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
Citation
MSc.Publisher
School of Biological Sciences, University of Nairobi
Description
Degree of Master of Science in Botany ( Plant Ecology)