Carbon stocks and cover change in the mangrove forest of Mtwapa creek, Kenya

Abstract

Most ecological assessments often focus on one ecosystem without investigating the integrity of contiguous ecosystems and implications thereof on the habitat of interest. From a conservation perspective, ignored ex situ processes may counteract management interventions in an ecosystem of interest if the former aren’t taken into account. This study sought to assess land-use cover change and its potential relationship or impact on the integrity of contiguous mangroves downstream. Carbon stocks data was collected from 36 systematically established 10 x 10 plots set along stratified transects. Biomass carbon was estimated based on general allometric equations applicable globally and wood densities derived within WIO region. Soil carbon estimation was done using soil samples extracted from the 36 plots. Loss on ignition procedure was used to get %SOM then calorimetric equation from adjacent creek was applied to get soil organic carbon. Mangrove cover and land use changes within Mtwapa system in Kenya was assessed using multi-temporal medium resolution Landsat TM (1992) and SPOT images taken in 2000 and 2009. Maximum likelihood classification method was used in ERDAS 9.1 and ArcGIS 10.0 softwares. Land-cover changes around the creek from 1990 to 2009 revealed a high rate of upland deforestation (3.85% yr-1) and an increase in agricultural land (13.9% yr-1). Between 1992 and 2009 the mangrove forest lost 21% of the cover, translating to 1.2% cover loss per annum which fell within the global mean of 1 – 2%. The stocking rates of mangroves in Mtwapa were estimated at 2870±295 stems/ha. The mean biomass carbon for the study area was 49.46±8.49 Mg C ha-1, with no significant variation between sites (p>0.5). Mean SOC of the study area was 196.09±19.31Mg C ha-1 giving a total ecosystem carbon of 245.54±20.95Mg C ha-1. This was quite low compared to in-country and off-shore carbon stocks and is likely due to poor forest structure in Mtwapa creek associated with anthropogenic disturbance as noted by high stump count 2,425±423 stumps/ha. There was a highly positive correlation between land use cover change (agricultural expansion – R2 = 0.70) and mangrove cover Although these mangroves recorded high stocking densities, high degradation rates and observed sedimentation due to poor land-use practices upstream have led to poor stand structure hence low carbon stocks. A landscape approach which combines sound land husbandry upstream and mangrove conservation is recommended.