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

Selected physical properties of the soils developed within a strongly-bande d grassland in arid New South Wales were investigated to reveal their possi ble significance for the hydrologic and erosional behaviour of the mosaic l andscape. Detailed surface microtopography, surface roughness, soil bulk de nsity 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 indi cate that the cross-pattern (downslope) variation in the soil parameters is systematically related to position within a particular component (grove, i ntergrove, etc.) of the mosaic. Compressive strength and bulk density incre ase downslope across intergroves, peaking at very high levels within the zo ne of forbs, while groves display lower but more uniform values. Surface ro ughness increases downslope through the intergrove and the zone of forbs at the upslope margin of a grove, reaching its maximum within the grove. Mosa ic components thus cannot be treated as uniform in their soil properties, a nd single samples from within a component are shown in general to be inadeq uate. The mapped pattern of soil properties implies a very stable configura tion for banded mosaics. Surface runoff is increasingly hindered during flo w from the intergrove onto the grove. At the same time, soil resistance to entrainment increases in opposition to the shear forces generated by the ru noff. In concert, these tendencies imply that little sediment transport is possible across the mosaic. The resulting landscape stability appears to co nfer robustness to the mosaic in the face of stresses such as drought and p astoralism, when plant cover may be temporarily thinned or absent. After dr ought, for example, water pending would again begin at the downslope margin of the concave topographic elements, fostering re-establishment of the gro ves. (C) 1999 Elsevier Science B.V. All rights reserved.