Artificial regeneration and sustainable yield management of mangrove forests at Gazi bay, Kenya

Abstract

Mangrove forests in Kenya are estimated to cover about 50,000 - 60,000 ha. However non-sustainable utilization, over-exploitation of resources and conversion to other land uses principally for fish pond, salt pans, infrustructure development are drastically removing this resource base at a highly alarming rate. Loss of mangrove forests in turn is affecting the local economy as indicated by shortage of firewood, building poles, decreasing fishery resources, destruction of corals and exposure of human settlements to tidal waves. Conservation alone is not enough. The damage can be overcome by rehabilitation and reafforestation of mangrove areas. A mangrove reafforestation project to rehabilitate degraded areas, restock denuded mudflats and transform disturbed forests into uniform stands of higher productivity was launched in October 1991 at Gazi bay. Basically three artificial regeneration techniques were employed: (a) use of propagules (seeds), (b) use of saplings (less than 1.0 m height) and (c) use of 'small trees' up-to 2.0m height. More than 7,000 'propagules, saplings and 'small trees' of Rhizophora mucronata, Ceriops tagal, Bruguiera gymnorrhiza, Avicennia marina, Sonneratia alba, Xylocarpus granatum and Heritiera littoralis wer~ planted/transplanted at different heights along the intertidal complex, and monitored-for their growth at 14 days, 1,2,3 or 4 months interval, for more than a year, depending 01) the experiment. Although there was no single technique that could be judged as best for all species and that no common habitat site was suitable for all species, successful reafforestation proved to be largely modified by: (a) a planting site with little or no wave action against the shore to dislodge plantings and (b) proper elevation within the intertidal zone. Survival rate of the planted propagules and saplings after 12 months varied between 10% in areas heavily exposed to wave action and more than 85% in well protected areas. S. alba growing at the most seaward plots, showed the highest growth rate among all the transplanted saplings, with a maximum annual diameter increment of 1.9em and height increment of 1.18 m. In almost all the observed parameters C. tagal showed the lowest growth rate. Some extensive, fast growing mangrove species e.g. S. alba, A marina, X granatum and R. mucronata (investments returning high rates of 'interests') can sustainably be planted for fuelwood or timber as well as supplying fish and wildlife to nearby human population. For slower growing species e.g. C. tagai and H littoralis exploitation must be slower. Beside field and nursery experiments, air-layering of L. racemosa, S. alba and X granatum is also mentioned as a promising technique of providing stock plants for transplanting without removing mangroves from source area. Rooting success was highest in S. alba followed by L. racemosa and X granatum. There is a strong argument that increasingly we should recognize the changing demands of the society and our long term dependence on practices that ensure minimum modification of the environment with maximum returns on a sustainable basis.