Estimation of belowground biomass in woody ecosystems is important because of its relevance to nutrient turnover and the potential to store carbon. Most studies on mangroves biomass have concentrated on standing biomass with very little on belowground biomass; this is particularly true for the eastern Africa region. The present study was conducted at Gazi bay in southern coast of Kenya. The main objective was to determine belowground biomass of mangroves in natural and replanted plantations of Rhizophora mucronata, Avicennia marina and Sonneratia alba. The study in addition investigated horizontal and vertical root distribution. Sampling was carried out in 10 m x 10 m plots where cores (65 cm length and 15.6 cm diameter) were made at the parent root base, between and away from the stem as far as the roots from individual stem could possibly extend based on the sampled tree canopy diameter. In R. mucronata below-ground biomass from different stands ranged between 4.9 to 37.7 t ha-1, S. alba 50.8 to 87.3 t ha-1 and A. marina 28.7 to 47.6 t ha-1. R. mucronata showed a clear effect of age with the amount of biomass increasing with age. Vertical root distribution showed high root densities in the top 20 cm layer decreasing gradually with depth in all species under study. Fine root «5 mm) density ranged between 20 to 52.8 % of the total live root biomass among all species. The information generated is useful in complimenting previous studies on above-ground biomass In Gazi bay and therefore contribute to determination of the potential amount of carbon sequestered by mangroves total biomass within the entire Gazi bay hence improving on forest management.

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