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Vegetation Stripes on Sheet Wash Surfaces

L. P. White
Journal of Ecology
Vol. 59, No. 2 (Jul., 1971), pp. 615-622
DOI: 10.2307/2258335
Stable URL: http://www.jstor.org/stable/2258335
Page Count: 8
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Vegetation Stripes on Sheet Wash Surfaces
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Abstract

Striped vegetation patterns occur on sheet wash surfaces of the arid and sub-arid zones. These are smooth surfaces on which the slope is too slight to allow rill or gully erosion to occur and run-off occurs as sheet flow. The maximum slope on which stripes have been described is 1 in 50 though most are on slopes of 1 in 100 or less. Rainfall in these areas is markedly seasonal. The maximum for any described occurrence of stripes is 750 mm per year and they occur in areas with only 50 mm annual rainfall. The stripes are not due to gross soil differences, and differences in soils between the vegetation stripes and the intervening lanes are due entirely to the influence of the vegetation itself. Soils within stripes have thickened A horizons with improved permeability and surface moisture penetration. Some have surface accumulations of permeable wind-blown material that has been trapped by the vegetation. In some cases there is a marked leaching of soluble salts within the arcs and the impression of carbonate and silica hard-pans has been described. Different factors have initiated the patterns in different areas. At the arid margin of the climatic zone in which the patterns occur, arcs probably originated by the extension of scattered individuals or colonies on land surfaces previously only sparsely vegetated. At the humid margin of the zone, patterns most probably developed by the coalescing of areas of bare ground formed by faunal activity. The patterns are apparently highly stable provided that environmental conditions remain unchanged. This does not necessarily preclude the migration of stripes within the patterns, or local reorientation due to accident. In most cases the vegetation is probably a climatic climax or grazing sub-climax, and severe degradation is likely to take place if the arcs are destroyed. As the development of the arcs is conditional upon the interaction of critical climatic and geomorphic factors they are likely to have been in existence as long as the surfaces on which they occur have been subject to the appropriate climatic conditions. If this is the case they will have been in existence throughout the Quaternary period and would have migrated with the extension and contraction of their critical climatic zone during the successive pluvials. At the arid margin of the zone they would have advanced onto sparsely vegetated ground, while at the humid margin they would have developed from previously fully vegetated surfaces.

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