THE INFLUENCE OF CLIMATIC FACTORS ON THE REACTION OF 

 DESERT SHRUBS TO GRAZING BY SHEEP 



Professor H. C. Trumble, and K. Woodroffe* 

 {V^aite Agricultural Research Institute, University of Adelaide) 



The arid pastoral areas of southern Australia with a mean annual rainfall of less 

 than 10 inches are characterized by species of Atriplex and Kochia which occur ex- 

 tensively as low growing shrubs. These plants are well adapted to withstand rain- 

 less periods of long duration, and they provide reserves of feed in unfavourable sea- 

 sons. The shrub cover has been greatly depleted by grazing sheep for approximately 

 a century, and much land has been completely denuded. As viable seed of these 

 shrubs no longer occurs in significant quantity, natural regeneration of perennials 

 is rare on such country. Artificial seeding is costly and hazardous owing to low and 

 uncertain rainfall. Sheep - raising in these areas is now largely dependent on annual 

 herbage, and thus has become more subject to the risks of unfavourable seasons. 



In other parts of the pastoral country, relatively undamaged associations of 

 perennial shrubs still occur. Fairly extensive areas carry a moderate to sparse popu- 

 lation of shrubs which produce seed in favourable seasons, making possible some 

 natural regeneration. Investigations of pasture management here should throw light 

 on the reaction of desert shrubs to grazing by sheep, and hence suggest more effec- 

 tive and permanent systems of utilization in arid regions. A previous investigation 

 (Osborn, Wood and Paltridge, 1932) has indicated that pastures of saltbush (Atriplex 

 vesicarid) may be improved under certain conditions of grazing. 



Researches were commenced in 1941 at Yudnapinna Station, approximately 250 

 miles north-west of Adelaide; this centre is within the extensive North-west Pas- 

 toral District of South Australia. Investigations of the ecological factors concerned 

 with the grazing management of bluebush {Kochia sedifolia) and associated species 

 were undertaken on a long- term basis. 



Climatic Factors 



A meteorological station (Fig. 1) was established at Yudnapinna in October, 

 1938 and daily records of air temperature, relative humidity, wind mileage, free water 

 evaporation and rainfall have since been maintained. Rainfall had been measured 

 previously from 1885 onwards, thus giving an uninterrupted record to date of 67 years. 

 The mean annual rainfall is 7.92 inches with a range from 2.36 inches to 18.08 inches 

 per annum, and a modal frequency of between 5 and 6 inches. 



Although the mean rainfall is fairly evenly distributed through the year, winter 

 rains are more frequent and dependable ; the mean monthly rainfalls for the period 

 May — August are slightly greater than for the remainder of the year. In summer the 

 precipitation tends to occur spasmodically as extremely fortuitous but heavy rains. 



* The authors have been jointly responsible for the planning of the work described and the 

 analysis of the data recorded ; but the detailed investigations and most of the observations 

 made are attributable to Mr K. Woodroffe, and will be the subject of a subsequent paper by him. 



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