Factors influencing the spatial distribution of dry forests in a forest-savanna mosaic in West Africa

Hennenberg Klaus ¹, Goetze Dethardt ¹, Fischer Frauke ², Porembski Stefan ¹

¹ Institute of Biodiversity Research, Department of Botany, University of Rostock;

² Department of Animal Ecology & Tropical Biology, University of Wuerzburg

The forest-savanna mosaic of the Comoé National Park region (CNP, Ivory Coast, transition zone between Guineo-Congolian rain forest and Sudanian woodland) includes dry and wet forest islands as well as gallery forests. A digital elevation model of the investigated area shows that the region is characterised by a shallow relief with slope inclinations normally less than 3%. The more elevated sites show shallower soils with coarser and more skeletal soil material than the slopes and the valleys.

Dry forest islands mostly are located at the more elevated sites in the relief, which can be shown by satellite images and GPS mapping. On the valley slopes, the dry forests adjoin to tree dominated savanna types which are linked with grass savanna or occasionally wet forest types in the temporally flooded valley bottoms.

The following question arises: Why are dry forests restricted to the elevated sites of the hills with less favourable soil conditions, and why does savanna dominate on the slopes?

To answer this question, eight transects of a length of 300m each and a width of 10 (-50)m were installed in the undisturbed area of the CNP. The transects are located at eight independent forest islands along the ecological gradient between forest and savanna. This gradient is characterised by a higher daily maximum of air temperature in the savanna and of relative humidity in the forest. Unexpectedly such clear edge effects are not apparent for the mean values. Tree individuals of all size classes have been mapped and will be controlled regularly. Especially Anogeissus leiocarpus is of crucial importance at the forest border. Composition and density of vegetation and site parameters are being analysed. Investigations of biomass (grasses, herbs and litter) show that grass biomass is clearly correlated to fire events. A modelling approach has been developed in order to test the relevance of different factors influencing the position of dry forests in the investigated area.

Our study is embedded in the project BIOTA (Biodiversity Transect Analysis), funded by the German Federal Ministry of Education and Research (BMBF).