondation. Cela confère un avantage à cette espèce sous le régime de perturbation actuel. Les perturbations incluent les eaux usées non traitées, les récoltes non-réglementées et l‟envasement. L'état actuel de la forêt est une phase de récupération, un stade de succession d‟espèces en multiples phases, suite à un événement majeur de perturbation, accompagné d‟une pression anthropogénique récurrente. Cette étude démontre que la composition d‟espèces et donc de la récupération de la forêt de mangrove après une perturbation dépend en partie de l'équilibre entre les perturbations naturelles à grande échelle et humaines à moindre échelle.

Le chapitre 4 évalue la dépendance humaine à l'égard de la mangrove périurbaine. Par des questionnaires et des travaux de terrains, on a constaté que l‟utilisation des questionnaires est délicate. En effet, ceux-ci ciblent des personnes impliquées dans l'exploitation des ressources sensibles. Ceci limite potentiellement sinon empêche le flux de l'information. Le bois de feu est la forme d‟utilisation la plus répandue du bois de mangrove, utilisé dans le cadre de la subsistance et du commerce. Les résultats indiquent qu‟il n‟y a pas de sites de récolte préférés. R. mucronata est l‟espèce la plus récoltée, probablement en raison de sa distribution et facilité d‟accès. La pollution par les eaux usées a appréciation mixte. Certains l‟apprécient pour l‟irrigation de leurs cultures de subsistance, malgré la capacité de filtration des mangroves. L‟étude montre que l‟exploitation des ressources est intense, dans un cadre urbain, dû à une conduite économique et une demande croissante. Par contre, dans le cadre rural, l‟utilisation inclus la production de charbon de subsistance ainsi que commercial. La limitation des moyens de vie dans le cadre rural, couplés à la pauvreté et le besoin d‟énergie domestique bon marché, créent une dépendance des ressources de

mangroves. Ceci a favorisé des niveaux d‟exploitation plus importants, destructeurs et non viables. Les gains associés à ces activités qui dégradent l‟écosystème, sont beaucoup moins importants que les biens et les services offerts par la mangrove. Nos observations, en accord avec d‟autres études, montrent que la gestion de l‟extraction du bois des mangroves dans des zones urbaines n‟est pas une option viable et/ou durable, puisqu‟il entre en conflit avec les fonctions et services essentiels de „filtration‟ et „d’approvisionnement pour l’habitation‟ de la mangrove. Ce phénomène résulte du manque d‟alternatives adéquates et des conflits d‟intérêts des zones urbaines en expansion. Il est recommandé de mettre en place une gestion „adaptative‟ et „participative‟ fondée sur les utilisations et utilisateurs multiples, ainsi qu‟une législation, une éducation et des interventions institutionnelles spécifiques, intégrant les connaissances écologiques locales, afin de faciliter des plans de gestion durable pour les mangroves périurbaines.

Le chapitre 5 présente un aperçu de la productivité de la mangrove périurbaine sous-évaluée, surexploitée et polluée par des eaux usées à travers des études de la litière de trois espèces communes de palétuvier, R. mucronata, A. marina et S. alba. L‟étude couvre une période de deux ans. La chute moyenne annuelle de litière a été estimée à 12 ± 3 t ha-1an-1 pour la totalité du secteur, ce qui correspond aux valeurs de forêts comparables occupant des latitudes similaires. La composition et la quantité de la litière changent fortement avec les saisons. Les taux élevés (ca. 5 ± 1 g DW m2 j-1) s‟observent au cours de la saison sèche (Janvier-Avril), sous le North Easterly Monsoon (NEM).Tandis que les taux inférieurs (ca. 3 ± 0,50 g DW m2 j-1) s‟observent au cours de la saison humide (Juin-Octobre), sous le South Easterly Monsoon (SEM). Il y a des variations significatives entre les espèces. R. mucronata a enregistré le taux annuel le plus élevé 15 ± 3 t ha-1an-1. Par contre, il n‟y a pas de différences significatives entre A. marina et S. alba, (11 ± 3 et 10 ± 5 t ha-1an-1, respectivement). Le niveau de pollution par les eaux usées ne semble pas influencer les taux de production de litière en dépit de la concentration plus élevées des nutriments dans les zones complètement exposées. Le δ15N a varié entre les espèces, A. marina (6,48 ± 0,03‰) et S. alba (6,76 ± 0,24‰) ayant une composition plus élevée que R. mucronata (3,88 ± 0,64‰). Cela implique que la forêt a un cycle de l‟N plus ouvert, favorisant l'accumulation dans le système. Cette étude démontre que l'exposition aux eaux usées ne se traduit pas nécessairement en une productivité élevée des palétuviers, mais peut changer le contenu de nitrogène total des feuilles, probablement en changeant la décomposition de la litière et le cycle des nutriments. Le chapitre 7 présente une synthèse liant les résultats aux implications possibles sur le statut général de l'écosystème des mangroves. Les perturbations principales observées pour les mangroves périurbaines de la baie de Tudor incluent (i) la pollution par les eaux usées domestiques, (ii) la récolte récurrente irrégulière et (iii) l‟envasement annuel pendant la saison des pluies. Nos observations indiquent que: (i) la pollution par les eaux usées non traitées n‟est pas nocive pour les palétuviers, mais peut affecter les conditions édaphiques par l'enrichissement en éléments nutritifs. Les effets de la pollution des eaux usées, quoique non prouvés qualitativement dans notre étude, augmentent la croissance des palétuviers. Ce par une augmentation des quantités de nutriments disponibles pour la formation de biomasse, observée lors de l‟étude de l‟efficacité de résorption des nitrates par les feuilles. Cependant, on rapporte que les eaux usées domestiques non traitées changent le système de décomposition aérobique-anaérobique naturel des mangroves vers un système uniquement anaérobique. Ce qui diminue l‟efficacité du cycle nutritif et engendre l'accumulation des nutriments dans les sédiments. Les observations sur le même site et sur d‟autres mangroves de l‟Afrique de l‟Est (sous le projet PUMPSEA), indiquent des effets négatifs des eaux usées sur la diversité des cyanobactéries du sédiment et une augmentation de l'abondance des micro-algues. En outre, à Mikindani (site affecté par des eaux usées dans la baie de Tudor), les taux élevés de production de ΣCO2 dans le sédiment indiquent un système sous pression (ou sous stress), suite à la présence de la matière organique facilement dégradable. (ii) L‟exploitation non réglementée va créer et agrandir des gaps. En fonction de la nature sélective de l‟exploitation, il y aura une diminution de la disponibilité des arbres produisant des propagules, un changement de la composition des espèces et de la distribution des tailles des arbres. De plus, elle réduit la régénération sous les larges gaps. Cela diminue fortement les taux de rétablissement après d‟importantes perturbations.

(iii) L'envasement est la cause majeure de la dégradation. L'envasement est extrinsèque par nature, parce qu‟il découle de mauvaises pratiques d‟exploitation de la terre. Ce phénomène est probablement le point de concertation majeur, dû à son impact sur la régénération. Un événement d'envasement majeur, lié à l‟ENSO de 1997-1998, est identifié comme étant une cause d‟agrandissement des gaps. Peu de récupération s'est produite lors des dix années qui suivirent l'événement, suite à la pression anthropogène récurrente. Les effets combinés de ces facteurs ont des implications importantes sur la croissance, la productivité et la récupération de l'écosystème des mangroves. Les effets incluent des variations ou des changements de la distribution des espèces de palétuviers. Ceci a diminué la diversité fonctionnelle et la diversité des réponses du système et a donc diminué la résilience de l‟écosystème - la viabilité de l'écosystème. Il est recommandé qu‟une gestion adaptative intégrée, fondée sur la connaissance orale du système, soit la meilleure approche. La participation des décideurs (institutions gouvernementales, secteur privé et communautés locales) est cruciale pour la gestion des mangroves périurbaines. Dans l‟absence d‟intervention, seul le pire scénario peut être envisagé. En particulier avec la crise financière globale actuelle, la population locale se tournera encore plus vers le bois de feu „bon marché‟ de la mangrove.
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Though globally recognised as ecosystems of ecological, biological and economical

remarkable importance, these ecotone formations are characterised by a continuously

increasing anthropization. However, very little studies have been focused on the impact of

various anthropogenic activities on the mangrove vegetation structure.

We have firstly (a) assessed the commercial and subsistence utilization of mangrove

wood products in the Littoral region (Cameroon estuary). Then, we have confronted the

subsistence usages of mangrove wood products in the Southern region (close to the mouth of

the Nyong River and Mpalla village (Kribi)) in comparison with the Littoral region. By doing,

we have compared the local residents‟ perceptions on environmental changes that occurred

within the two regional mangrove forests. Also, we have (c) studied the structural dynamic of

mangrove vegetation neighbouring the Douala city (Cameroon). Always in the vicinity of this

town, we have (d) reconstructed the original structure of largely disturbed mangrove forests.

Moreover, we have (e) map the mangrove structure in a non peri-urban setting located within

the Cameroon estuary. Here, we have finally (f) analysed the spatial distribution of a black

mangrove namely Avicennia germinans (L.) Stearn.

Our results underlined an excessive utilization of mangrove wood products in the

Cameroon estuary. We have showed that the frequency of mangrove harvesting was relatively

fewer in Kribi (Mpalla) and the mouth of the Nyong River. The local people inhabiting these

two localities perceived mangroves as less degraded areas. In contrast, those established

within the Cameroon estuary stated that mangroves were largely disturbed. When combining

the local people statements with our field observations, we recorded that it a complex mix of

causes (e.i. clear-felled corridors, agriculture, sand and gravel extraction, over-harvesting and

anarchic urbanization) that have led to the largely degradation (vegetation and sediment) of

the peri-urban mangroves in Cameroon. A diachronic analysis (1974, 2003, 2009) of their

coverage revealed that over the 35-year period, mangrove had decreases in cover of 53.16%

around Douala. We have also showed that in the peri-urban settings, wood harvesting was

commonly applied on the structurally more complex (highly dense stands neighbouring the

habitations) mangrove forests (Mboussa Essengue) and, in a lesser extent, on the structurally

more developed mangrove stands (fewer dense stands faraway from Douala). On the other

hand, the mapping analysis of the non peri-urban mangroves (distant from Douala) has

revealed that the structure of these intertidal forests was relatively less impacted. In the

Cameroon estuary, we also showed that A. germinans trees were randomly distributed on

almost one-half of the sampling plots and clumped at some scales on the remaining plots.

Accordingly, this species might play a significant role in the recovery process of artificial

gaps found in the non peri-urban areas.

The multi-disciplinary approach employed in this study has allowed a better

understanding of the direct and indirect impacts of anthropogenic activities on the mangrove

vegetation structure in Cameroon. These results constitute a fundamental data base quite

useful for the multi-temporal monitoring of these littoral ecosystems perpetually disturbed.

The application of similar approach in other mangroves facing high anthropogenic pressures

appears important.
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