Australian Journal of Ecology (1983) 8. 77-79 Short note The role of soil mobility in controlling a vegetation gradient RALF C. BUCKLEY Department of Biogeography and Geomorphology, Research School of Pacific Studies, Australian National University. PO Box4, Canberra Australia 2600 Abstract Mature plants of species which characteristically occupy the mobile crests and upper slopes of the central Australian sandridges are more tolerant of sand burial or undercutting than those of the lower slopes and swales. Seedlings growing on the mobile crest areas are susceptible to sand burial, even during wet years. The young shoots of vegetatively spreading species are less susceptible than indi- vidual seedlings of the same size. Introduction Much of arid central Australia is covered by longi- tudinal dunes or sandridges 5-25 m high, separated by interdune corridors or swales 0,2-1,5 km across. They are closely comparable to the arid longitudinal dunefields of India and southern Africa (Buckley 1981a), and an understanding of the pattern and dynamics of their plant cover is therefore of general interest. As in many desert areas, rainfall in central Australia varies greatly from year to year. Droughts comprising several successive dry years alternate with wetter periods. Historical records, reviewed by Buckley (1979), indicate that plant cover is reduced and sand mobility increased during drought. Soil mobility is much greater on the crests and upper slopes of the sandridges than on the lower slopes and swales, even during wet periods (Buckley 1982), There is also a dune-swale floristic gradient on the sandridges (Wiedemann 1971, Fatchen & Barker Con-cspondcnce: AMDEL (Personal). PO Box 114. EaMwood. SA 5063. Australui. 1979, Buckley I981b,c). Are plants from different topographic zones equally tolerant of soil mobility? The overall relations between wind regime and other climatic factors, topography and soil para- meters, plant growth form, wind flow patterns and soil mobility have been reviewed recently by Ash & Wasson (1982). Here I examine one aspect of the overall problem experimentally: the effects of sand accumulation and erosion on the survival of indi- vidual plants. Methods I poured loose sand on to 40 mature plants of each species studied, partially burying them, and fol- lowed their growth or death over the next six weeks. I filled ten of these completely, ten to 0.75, ten to 0.5 and ten to 0.25 their overall height. I marked these levels on stakes beside the plants and returned every 1-8 days to adjust sand levels altered by the wind. A further ten plants of each species served as controls. Partial burial in this manner is directly analogous to that experienced by a plant which accumulates windblown sand. I excavated soil around the root systems of ten further plants of each species until they fell sideways, though still con- nected to the root system, and followed their sub- sequent fate. This is analogous to "moating" or erosion of the sand surface by wind, around indi- vidual plants. The species studied are listed in Table 1, together with their characteristic topographic ranges on the sandridges and their growth forms, which affect the degree to which they accumulate sand. All are perennial. Only the smallest Zvt,'r)(7)/f>rt mounds were used: the largest may be up to 3 m high and are not easily buried. I also buried or excavated seedlings of Acacia ligulata and Chamaesyce wheeleri, these being the only species whose seed- lings were sufficiently abundant, and the young vegetative shoots of the rhizomatous Ptychosema anomalum. These three species are all characteristic of the dune crests. All the experiments were carried