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 mice, gophers, and rabbits (Plate IV, Rg. 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. 
