PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



171 



Figube 4. — Network of sheep tracks at Benilkie Ridge, 137 km. N.N.E. of Balranald, N.S.W., typical of the site on 

 which variation of sorptivity with trampling intensity was investigated (fig. 5). A trampling index (T.I.) was 

 assigned to the tracks depending on their prominence. Trampling indices illustrated include T.I. 0: soil between 

 shrubs traversed by tracks; T.I. 0.5: track barely discernible (for example, track from bottom right to top 

 left) ; T.I. 1: track clearly defined but with little or no difference in level with surrounding soil surface (for 

 example, tracks in foreground from left to right) ; T.I. 4: track well defined with surface more than 2 cm. lower 

 than the surrounding soil surface (for example, track in background from top left to top centre). In the study, 

 intermediate and higher T.I. values were obtained at track intersections. 



45-percent increase in sediment yield from 

 grazed compared with ungrazed catchments over 

 a 14-y ear-study period (39). The importance of 

 reduced infiltration due to trampling has also 

 been noted by Jackson {28). 



Natural resilience of the soil is of some impor- 

 tance in its reaction to trampling. Thus, the high 

 swelling and shrinking capacity of many of 

 the sodic clay soils of the Riverine Plain of south- 

 eastern Australia and possibly also the soils of 

 much of the Mitchell grass (Astrebla spp.) coun- 

 try, in addition to their inherently low infiltra- 

 tion rates, are likely to reduce the effects of tram- 

 pling. The more loosely aggregated, calcium rich, 

 desert sandy loams have less natural resilience. 

 In climates where frost heaving occurs, this can 



reverse the effect of trampling by its seasonal 

 effect on infiltration (55). 



Change In Dominant Species 



Efficiency in the use of water by vegetation 

 depends in part on the soil volume exploited by 

 plant roots, the plant rooting zone M in equation 

 1. The differential grazing treatments of Wilson 

 and others (73) resulted in adjacent plots with 

 and without Atriplex vesicaria (fig. 1). Changes 

 in soil moisture down to 54 cm. were followed 

 over a 33-day period in these plots after re- 

 charge of the soil profiles by rain during Novem- 

 ber 1970. The results shown in figure 7 indicate 

 that the saltbush roots exploited the water in the 



