Oblique vegetation banding in the Australian arid zone: implications for theories of pattern evolution and maintenance

Abstract

This paper describes strongly developed vegetation bands that are not aligned along the contour. This previously unreported oblique vegetation banding is developed in an arid region of mixed tussock grasses and chenopod shrubs in western New South Wales, Australia. In two of four contour-mapped, 0·5 ha quadrats, the vegetation bands are oriented at angles of 45–70° to the direction of slope. Various soil features indicative of a runoff and runon hydrology, very similar to those classically reported from contour-parallel vegetation banding, suggest that the oblique configurations have been stable for a considerable time. In addition to the unusual band orientation, marked variations in the ratio of intergrove width to grove width were found within the study area, spanning the range 0·21–4·95, and there was generally no correlation between intergrove width and the width of the grove receiving runoff water from it. These findings cast doubt on the supposed close adjustment of intergrove dimensions to grove water requirements. In nearby plots having ordinary contour-parallel vegetation banding, the absence of any upslope motion of the pattern was demonstrated using monuments installed at band margins 6 years previously. These properties do not accord with the predictions of many models of the development of vegetation banding. However, soil changes associated with the runoff–runon system operating within vegetation bands may be involved in stabilizing their configurations. Thus, future investigations into the origin of vegetation banding must distinguish between their form and hydrologic functioning once established, and the fundamental origin of the vegetation pattern, which may be altogether different. In addition, pedologic and other processes which may inhibit free pattern adjustments must be incorporated into models addressing the development of vegetation banding.