Structure,Biomass and cover change of change of a Peri-urban mangrove forest in Kenya, A case study of Mwache Creek

Abstract

Mangrove forests provide an array of ecosystem goods and services which support the livelihoods of millions of people in the tropics and sub-tropics. However, these unique intertidal forests are threatened by both anthropogenic and natural impacts (more recently climate change), with such pressures being disproportionately high on peri-urban mangroves. The objective of this study was to assess the status and regeneration of Mwache creek mangrove forest (a peri-urban ecosystem in Mombasa) as a function of vegetation structure, biomass and cover change between 1992 and 2009 using high resolution SPOT multispectral images and digitized aerial maps on a Geographical Information Systems’ platfonn. Forest structure and species composition were studied based on stratified transects using the plot method; whereas natural regeneration was analyzed based on Linear Regeneration Sampling. Biomass was derived based on allometric equations that can be used globally for estimation with application of species specific densities. A total of 67 plots measuring l0xl0 m were sampled during this study. The sampled sites included Mwakuzimu, Ngare, Mashazani, Maguzoni and KPA. The mangrove species encoimtered in the study area included Avicennia marina, Bruguiera gymnorrlziza, Ceriops raga], Rhizophora mucronata and Sonneratia alba. There was a significant difference (p<0.05) in stand density in all sites with Mashazani being the lowest with 84i9 stems/ha. In terms of complexity index, Ngare (25.6) was the most structurally developed whereas Mashazani (0.03) was subordinate. The mean tree heights did not vary significantly (p>0.05) among the sites and it ranged from 6.4 i 0.8m in Ngare to 4.6 i 0.7m in Maguzoni. The total live biomass in the creek was 32l.85i56. l0 t ha". In terms of forest cover, from 1992 to 2009 the extent had reduced from 1861.4 ha to l016.9 representing 45.4% reduction. Rhizophora mucronata and C. tagal reduced by 70.63% and 71.97% from the original 978.33ha and 685.46 ha respectively, whereas S. albu increased by ll98.99%. The site with significantly high (p<0.05) rejuvenation was Maguzoni (17,967 juveniles ha“) compared to KPA with the least 817 juveniles ha'1 . This stud indicates Y that these peri-urban mangroves are being degraded at a higher rate of 2.7% per year than the global average of l-2% due to both intense anthropogenic pressure and indirect impacts of climate change. This is concurrent to loss of biomass in the forest. Implementation of management regimes such as sustainable harvesting will reduce these pressures making the mangroves more resilient to climate change impacts and support recovery of the ecosystem.