210 



MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



The grazed vegetation averages between 30 and 

 50 percent shorter than the ungrazed vegetation 

 throughout the season. 



Below ground biomass and soil bulk density 

 also are affected by grazing. Five 1-inch -cores 

 were taken on each clipped plot immediately after 

 clipping, a total of 60 cores at each date repre- 

 senting 12 individual samples. The results of 

 this sampling were remarkedly consistent and 

 showed that at all dates of sampling the weight 

 of subterranean material (roots and rhizomes) 

 was appreciably greater on the grazed site than 

 on the ungrazed site (table 4). For the season, 

 underground plant material averaged 2.519 g./m. 2 

 on the grazed site and 1,643 g./m. 2 on the un- 

 grazed site, a little over 53 percent greater weight 

 of material occurring under grazing. 



The compacting influence of animal trampling 

 under grazing had altered the bulk density of 

 the soil to a depth of about 20 cm. (table 5). The 

 increase in bulk density under grazing was espe- 

 cially marked in the first 10 cm. of the soil pro- 

 file. Bulk density values were determined from 

 small cores (2.5 cm. diam.) and were highly vari- 

 able. The core values obtained in the 1970 season 

 do. however, correspond closely to values obtained 

 with larger cores (6.35 cm. diam.) in previous 

 years. No attempt was made to assess directly the 

 significance of either the greater bulk of root 

 material under grazing or the increase in soil 



bulk density in relation to microenvironmental 



influences. 



Microenvironmental Influences 

 Air Temperatures 



Air temperatures were determined with shield- 

 ed thermocouples mounted at various heights 

 above the grazed and ungrazed grassland ; tern- 

 peratures were recorded hourly by a strip-chart 

 recording potentiometer. The thermocouples were 

 mounted at heights of 2.5, 7.5, 15, 25. 55. 75, and 

 120 cm. above the ground surface on both areas, 

 with an additional unit at 130 cm. on the grazed 

 area, and at 145 cm. on the ungrazed area. On 

 both sites, greatest height of measurement above 

 ground was considered to be mature canopy 

 height plus 1 meter. 



Temperatures above the grazed grassland aver- 

 aged 1.5° C. higher than the temperatures above 

 the ungrazed vegetation on the season — long basis, 

 and. in general, showed about the same difference 

 in each of the summer months (table 6). Highest 

 average air temperatures occurred in July, with 

 temperatures above the grazed and ungrazed sites 

 averaging 23.7° C. and 22.0°, respectively. 



The nature of the gradients in air temperature 

 that exist above the grassland in the Northern 

 Plains have been discussed by "Whitman and 

 Wolters (23) in some detail. The gradients found 



Table 3. — Average leaf heights of major species and average 

 canopy heights of grazed and ungrazed vegetation at approx- 

 imate monthly intervals during the 1970 growing season. 







Leaf heights 



; in cm 



at different dates 





Species 





Grazed 







Ungrazed 







5/24 



6/22 



7/22 



8/18 



5/24 



6/22 



7/22 



8/18 



Stipa comata 



6 



12 



14 



21 



11 



25 



33 



35 



Agrnpyron smithii 



9 



25 



25 



27 



16 



32 



34 



35 



Bouteloua gracilis 



2 



T 



9 



9 



4 



13 



16 



16 



Care j- eleocharis 



6 



9 



9 



10 



8 



15 



17 



16 



Calainagrostis mont. 



10 



13 



18 



17 



— 



— 



— 



— 



Galamovilfa long. 













30 



42 



47 



Average 



i 



13 



15 



17 



10 



23 



28 



30 



Approximate canopy 



ht. ' 9 



25 



32 



35 



12 



32 



47 



53 



1 Combined leaf and stalk heights of all standing vegetation — estimated. 



