(1) Associations of perennial grasses on dune slopes in the Namib Desert were studied across a complex environmental gradient from the Atlantic Ocean (west) to the base of the Great Escarpment of Africa (east). (2) The number of storms with measurable precipitation increased from the sea inland. In contrast, the rate of sand movement decreased from the west to the east. (3) The standing crop of the perennial grasses and the rate at which Stipagrostis sabulicola, a species found across the dune fields, recharged its xylem water potentials increased inland as a result of the former gradient of increasing rainfall. (4) Complex environmental gradients also occurred over each dune slope. The rate of sand movement increased from the edge of the inter-dune valley to the top of the dune. However, the moisture gradient was more complex and is poorly understood. There appears to be interflow of soil moisture within the dune such that the upper dune is drier than the mid-dune. At the base of the dune, there is a reduction of soil moisture due either to increased compaction of the sand and subsequent increased surface run-off or, as a result of the closeness of the hard inter-dune substratum, to a smaller sand volume from which roots can extract water. (5) In consequence, the average standing crops for perennial grasses are greatest in the central portions of the dune slope across the dune fields. (6) These geographical and local environmental gradients influence the number of species of perennial grasses found forming associations across the dune fields as well as their growth strategies and distributions over a dune slope. (7) The number of perennial grass species forming associations increases from a single species in the western dunes to four species in the east. Species establishing predominantly from rhizomes are found where rates of sand movement are highest (i.e. in the western dunes and on the upper sections of the dune slope). In contrast, species establishing predominantly from seed are found in the easternmost dunes and at the bases of the dune slopes. (8) There is an inverse correlation between density of the vegetation and the rate of sand movement. Thus, the ultimate factor controlling the stabilization of these dunes and the dynamics of these grasslands is the amount of rainfall which, in turn, determines the establishment of new individuals from seed.