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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. [Copyright &y& Elsevier]

Countries of semi-arid West Africa are experiencing growing populations, expansion of cultivated land and intensification of crop and livestock production an ever-increasing burden on the region''s limited natural resources, consequently increasing degradation rates. A broad range of technologies combating degradation currently exist. This paper presents and discusses both traditional techniques as well as modern techniques which are derived from the traditional ones. Various methods of mulching and application of manure or mineral fertilizers are used to maintain or increase soil fertility. The use of mulch as well as various methods of integrating shrubs, trees and herbaceous vegetation into the cultivation systems are the technologies currently used to decrease the effects of wind erosion. Similarly, water erosion can be prevented with lines of stones, mulch and grasses. The most promising methods which impede decreasing natural vegetation are promotion of natural regeneration and to some extent the implementation of agricultural parks. In contrast, community-based management of grazing land or forested areas are difficult to establish.Most conservation strategies are limited by the availability of mulch, fertilizer or manure to fertilize fields or to protect sufficient land areas from wind and water erosion. Household constraints of individual farmers also play a crucial role in increasing such protection. Great efforts are needed to distribute the knowledge of the various conservation strategies throughout the different regions of semi-arid West Africa, and to develop new technologies, preferably with farmers'' participation, to enable adoption. As degradation continues and populations increase, this must be accomplished in the near future in order to hinder devastation of land resources. [Copyright &y& Elsevier]