Structure and Biomass Accumulation of Natural Mangrove Forest at Gazi Bay, Kenya

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.