Tridecadal assessment of mangrove cover and cover change in the trans boundary areas of Kenya and Tanzania between 1986-2018

Abstract

Mangrove forests are among the most threatened ecosystems on earth. Some of these forests transverse national boundaries complicating their management due to differences in governance structures between countries. In order to improve the management of trans-boundary mangroves and associated biodiversity, regular monitoring of status and conditions of these forests is essential. The aim of the study was to create a better understanding of trans-boundary mangrove cover/ change and the current conditions of the ecosystem, with the aim of supporting joint management plans, regional policy formulation and better conservation strategies. The study adopted correlational research design. A systematic random sampling design was used in selection of sampling sites, where 120 plots were selected along the transboundary area. Remotely sensed data was used to estimate mangrove forest cover and analyze changes in forest conditions guided by the following study variables; area coverage of land use/cover, tree density, time, tree diameter at breast height, tree quality, number of stumps, species importance value, transboundary mangroves of Kenya and Tanzania. Image classification and analysis was conducted on Landsat images dating from 1986-2018 sentinel image, using hybrid unsupervised and supervised maximum likelihood classification algorithm. This was complemented by a detailed ground-truthing. Results indicate that the transboundary mangroves cover an estimated area of 11,906ha; 55% of which is in the Kenyan side and the rest in Tanzania. Generally transboundary mangroves recorded a continuous decline of 0.69%/year. Mangrove areas near human settlements within Tanga and Moa in Tanzania and Vanga and Funzi in Kenya, faced a higher degradation level over the last three decades. The Kenyan side of the transboundary area recorded more losses and degradation in comparison to the Tanzanian side. Over the entire study period, Vanga recorded the highest rate of mangrove loss at 27ha per annum followed by Tanga and Funzi at 14.5ha per annum and 12.3ha per annum respectively. Structural data indicated highest importance value of 113% and 100% for Rhizophora mucronata and Ceriops tagal respectively. The vegetation map obtained showed that the forest is dominated by Ceriops and Rhizophora with the mixed stand of Ceriops leading with 17% of the total area. A relationship was observed between other land use/ cover types change and mangrove cover change. Harvesting of mangroves for building poles and energy has contributed to major loss of mangroves in the region. For effective protection and conservation of mangroves along transboundary area of Kenya and Tanzania, the study recommends provision of alternative sources of energy and materials for construction without which mangrove loss and degradation is expected to continue.