Structure and natural regeneration of mangroves in areas exposed to different agents of degradation in Kenya

The structure and regeneration of mangrove forests in three coastal areas of Kenya were investigated in this study. This study assessed species diversity, species distribution and regeneration in the three sites. In Mida Creek mangroves, data were collected within a 10 m x 10 m quadrat sample from two stratum of mangrove type: open canopy mangroves and closed canopy mangroves. At Lamu (Manda Island) and Mwache Creek, belt transect measuring 270 x130 m was established running from hinterland towards the seaward cutting across different mangrove zones. In all the three sites, Diameter at Breast Height (DBH) which is 1.3 m above the ground (where the highest prop-roots reach) was measured using a diameter tape. Tree height was measured using a Laser Ace for each mangrove tree to a minimum diameter of 7cm. Saplings and seedlings were enumerated per species within 3 m x 3 m and 1 m x 1 m subplots respectively laid at the center of the main plot for determination of species regeneration. The result demonstrated higher species richness in Manda (seven mangrove species) followed by Mida creek (five species). C. tagal was the most dominant species in Mida creek followed by R. mucronata as demonstrated by their frequency. Whereas, in Manda Island, the importance value index revealed that R. mucronata dominated species followed by A. marina. In Mwache Creek, R. mucronata and A. marina exhibited high importance value index (223.8, and 159.3). No significant difference was observed in DBH (p<0.001; LSD=16.49) and Height (p>0.432; LSD=0.6310) among the three sites. However, a significant difference observed in the stem density (p<0.001; LSD= 2911) and basal area (p<0.001; LSD=38.76) among the sites. Mida had big sized trees compared with Manda and Mwache and hence the mean basal area was high in Mida followed by Mwache and Manda. Seedlings and saplings densities indicated significant different among the sites (p<0.001). Mwache exhibited the highest seedling density (70,000 seedlings ha-1) followed by Manda (46,667 seedlings ha-1) while Mida had the least (21,694 seedlings ha-1). Generally, the mangrove forest in the three sites had different structural formation and species composition which could be attributed to the different agents of degradation i.e. overexploitation, oil spillage and siltation and stochastic weather events associated with climate change. Therefore, understanding the structure and regeneration patterns of the mangroves exposed to different agents of degradation is critical for developing site specific management strategies that enhances and preserves the ecological functionality of mangrove ecosystems.