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This work, carried out in the pre-Saharan region of Tunisia, was designed to investigate the changing characteristics of sandy steppe during its progressive invasion by wind-borne sand from disturbed neighbouring zones (regressive evolution). In addition, the development of the vegetation in such regions after the disturbance had ceased has been determined (progressive evolution). The method consisted of studying differences in the main perennial species in the presence of mobile sand, in order to establish whether these species show special adaptations to the mobility of the sand and their role in sand fixation. Only two species show ability to tolerate sand accumulation and are thereby able to fix drifting sand. These are Rhanterium sauveolens Desf. and Aristida pungens Desf. The most effective mode of resistance to sand mobility (accumulation and deflation) appears to be a change in the vegetative growth, i.e. the formation of adventitious roots in the accumulated sand and the development of aerial structures at a level corresponding with the new soil surface.

Measurements carried out between 1971 and 1977 indicate the effect of rainfall on above-ground plant production. The site investigated consists of steppeland on sandy soil, dominated by the perennial dwarf shrub Rhantherium suaveolens with associated annual species. The average precipitation in this region (180 mm), produces about 1000 kg dry matter/ha. The ARFEJ model described, uses the most pertinent characteristics of the system so that production can be predicted under the assumption that water is the limiting factor. Seven model coefficients were calculated with the data of 1971–75. The results show (a) the considerable effect of rainfall distribution on evaporation from bare soil. The mean of this evaporation was 55 per cent (range 40–69 per cent) of total evapotranspiration; (b) during the short annual period of fast growth, the ratio of dry matter to water transpired would be about 1·26 g dry matter/kg H2O for annuals, and 1·39 g dry matter/kg H2O for perennial species. Over the whole year, the average efficiency of water use is 0·45 g dry matter/kg H2O for all species; (c) above 250 mm precipitation, the availability of nitrogen and/or phosphorus becomes the limiting factor for plant production and efficiency of water use decreases.