Banded vegetation near Broken Hill, Australia: significance of surface roughness and soil physical properties

Selected physical properties of the soils developed within a strongly-banded grassland in arid New South Wales were investigated to reveal their possible significance for the hydrologic and erosional behaviour of the mosaic landscape. Detailed surface microtopography, surface roughness, soil bulk density and the unconfined compressive strength of the soils were determined using linear transects across the banded mosaic. The landscape is shown to consist of a tier of concave-upward microtopographic elements. Results indicate that the cross-pattern (downslope) variation in the soil parameters is systematically related to position within a particular component (grove, intergrove, etc.) of the mosaic. Compressive strength and bulk density increase downslope across intergroves, peaking at very high levels within the zone of forbs, while groves display lower but more uniform values. Surface roughness increases downslope through the intergrove and the zone of forbs at the upslope margin of a grove, reaching its maximum within the grove. Mosaic components thus cannot be treated as uniform in their soil properties, and single samples from within a component are shown in general to be inadequate. The mapped pattern of soil properties implies a very stable configuration for banded mosaics. Surface runoff is increasingly hindered during flow from the intergrove onto the grove. At the same time, soil resistance to entrainment increases in opposition to the shear forces generated by the runoff. In concert, these tendencies imply that little sediment transport is possible across the mosaic. The resulting landscape stability appears to confer robustness to the mosaic in the face of stresses such as drought and pastoralism, when plant cover may be temporarily thinned or absent. After drought, for example, water ponding would again begin at the downslope margin of the concave topographic elements, fostering re-establishment of the groves.