Species accumulation within land use and tree diameter categories in Burkina Faso, Mali, Niger and Senegal

Although farmers have managed west African parkland savanna systems for 1,000 of years, concerns have been raised about the sustainability of these agro-ecosystems due to human population growth, shortening of fallow periods, droughts, desertification and new orientations towards cash generation away from subsistence farming. We conducted a tree diversity survey in 16 villages from Burkina Faso, Mali, Niger and Senegal, recording total species composition for 300 quadrats (mainly 50 x 20 m(2)) that were randomly sampled from the main landuse categories of parklands of village fields (VF), bush fields (BF), sylvopastoral zone (SP) and forest reserves (FR). About 110 tree species were encountered, including 100 indigenous species. The results from balanced species accumulation curves (based on randomized subsampling of the same number of quadrats from each village) showed that the accumulated number of species was smallest in VF, largest in FR and almost equal in BF and SP, whereas classical (unbalanced) species accumulation curves yielded different results. Although there was a significant within-village reduction in species richness with increasing diameter class [averaging 8.5-13.1 more species in the smallest (< 5 cm) compared to the largest diameter class (> 80 cm)] for the different landuse categories, new species were also encountered in larger diameter classes (2.7-7.2 species). The evidence for tree regeneration problems (including problems in FR) suggest that farmer-managed tree regeneration should be further explored and that advances in domestication and marketing of indigenous tree species may be crucial to tree conservation in parkland systems.