Inter-annual variation of species composition of fallow vegetation in semi-arid Niger

To monitor inter-annual changes of species composition, ten permanent plots were investigated on six fallow sites on different soil types in Niger, West Africa, from 1994 to 1998. Based on the behaviour of species occurrence, four groups of plant species were distinguished for each site: (i) a group of species that occurred in one of the years but was absent in others, (ii) a group of species that occurred in all years but changed enormously in abundance from year to year, (iii) a group of species of constant occurrence over the years and (iv) a group of species that could not be attributed to any of the other groups because they either occurred in too small numbers or showed a contradictory behaviour between the different sites. In total, 43 species, most of them restricted to one or two sites, showed inter-annual fluctuations in occurrence. This constitutes a considerable problem for vegetation studies in semi-arid environments, because for phyto-sociological approaches as well as for the analysis of indicator species, species that are abundant in some years but absent in others cannot be used as character species or differential species for the distinction of plant communities. For this purpose we identified five groups of species, that, in relation to soil type and grazing intensity, can be used as indicator, character or differential species groups for vegetation studies in the Sahel.