Seed dispersal mechanisms and the vegetation of forest islands in a West African forest-savanna mosaic (Comoé National Park, Ivory Coast)

Abstract

The transition zone between forest and savanna is typically characterized by a dynamic patchwork of forest and savanna. We studied the woody plant species composition of 49 forest islands, 18 savanna, and 3 gallery forest plots in the Comoe National Park (Ivory Coast), West Africa's largest savanna reserve. TWINSPAN makes a clear distinction in vegetational composition between these three major habitat types but, nevertheless, more than 50% of the 292 species occur in at least 2 of them. The gallery forest is dominated by Cynometra megalophylla (Caesalpiniaceae), a genus known to dominate humid forests in other afrotropical regions. Ordination reveals four distinct categories of forest islands, (1) humid forests comparable to the gallery forest in their species composition, (2) dry disturbed and (3) dry undisturbed forests and (4) forests formerly inhabited by humans. Disturbed forests harbor more savanna species but also a distinct group of disturbance-tolerant forest species. Compared to other forest species, in this latter group we found an exceptionally high fraction of animal dispersed species (80% vs. 58%), while wind dispersed species or species lacking long distance seed dispersal mechanisms were correspondingly rare. This pattern occurs in spite of the fact that the frequency of wind dispersed species in general increases from dense humid forests to open dry forests and savanna. Species lacking long distance dispersal mechanisms are most abundant among those specialized on humid forests. These observations suggest that the species composition of forest islands is to some extent determined by the seed dispersal abilities of the different species.