EFFECT OF GRAZING UPON ASPEN REPRODUCTION. 27 
eliminated shortly after its appearance by adverse climatic factors, 
chiefly frost. 
(10) A surprisingly large proportion of the reproduction produced 
even on the most favorable sites is killed during the first three years 
of its growth by causes other than grazing. Frost and bark-eating 
mammals, notably gophers, field mice, and rabbits, are mainly respon- 
sible for such mortality. Much of the damage caused by gophers 
and mice is done under the snow during winter or early spring. 
(11) The factor that chiefly determines the time of exemption of 
the reproduction from destruction by sheep and cattle is the height 
of the sprout. The average maximum height at which sheep browse 
is approximately 42 inches. Sprouts averaging 45 inches in height 
are found to be exempt from destructive browsing by sheep. In 
the case of cattle there is some damage to sprouts between 55 and 60 
inches in height, but seldom to those of any greater height. Repro- 
duction averaging 5 feet in height, therefore, is practically free from 
damage by cattle browsing. 
(12) The annual rate of height increment of the aspen reproduction 
averages about 15 inches. Hence sprouts 3 years of age are exempt 
from serious injury by sheep, and those from 4 to 5 years of age are 
free from serious injury by cattle. 
(13) Aspen is practically unable to reproduce under its own shade, 
and the best means of obtaining vigorous and dense reproduction, 
and at the same time of harvesting the timber economically, is to 
clear-cut the lands or to thin the stand heavily. 
(14) In the logging operations various methods are used in dis- 
posing of the branches and tops, the most common practice being to 
pile and burn them. This method of brush disposal is not conducive 
to the best results on typical aspen lands, as it exposes the repro- 
duction unduly both to grazing and to unfavorable climatic conditions. 
Protection from live stock and from frost, excessive insolation, 
evaporation, and other adverse factors is obtained by arranging the 
unlopped tops about the stumps so as to protect the on-coming 
sprouts. In three or four years, when the reproduction is practically 
exempt from serious browsing, the brush is for the most part decayed 
and out of the way. Furthermore, such a disposition of the brush 
does not make the danger from fire any greater. 
RECOMMENDATIONS. 
From the results given in the preceding pages certain recommenda- 
tions may be made for managing grazing in the aspen type so as to 
secure the highest possible economic use of the lands, both in the way 
of timber output and meat production. If properly handled, live 
stock will not prove antagonistic to the establishment of the repro- 
duction, the understory of palatable forage will not be wasted, and 
