Ecology and restoration of mangrove systems in Kenya

This Ph.D. dissertation marks the climax of my academic work that started way back in 1973 when I was only allowed to write on a ground ‘tablet’. Reminiscing my days as a young boy, myths prevented us from collecting or even touching eggs laid by wading birds that nested in the ‘massive’ wetland that bordered my rural home in Kenya. Not until my university years in Nairobi did I come to learn and appreciate the important roles played by wetlands - both ecological, economic and environmental. But it was too late! The ‘massive’ wetland in my rural home, had been reclaimed for agriculture and human settlement. The crane birds that used to perform ritual dances in the wetland were gone! In their place I see buildings, dykes, hamlets and a poorer generation. Where did the birds go? My quest to restore the degraded wetlands started here. My thesis is not about the fascinating wetland birds, but rather about mangroves - ‘forests growing at the edge of tropical and subtropical seas’. In addition to providing a range of products that people need, including building materials, firewood, tannins, fodder and herbal medicine, mangroves are of invaluable local and global ecologic, economic and social importance. Mangroves serve as nursery and feeding sites for many species of fish, mollusks and crustacean. Mangroves also serve as filters for sediments that threaten siltation of coral reefs and help to control water quality (Odum & Heald, 1972; Robertson et al. 1992). Despite the growing recognition of the economic and ecological importance of mangroves, these forests are disappearing fast from the face of the earth. A rate exceeding 1% by area per annum (Robertson & Alongi, 1992). This thesis concerns the assessment of mangrove forests in Kenya in terms of wood resources, and their regeneration potential. The work is divided into 6 chapters. C hapter 1 presents a global picture of mangroves, what they are, their value, threats and efforts being made to address the problems. Mangroves once occupied 75% of the tropical coasts worldwide (McGill, 1959), but anthropogenic pressures have reduced the global range of the forests to less than 50%. Based on remote sensing technology, the current area of mangrove in the world is estimated to be between 180,000 km2 and 200,000 km2 (Spalding et al. 1997). Mangrove forests in Kenya are estimated to occupy about 54,000 ha, 70% of which occurs in Lamu district. There are 9 recorded mangrove species in Kenya. The principal species are Rhizophora mucronata and Ceriops tagal which form more than 70% of the forests. The most important use of mangrove forests in Kenya is as wood for building and heating. The coastal people are largely dependent on mangrove poles for the framework of their houses. Historically mangrove poles were an important export item from the Kenyan coast to the treeless Arab countries (Rawlins, 1957). Over-exploitation led to a ban of mangrove exportation in 1982, a move that affected the coastal economy enormously (Kokwaro, 1985). C h ap te r 2 provides a description of the study area - the Kenyan coast. The coastline runs for approximately 574 km in a NNE and SSE direction, between latitudes 1°40’S and 4°25’S and longitudes 41°34’ E and 39°17’E. The agro-climatological zones along the Kenyan coast differ markedly from the north to the south. The relative humidity is higher in the south than in the north. The ocean current regime also differs from the south to the north, providing nutrient poor water in the south and nutrient rich water in the north (McClanahan, 1988). These differences in climate and ocean currents cause a strong divide between the vegetation types such that the northern mangroves in Lamu are structurally more complex than the southern mangroves in Mida creek. What structural parameters best describe a ‘healthy mangrove forest’? Is the measurement of forest cover enough indicator of guaranteeing a ‘healthy system? What is the minimum number of juveniles required to ensure adequate natural regeneration of the forest after logging? These are among the issues addressed in chapters 3 and 4 of my thesis. Chapter 3 details mangroves of Mida creek, defined in this study as ‘young secondary mangrove stand that is vigorously growing, but subjected to periodic harvest. While we may be contented with the good natural regeneration that has taken place in Mida, close analysis reveals that Mida mangroves are in fact degenerating. What was harvested is not what is coming up. Mangrove harvesting in Kenya proceeds in a selective manner. Rhizophora mucronata is the preferred mangrove species because it produces poles that are hard, tall and straight. The most merchantable pole size is the boriti, with butt diameter range of 11.0 - 13.5 cm. Others are mazio (diameter 7 .5- 11 cm) and pau (5.0 - 7.5 cm). Poles greater than 15.0 cm diameter (banaa) are of less economic value and are therefore left standing in the forest. Excessive removal of boriti and mazio sized poles has created complex mangrove silvicultural problems in Kenya. The overgrown banaa canopy shade out juveniles and young trees and cause them to be crooked as they try to grow in an open space inside the closed forest canopy. Chapter 4 is about the application of remote sensing and GIS technology in mapping the mangrove forests within and adjacent to the Marine Protected Area (MPA) of Kiunga, Lamu. Remote sensing and GIS are increasingly used in mangrove forestry worldwide to assist in gathering and analysing images acquired from aircrafts, satellites and even balloons The notable advantages of using GIS include the ability to store, retrieve and analyse various types of information rapidly and making this information available as required. Thise study revealed the presence of 2.4 x 106 m3 of mangrove wood within and adjacent to Kiunga Marine National Reserve (KMNR), in 16,035.94 ha. The stand volume ranged from 6.85 m3/ha to 710.0 m3/ha. The average stand volume was 145.88 m3/ha, which corresponds to a stocking rate of 1736 stems/ha. Given its high potential productivity and regeneration, mangroves within and adjacent to KMNR have excellent prospects for sustainable exploitation. The management of mangroves as renewable resources poses severe problems in that natural regeneration seems to be insufficient where large-scale operations have taken place. To sustain the yield of these forests there is a need to address both artificial and natural regeneration methods. Artificial mangrove planting in Asia has been promising in solving the problems of limited supply of mangrove products as well as maintaining the overall ecological balance of the coastal system. In C h ap te r 5, assessment is made of the above ground biomass increment of mangrove plantations that were established at Gazi bay in 1991. The above ground biomass of a 5-year old Rhizophora plantation was calculated at 20.25 t dry matter ha for trees with stem diameter greater than 5.0 cm. Finally in Chapter 6, a comparative analysis of mangrove forests along the Kenya coast is provided. Emphasis is given to the mangrove areas where this study was done. The variation ot mangrove forest structure in Kenya occurs due to differences in environmental settings as well as differences in the levels of human pressure. Mangroves north of Tana river are river and tidal dominated systems, with a lower human pressure than mangroves south of the Tana river

Growth rings, growth ring formation and age determination in the mangrove <i>Rhizophora mucronata</i>

Background and Aims The mangrove Rhizophora mucronata has previously been reported to lack annual growth rings, thus barring it from dendrochronological studies. In this study the reported absence of the growth rings was reconsidered and the periodic nature of light and dark brown layers visible on polished stem discs investigated. In addition, the formation of these layers in relation to prevailing environmental conditions, as well as their potential for age determination of the trees, was studied.Methods Trees of known age were collected and a 2.5-year cambial marking experiment was conducted to determine the periodic nature of the visible growth layers.Key Results Annual indistinct growth rings were detected in R. mucronata and are defined by a low vessel density earlywood and a high vessel density latewood. The formation of these growth rings and their periodic nature was independent from site-specific environmental conditions in two forests along the Kenyan coast. However, the periodic nature of the rings was seriously affected by slow growth rates, allowing accurate age determination only in trees with radial growth rates above 0.5 mm year(-1). The onset of the formation of the low vessel density wood coincided with the onset of the long rainy season (April-May) and continues until the end of the short rainy season (November). The high vessel density wood is formed during the dry season (December-March). Age determination of the largest trees collected in the two studied forests revealed the relatively young age of these trees (+/-100 years).Conclusions This study reports, for the first time, the presence of annual growth rings in the mangrove R. mucronata, which offers further potential for dendrochronological and silvicultural applications

Carbon storage in the seagrass meadows of Gazi Bay, Kenya

Vegetated marine habitats are globally important carbon sinks, making a significant contribution towards mitigating climate change, and they provide a wide range of other ecosystem services. However, large gaps in knowledge remain, particularly for seagrass meadows in Africa. The present study estimated biomass and sediment organic carbon (Corg) stocks of four dominant seagrass species in Gazi Bay, Kenya. It compared sediment Corg between seagrass areas in vegetated and un-vegetated ‘controls’, using the naturally patchy occurence of seagrass at this site to test the impacts of seagrass growth on sediment Corg. It also explored relationships between the sediment and above-ground Corg, as well as between the total biomass and above-ground parameters. Sediment Corg was significantly different between species, range: 160.7–233.8 Mg C ha-1 (compared to the global range of 115.3 to 829.2 Mg C ha-1). Vegetated areas in all species had significantly higher sediment Corg compared with un-vegetated controls; the presence of seagrass increased Corg by 4–6 times. Biomass carbon differed significantly between species with means ranging between 4.8–7.1 Mg C ha-1 compared to the global range of 2.5–7.3 Mg C ha-1. To our knowledge, these are among the first results on seagrass sediment Corg to be reported from African seagrass beds; and contribute towards our understanding of the role of seagrass in global carbon dynamics

Mangroves in peril: unprecedented degradation rates of peri-urban mangroves in Kenya

Marine ecosystems are experiencing unprecedented degradation rates higher than any other ecosystem on the planet, which in some instances are up to 4 times those of rainforests. Mangrove ecosystems have especially been impacted by compounded anthropogenic pressures leading to significant cover reductions of between 35 and 50% (equivalent to 1–2% loss pa) for the last half century. The main objective of this study was to test the hypothesis that peri-urban mangroves suffering from compounded and intense pressures may be experiencing higher degradation rates than the global mean (and/or national mean for Kenya) using Mombasa mangroves (comprising Tudor and Mwache creeks) as a case study. Stratified sampling was used to sample along 22 and 10 belt transects in Mwache and Tudor respectively, set to capture stand heterogeneity in terms of species composition and structure in addition to perceived human pressure gradients using proximity to human habitations as a proxy. We acquired SPOT (HRV/ HRVIR/ HRS) images of April 1994, May 2000 and January 2009 and a vector mangrove map of 1992 at a scale of 1:50 000 for cover change and species composition analysis. Results from image classification of the 2009 image had 80.23% overall accuracy and Cohen's kappa of 0.77, thus proving satisfactory for use in this context. Structural data indicate that complexity index (CI) which captures stand structural development was higher in Mwache at 1.80 compared to Tudor at 1.71. From cover change data, Tudor lost 86.9% of the forest between 1992 and 2009, compared to Mwache at 45.4%, representing very high hitherto undocumented degradation rates of 5.1 and 2.7% pa, respectively. These unprecedentedly high degradation rates, which far exceed not only the national mean (for Kenya of 0.7% pa) but the global mean as well, strongly suggest that these mangroves are highly threatened due to compounded pressures. Strengthening of governance regimes through enforcement and compliance to halt illegal wood extraction, improvement of land-use practices upstream to reduce soil erosion, restoration in areas where natural regeneration has been impaired, provision of alternative energy sources/building materials and a complete moratorium on wood extraction especially in Tudor Creek to allow recovery are some of the suggested management interventions

Mangroves facing climate change: landward migration potential in response to projected scenarios of sea level rise

Mangrove forests prominently occupy an intertidal boundary position where the effects of sea level rise will be fast and well visible. This study in East Africa (Gazi Bay, Kenya) addresses the question of whether mangroves can be resilient to a rise in sea level by focusing on their potential to migrate towards landward areas. The combinatory analysis between remote sensing, DGPS-based ground truth and digital terrain models (DTM) unveils how real vegetation assemblages can shift under different projected (minimum (+9 cm), relative (+20 cm), average (+48 cm) and maximum (+88 cm)) scenarios of sea level rise (SLR). Under SLR scenarios up to 48 cm by the year 2100, the landward extension remarkably implies an area increase for each of the dominant mangrove assemblages except for Avicennia marina and Ceriops tagal, both on the landward side. On the one hand, the increase in most species in the first three scenarios, including the socio-economically most important species in this area, Rhizophora mucronata and C. tagal on the seaward side, strongly depends on the colonisation rate of these species. On the other hand, a SLR scenario of +88 cm by the year 2100 indicates that the area flooded only by equinoctial tides strongly decreases due to the topographical settings at the edge of the inhabited area. Consequently, the landward Avicennia-dominated assemblages will further decrease as a formation if they fail to adapt to a more frequent inundation. The topography is site-specific; however non-invadable areas can be typical for many mangrove settings

Biomass and productivity of seagrasses in Africa

There is growing interest in carbon stocks and flows in seagrass ecosystems, but recent global reviews suggest a paucity of studies from Africa. This paper reviews work on seagrass productivity, biomass and sediment carbon in Africa. Most work was conducted in East Africa with a major geographical gap in West Africa. The mean above-ground, below-ground and total biomasses from all studies were 174.4, 474.6 and 514 g DW m-2, respectively with a global range of 461-738 g DW m-2. Mean annual production rate was 913 g DW m-2 yr-1 (global range 816 - 1012 g DW m-2 yr-1). No studies were found giving sediment organic carbon, demonstrating a major gap in seagrass blue carbon work. Given the small numbers of relevant papers and the large geographical areas left undescribed in Africa, any conclusions remain tentative and much remains to be done on seagrass studies in Africa

Sustainable natural resource management must recognise community diversity

Deforestation and overexploitation of mangrove forests are affecting the livelihoods of millions of families that rely on their ecosystem services. Understanding local perceptions about the status and threats to mangroves is therefore crucial in addressing this issue. This research aims to enhance understanding of how sociodemographic factors influence resource use and perceptions of environmental changes through a questionnaire survey (n = 592 households) in five locations in Lamu County, home to 62% of Kenya’s mangroves. The results highlight the variability of mangrove use, ecosystem service recognition, and perceptions and drivers of change across locations, which are influenced by sociodemographic factors such as gender, education, and occupation. Although 89% of respondents reported using mangrove products, only 56% were able to identify mangrove ecosystem services, with those without formal education being less likely to recognize them. Interestingly, 50% of respondents perceived an increase in mangrove cover, contrary to research showing mangrove loss in the area over the last decade. Results show that communities are diverse and perceptions vary between groups, suggesting that implementing uniform management measures may be incomplete or ineffective. Awareness campaigns and capacity-building efforts must be tailored to reduce misperceptions about the state of local resources and to address the specific needs and challenges faced by different groups. Recommendations made here are widely applicable to promote more inclusive and sustainable community engagement in the management of natural resources in developing countries worldwide

Seagrass removal leads to rapid changes in fauna and loss of carbon.

Seagrass habitats are important natural carbon sinks, with an average of ∼14 kg C m−2 buried in their sediments. The fate of this carbon following seagrass removal or damage has major environmental implications but is poorly understood. Using a removal experiment lasting 18 months at Gazi Bay, Kenya, we investigated the impactsof seagrass loss on sediment topography, hydrodynamics, faunal community structure and carbon dynamics. Sediment pins were used to monitor surface elevation. The effects of seagrass removal on water velocity was investigated using Plaster of Paris dissolution. Sediment carbon concentration was measured at the surface and down to 50 cm. Rates of litter decay at three depths in harvested and control treatments were measured using litter bags. Drop samples, cores, and visual counts of faunal mounds and burrows were used to monitor the impact of seagrass removal on the epifaunal and infaunal communities. Whilst control plots showed sediment elevation, harvested plots were eroded (7.6 ± 0.4 and −15.8 ± 0.5mm yr−1 respectively, mean ± 95%CI). Carbon concentration in the surface sediments was significantly reduced with a mean carbon loss of 2.21Mg C ha−1 in the top 5 cm. Because sediment was lost fromharvested plots, with a mean difference in elevation of 3 cm, an additional carbon loss of up to 2.54Mg C ha−1 may have occurred over the 18 months. Seagrass removal had rapid and dramatic impacts on infauna and epifauna. There was a loss of diversity in harvested plots and a shift toward larger bodied, bioturbating species, with a significant increase in mounds and burrows. Buried seagrass litter decomposed significantly faster in the harvested compared with the control plots. Loss of seagrass therefore led to rapid changes in sediment dynamics and chemistry driven in part by significant alterations in the faunal community