Dynamics of forest-savanna mosaics in north-eastern Ivory Coast from 1954 to 2002

Aim In the transition between the southern Sudanian and northern Guinean zones of West Africa, numerous islands of predominantly semi-deciduous forests are interspersed in extensive savannas. During the past decades, human population and intensity of land use have increased. At the same time, almost nothing is known about the natural dynamics of this widespread forest-savanna mosaic and how they are altered by human activities. This was investigated with respect to past, present and future forest-savanna dynamics, which presumably influence northern Guinean biodiversity. Location The Comoe National Park (CNP) region in north-eastern Ivory Coast. Methods Landscape dynamics and the rates of anthropogenic deforestation and natural reforestation were retrospectively analysed by means of historical and recent aerial photographs and satellite images, directly relating the semi-natural conditions in the CNP to neighbouring, traditionally utilized countryside. Results From 1954 to 1996, the studied forest-savanna pattern proved to be remarkably stable, even with extensive land utilization outside the CNP. The contour towards the surrounding savanna and, thus, the size of 95.4% of 653 forest islands remained unchanged. Unvegetated surfaces also remained remarkably constant during the period from 1967 to 1996. Although dynamics were clearly higher outside the CNP as a result of human activities, the 913 unvegetated surfaces studied showed no general trend of decrease or increase, i.e. there was no response over the study period to an identified climatic shift. Anthropogenic deforestation and subsequent natural reforestation within the contours of the existing forest have occurred to a noteworthy extent only outside the CNP. For the period 1988-2002, the extent of deforestation was greater than the extent of reforestation (40% vs. 14%). By 2002, 62% of the original gallery forest along a 75-km section of the Comoe river had been cleared. Main conclusions With its remarkable overall stability, the landscape pattern inside the CNP has responded with resilience to varying influences (in particular climatic variations) and disturbances (e.g. episodic mass herbivory). A possible natural succession from savanna to forest appears to proceed only very slowly due to the counteracting effects of annual savanna fires and the lower climatic humidity of the area compared to the southern Guinean zone. Consequently, the forests should be considered as habitat islands rather than as habitat fragments. As deforestation outside the CNP has increased considerably, the pre-existing pattern of forest insularization is now becoming overlain in these non-CNP areas by a pattern of forest fragmentation. This will aggravate the ecological and genetic isolation of the undisturbed forests in the CNP, which are among the last remaining natural forests of the entire Guineo-Sudanian transition zone.