Ecology and Restoration of Mangrove Systems in Kenya.

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%. 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. Chapter 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. 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. 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. In open canopy areas, the less preferred Ceriops seedlings are regenerating at the expense of the removed Rhizophora trees. 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. The stand volume ranged from 6.85 m super(3)/ha to 710.0 m super(3)/ha. The average stand volume was 145.88 m super(3)/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 Chapter 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 super(-1) 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.