EFFECTS OF DIFFERENT SYSTEMS OF GRAZING. 



25 



4 meters square, and the number of plants of A., frigida in each 

 square meter was listed in 1915 and 1920. The results are presented 

 in Table 5. This table shows that there has been a heavy increase 

 of this species in the 30-acre pasture. The slight decrease in the 

 100-acre pasture is not great enough to be of any particular impor- 

 tance. There will be found a fluctuation in number from year to year, 

 as some plants die normally, while others meet with accidents, such 

 as being cut off by gophers or pulled up by the cattle. Under normal 

 conditions a few new plants will be established each year, so that 

 the total number remains about the same. However, as soon as some 

 disturbing factor enters, such as the continuous intense grazing of 

 the 30-acre pasture, this is the plant that is ready to take advantage 

 of the weakened condition of other species. 



Table 6 shows the number of plants of A. frigida in the units of 

 the isolation transects. These show very clearly the effects of graz- 

 ing upon this species. In the 100-acre transect practically no dif- 

 ference between the different units appears, but in the 30-acre tran- 

 sect there is a striking difference that can be accounted for only by 

 overgrazing. The units that are annually closed to grazing contain 

 the largest number of plants. The individual plants in these units 

 also show pronounced differences, depending upon the length of time 

 the unit has been closed. Table 7 presents these differences clearly. 



Table 6. — Number of Artemisia frigida plants per square-meter quadrat in the 

 isolation transects in the 100-acre and 30-acre pastures in 1921. 



[Series A was fenced in 1915; the units are opened in succession to grazing, beginning with unit 1 in 1918. 

 Series B is permanently closed to grazing. Series C is open to grazing except as the units are closed in 

 succession, beginning with unit 1 in 1918.] 





Se- 

 ries. 



Number of plants per square meter. 



Pasture. 



Unit 

 1. 



Unit Unit 

 2. 3. 



Unit 

 4. 



Unit 

 5. 



Unit Unit 

 6. 7. 



Unit Unit 

 8. 9. 



Unit 

 10. 



Total. 



100-acres 



It 

 It 

 It 

 It 



1 

 5 

 3 



1 

 5 

 2 



21 

 1 

 5 



29 

 

 6 



2 

 11 

 5 



4 

 3 



i 



'! 



23 



5 

 4 



3 



' 



7 

 3 



32 



2 



30 



8 

 5 

 3 



1 



11 

 1 



1 

 25 



4 







24 



9 

 6 

 13 



1 

 5 

 11 



2 

 2 

 9 



2 

 2 

 16 



2 

 ? 



5 

 1 

 1 2 



1 



2 3 

 5 4 



1 



3 

 14 12 







5 



6 10 



2 

 2 

 2 



1 

 

 1 







1 

 11 



1 



1 



9 



301 

 37J-107 



' 100-acres (duplicate) 



30-acres 



30-acres (duplicate) 



5 





 3 

 4 



1 

 

 12 



1 

 

 11 



2 



1 

 5 

 12 





 



7 



1 

 3 



6 



40J 



13] 



37^101 



51] 



40) 



16^169 

 113] 



841 



14^239 

 14JJ 



From Table 7 it is very evident that not only the number of plants 

 is influenced by overgrazing, but also the size of the plants, which is a 

 good index of their physiological vigor. 21 This condition is brought 

 about by reduction of the competition of other species. The vigor of 

 the other species has been reduced by the continuous severe grazing, 

 while the A. frigida has been favored, because the cattle do not like 

 it and will not eat it until forced late in the season after it has had 

 a chance to make its normal growth. Since the other species of 



a Sampson, Arthur W. Plant succession in relation to range management. U. S. 

 Dept. Agr. Bui. 791, 76 pp., 26 figs., 2 pis. 1919. 



