PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



189 



simulated rainfall. Organic matter content was 

 found to be the most important soil factor in- 

 fluencing soil erosion, and soil organic matter 

 was related to litter accumulation. 



These studies emphasize the importance of 

 vegetation and litter in maintaining infiltration 

 rate and reducing runoff and soil erosion. Herb- 

 age and mulch yields were determined in con- 

 nection with an infiltration study (38) on mixed 

 prairie in North Dakota when grazed at three 

 intensities for over 40 years. Total air-dry herb- 

 age yields were 815. 1,708, and 2,470 kg./ha. for 

 the heavily grazed, moderately grazed, and un- 

 grazed areas, respectively. Mulch weights were 

 383, 2,008, and 4.652 kg./ha., respectively, for 

 these treatments. Intake rates were 3.76, 6.10 and 

 10.85 cm./hr., respectively, for the initial hour of 

 precipitation. Total herbage and mulch accounted 

 for 88 percent of the variation in water intake 

 rate. 



Mazurak and Conrad (31) compared intake 

 rates on three soil types for pure stands of six 

 cool-season and four warm-season native and in- 

 troduced species. In general, the cool-season spe- 

 cies maintained a higher level of water intake 

 than did the warm-season species. This was at- 

 tributed to their more favorable effect on soil 

 structure. We can assume, therefore, that change 

 in species composition through grazing manage- 

 ment which changes a grassland from cool- 

 season to warm-season species (47) may result in 

 reduced water intake rate, without considering 

 other effects such as trampling or reduction in 

 mulch. 



The inverse relationship between grazing in- 

 tensity and infiltration rate has been reported by 

 others (5, 18, 24, 26, 27, 32, 33, 37, 38, 39, 40, 4h 

 4-2, 43, 44, SO). A comprehensive study of intake 

 rates on midcontinental rangelands (39) showed 

 that water intake rate was more closely cor- 

 related with both new and old vegetation than 

 with other variables studied. Vegetation factors 

 influenced intake rate more than did soil factors 

 on saline-alkali upland, dense clays, silty soils of 

 various sites and on most sandy soils. The soil 

 factors influenced intake rate more than did vege- 

 tation factors only on extreme sands and on over- 

 flow sites, while both factors were of equal im- 

 portance on shallow range soils and on some clay 

 soils. Structure was the most important soil factor 

 whenever soil factors were of major concern. 



Other factors have been found to affect infil- 

 tration. Some of these are animal induced, others 

 are not. Thompson (50) suggested that changes 

 in surface soil characteristics produced by the 

 shrink-swell action of frost during the winter 

 improves infiltration. Heavy summer thunder- 

 storms often destroy this condition. He considers 

 these factors to be more important to overall in- 

 take ability in many cases than are the grazing 

 effects. 



The physical trampling by grazing animals 

 was discussed by Packer (34)- Working in the 

 Boise River Watershed in Idaho, he used two in- 

 termixed types of foothill spring-fall range. Part 

 of the area was dominated by bluebunch wheat- 

 grass (Agropyron inerme [Scribn and Smith] 

 Rydb.) and another part by cheatgrass (Bromus 

 tectorum L.). From an infiltration study, he es- 

 tablished specific ground cover conditions for 

 preventing water erosion during typical major 

 summer storms. No more than 30 percent of the 

 soil surface could be bare, and the other 70 per- 

 cent had to be covered by either plant foliage 

 or litter. The bare soil between the plants and 

 the litter patches could be no larger than 10 cm._ 

 in diameter on wheatgrass range, and no larger 

 than 5 cm. on cheatgrass range. However, he 

 found these specifications to be inadequate if the 

 range was subjected to trampling. Using a steel 

 "hoof," he imposed simulated trampling disturb- 

 ance on 0, 10, 20, 40. and 60 percent of the plot 

 surface. All levels of trampling reduced the 

 amount of ground cover and increased the size 

 of the bare soil openings on both range types. 

 He found that 60 percent trampling caused little 

 more reduction in cover than did 40 percent, be- 

 cause the "hoof" impacts often occurred on litter 

 that had been moved by previous impacts. This 

 resulted in redistribution of litter with little ad- 

 ditional reduction in soil surface cover. All levels 

 of trampling increased the amount of overland 

 flow and the amount of soil erosion. Thus, he rec- 

 ommended that more vegetation and litter should 

 remain when trampling is involved. 



Quinn and Hervey (36) compared forage loss 

 from trampling by yearling steers under various 

 intensities of grazing. Losses ranged from 1 per- 

 cent under light grazing to 5 percent of the total 

 production under heavy grazing, or from 22 to 

 67 kg./ha. Susceptibility to trampling loss varied 

 with species, blue grama being less susceptible 



