Browsing Journal of Range Management, Volume 54, Number 4 (July 2001) by Subjects
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Low density of prickly acacia under sheep grazing in QueenslandPopulations of an introduced woody weed, prickly acacia (Acacia nilotica (L.) Delile ssp. indica (Benth.) Brenan syn. Acacia arabica (Lam.) Willd. ssp. indica Benth.), were surveyed at 4 sites in central Queensland. There is a significantly lower frequency of plants of 3 m in height within populations which have been grazed by sheep, indicating that browsing by sheep reduces regeneration. There were higher losses of seedlings at a sheep-grazed site than at cattle-grazed sites. These results support previous assertions that prickly acacia is regenerating more successfully on cattle properties, because cattle both disperse seeds and are less effective herbivores. In regions of low annual rainfall, prickly acacia is capable of forming dense stands (up to 2,700 shrubs ha(-1)) in lowland landscape types. Stands are less dense in upland landscapes (maximum of 718 shrubs ha(-1)). Of most concern is that in regions of high annual rainfall prickly acacia can form extremely dense thickets across most landscape types (up to 3,400 shrubs ha(-1)). We suggest that prickly acacia is most likely to become a management problem on cattle properties, and an extreme problem in high annual rainfall areas. The inclusion of sheep in livestock rotations may be an effective control measure in the Mitchell Grasslands, but this may not always be possible. A high priority is to prevent prickly acacia from expanding its range into equivalent high rainfall areas within Queensland, and also in the Northern Territory, northern New South Wales, and Western Australia. This could be achieved by quarantining livestock which have come from infested properties until seeds have passed through the digestive tract, after about 6 days. Management strategies at the property level should aim to prevent further spread of prickly acacia by controlling cattle movements between paddocks during periods when cattle are ingesting pods and seeds.
Upland erosion under a simulated most damaging stormA 2 year study was conducted to determine the effects of surface cover and roughness on sediment yield from plots subjected to a simulated most damaging storm. This storm, based on long term sediment records from 3 Wyoming streams, produced approximately 18 mm of precipitation in 15 min with an intensity of 97 mm hour(-1). The rainfall simulator covered 2 plots; each 0.6 by 2 m. Plots were on 9% slopes with highly erosive soils (silt and fine sand texture) on native rangeland in 3 areas of Wyoming. Cover and surface roughness were measured with a point frame. Sediment production typically peaked approximately 120 sec after runoff started and reached steady state within 6 min. Plots with no cover (tilled) seldom produced runoff due to high infiltration and the short duration rainfall. Sediment yield was moderately correlated with total cover for total cover less than 30%, and sediment yield decreased to 0.1 tonnes ha(-1) (assumed allowable soil loss) or less for greater than 30% cover. There was a weak correlation between surface roughness and sediment yield, and surface roughness was slightly correlated with total cover. These results suggested that maintaining at least 30% total cover could control sediment yields from short duration-intense storms. Experimental results also indicated considerably higher sediment yields than those predicted by the Revised Universal Soil Loss Equation or a modified version of that equation.