Abstract:
Mangroves occupy only 0.4% of forested areas globally but are among the most productive
ecosystems on earth. They account for about 11% of the total input of terrestrial carbon into the
oceans. The above ground carbon stock in mangroves in some parts of the World has been
estimated to be as high as 8 kg C m
-2
; with a similar amount reported for below ground
components. Although a lot of research has been done on estimates of mangrove biomass in
Kenya, there is no information on biomass accumulation across the zones. The present study
aimed at determining the forest structure and estimating above and below ground biomass
accumulation in Gazi Bay mangrove forest. Forest structure was determined in the western,
middle and eastern forest blocks of the Gazi Bay mangrove forest while biomass accumulation
studies were done in the western forest block. In-growth cores of 80 cm long × 20 cm wide and
60 cm-depth were used to estimate below ground biomass accumulation. Data on tree height and
stem diameter at breast height (DBH-130) were used to estimate above ground biomass
accumulation. Shoots were tagged for monitoring leaf phenology. Periodic measurements of
environmental variables across four mangrove species zones were done at the beginning,
thereafter every four months for a year. Composition and distribution pattern of natural
regeneration was obtained using the method of linear regeneration sampling (LRS). Among the
soil environment properties investigated, salinity had a significant negative correlation with
above ground biomass accumulation. Comparing the four forest zones, Sonneratia alba had the
highest biomass accumulation rate of 10.5 ± 1.9 t ha-1
yr
-1
. This was followed by Rhizophora
mucronata (8.5 ± 0.8 t ha-1
yr
-1
), Avicennia marina (5.2 ± 1.8 t ha-1
yr
-1
), and Ceriops tagal (2.6
± 1.5 t ha-1
yr
-1
). There were significant differences in above ground and below ground biomass
accumulation across zones (F (3, 8) = 5.42, p = 0.025) and (F (3, 8) = 16.03, p = 0 001) respectively.
Total biomass accumulation was significantly different across zones (F (3, 8) =15.56, p = 0.001).
A root: shoot biomass accumulation ratio of 2:5 was computed for the whole forest. The finding
of this study gives better estimates of mangrove carbon capture and storage which can be used in
negotiations for carbon credits in the evolving carbon market.
