Laboratory of General Botany and Nature Management, VU University, 1081 HV Amsterdam, THE NETHERLANDS.
This Ph.D. dissertation marks the climax o f 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 o f 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.
Chapter 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).
Chapter 2 provides a description o f 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 o f 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 o f 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 o f the coastal system. In Chapter 5, assessment is made o f 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 of 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.