EFFECT OF GRAZING UPON ASPEN REPRODUCTION. 11 



grazed clear-cut aspen plots, (2) the number of sprouts (a) injured 

 and (b) killed by browsing, (3) the number uninjured, and (4) the 

 number injured, by causes other than grazing. Figure 3 shows 

 seasonally (1) the sprouting ability of clear-cut aspen protected from 

 grazing, (2) the number of sprouts injured (a) by barking by rodents 

 and (b) by frost, and (3) the number killed by (a) barking by rodents 

 and (b) by frost. 



In 1913 practically the same number of sprouts per acre were pro- 

 duced on the protected and on the unprotected plots — namely, 

 about 80,000 per acre. In the summer of 1914, when the sprouts 

 were retallied, the number had increased to 105,589 per acre on the 

 unprotected plots and to 90,480 on the protected plots. In the fall 

 of 1914, 103,241 specimens were recorded on the unfenced plots — a 

 slight decline over that of the previous count — while on the fenced 

 areas the number had decreased to 58,324. From that time on 

 there was a sharp but uniform decline on both sets of plots until in 

 the fall of 1916 not one living sprout remained on the grazed plots 

 (Plate III), though 2,646 vigorous sprouts per acre were recorded 

 on the protected plots, most of which had attained a height beyond 

 that at which sheep browse (Plate IV, fig. 1). While this number 

 is ample for the establishment of a full aspen stand, a great many 

 more sprouts have been found on plots established elsewhere. 



The number of uninjured sprouts was notably greater on the 

 ungrazed than on the grazed plots (Figs. 2 and 3). On the grazed 

 plots there was a drop in the number of uninjured sprouts in the 

 summer of 1914, due probably to slightly more intensive grazing at 

 that time. This was followed by an' increase and then from the 

 fall of 1914 to the summer of 1916 the number of sprouts declined 

 rather uniformly to zero. On the protected plots the number of 

 uninjured sprouts decreased at approximately the same ratio as the 

 total number produced. On the grazed plots there were no unin- 

 jured sprouts in the summer of 1916, but on the plots protected from 

 grazing there remained 2,646 sprouts per acre. 



The rate of mortality even on the ungrazed plots is surprisingly 

 high. Usually a large proportion of the sprouts are killed, often 

 within a season, by the, injuries caused by bark-eating rodents, 

 chiefly field micej gophers, and rabbits (Plate IV, fig. 2). Besides, a 

 very large proportion of the specimens recorded as merely injured 

 by rodents and by frost died later from such injuries. The repro- 

 duction is often completely girdled, and not uncommonly several 

 belts of bark an inch or more in width are removed. While the 

 adverse factors of inclement weather and bark-eating rodents are 

 active in the elimination of reproduction on the fenced and unfenced 

 plots alike, such injury is insignificant compared with the injury 

 chargeable to sheep grazing on the unfenced plots. 



