Experiments were conducted in the Patagonian steppe in southern South America to test the following hypotheses: (a) grasses take up most of the water from the upper layers of the soil and utilize frequent and short-duration pulses of water availability; (b) shrubs, on the contrary, take up most of the water from the lower layers of the soil and utilize infrequent and long-duration pulses of water availability. Grasses and shrubs were removed selectively and the performance of plants and the availability of soil resources were monitored. Results supported the overall hypothesis that grasses and shrubs in the Patagonian steppe use mainly different resources. Removal of shrubs did not alter grass production but removal of grasses resulted in a small increase in shrub production which was mediated by an increase in deep soil water and in shrub leaf water potential. The efficiency of utilization of resources freed by grass removal was approximately 25%. Shrubs used water exclusively from lower soil layers. Grasses took up most of the water from upper layers but they were also capable of absorbing water from deep layers. This pattern of water partitioning along with the lack of response in leaf nitrogen to the removal treatments suggested that shrubs may be at a disadvantage to grasses with respect to nutrient capture and led to questions about the role of nutrient recirculation, leaching, and nitrogen fixation in the steppe.

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Two patch types were recognized in the Occidental District of the Patagonian arid steppe: i) shrubs encircled by a ring of tussock grasses, and ii) tracts of scattered tussocks. Completeness of the ring of grasses around the three dominant shrubs was a function of shrub size. Average completeness was 62, 71 and 83 ~o, respectively for the three dominant shrubs (Senecio filaginoides, Mulinum spinosum and Adesmia campestris). A model for the cyclic dynamics of the two patch types was proposed. It includes a building phase (grass ring construction), a mature phase (maximum ring completeness) and a degenerate phase. In this last phase, triggered by shrub death, completeness of the ring progressively decreases until remnant grasses become undistinguishable from the scattered tussocks patch type. Ring formation occurred independently of shrub species. Grass species were differentially associated to the two patch types and to rings of different shrub species. Cyclical patch dynamics influenced the pattern of resource utilization, since the shrub-ring patch, with a share of only 18 ~o of cover, contributed 44~o of the total primary productivity.Abbreviations." Shrub-ring patch (ST), M = asymptote, maximum completeness, D = parameter related to the initial lag, B = rate of increase in ring completeness, z = shape parameter, r = radius.Nomenclature: The taxa names are given according to Correa, M.N., 1971Correa, M.N., -1984. Flora Patag6nica. Colecci6n Cientifica del I.N.T.A.

The vegetation in the Coironal arid steppe consists of grasses and shrubs. The objective of this paper was to test Walter's hypothesis that woody vegetation and grasses compete for water in the upper layers of the soil, but woody vegetation has exclusive access to a source of water at deeper levels.Analysis of root profiles and patterns of leaf and soil water potential led us to accept the hypothesis for this arid steppe. Additional information on phenology and on the ability of the major grass species to respond to watering permitted to identify two ecological strategies corresponding to grasses and shrubs. Grasses behave as opportunists having always leaves ready to grow as soon as water becomes available. They have a shallow root system and are able to respond very rapidly to increases in soil water availability. In contrast, woody species have a clear-cut periodic pattern of growth and dormancy. They possess thick horizontal roots running below 35-40 cm and utilized water stored in lower layers of the soil.A diagrammatic model summarizes the role of periodic and opportunistic species upon water circulation in the ecosystem. The effect of changes in the proportion of the two groups upon water dynamics is also discussed.