The formation of degraded areas in the dry savanna woodlands of northern Australia

Abstract

The perennial tall grass understory of a eucalypt woodland on a commonly occurring red earth (Northcote Principal Profile Form Gn2.11) in northern Australia was burnt during the dry season and subjected to weekly clipping during the following two wet seasons to simulate overgrazing. Clipping killed many plants during the first wet season and almost all of them during the second. Infiltration measurements showed that sorptivity and hydraulic conductivity were greatly reduced after the first wet season following burning only, and micromorphological examination showed surface sealing and structural collapse. There was recovery of sorptivity after the second wet season, but not of hydraulic conductivity, and this was accompanied by an increase in the macropore space of the upper 5 mm of surface soil. With burning plus clipping, structural rearrangement formed a vesicular porous layer during the second wet season and infiltration rates remained low. Since no organic carbon was lost from the surface soil following burning only, the structural collapse was attributed to raindrop impact rather than loss of structural bonds in the oil. In the burn+clip treatment, organic carbon was lost only during the second wet season when most of the grass tussocks had died. Respiration measurements showed that there was little difference in biological activity between the burnt and unburnt grassland during the first wet season and that an estimated 600 g m-2 year-1 of carbon was respired. Overgrazing pastures on these soils results in complete degradation within two set seasons and the formation of bare areas with surface seals.