Change-detection in Western Kenya – the Documentation of Fragmentation and Disturbance for Kakamega Forest and Associated Forest Areas by Means of Remotely-sensed Imagery

In order to understand causes and effects of disturbance and fragmentation on flora and fauna, a time series on land cover change is needed as basis for the BIOTA-East Africa project partners working in western Kenya. For 7 time steps over the past 30 years Landsat data were collected for Kakamega Forest and its associated forest areas. Preprocessing involved georeferencing and radiometric corrections. In a first step the time series is evaluated via a threshold analysis distinguishing between “forest” and “non-forest”. Even though a temporally changing pattern of forest losses and replanting is observed, in total no major change in forest-covered area is revealed. Therefore, a supervised multispectral classification is performed distinguishing between classes at the ecosystem level. Ground truthing for the historical imagery is done with the help of maps showing vegetation types or land cover. Actual land cover verification is based on amateur photographs taken from an aeroplane as well as on terrain references. For classification the maximum-likelihood decision rule is applied considering bands 3, 4, 5, 7 plus 7/2 for TM/ETM+ imagery and 1, 2, 3 and 4 for MSS-data, repectively. If available, scenes from both the rainy and dry seasons are made use of. From planned 17 land cover classes 12 can be realized, of which 6 belong to forest formations. A shortcome is that plantation forest of Maesopsis eminii (planted mixed in with other indigenous tree species) cannot be separated. Nevertheless, the classification results form a solid basis for a consistent and detailed evaluation of forest history between 1972 and 2001. Analyses presented include graphs of change in land cover class areas over time as well as such allowing for true change detection with transitions between the different classes.