18 
BULLETIN 1291, U. S. DEPARTMENT OF AGRICULTURE 
sprouts in various height classes as an aid in determining the period 
of protection necessary to insure satisfactory regeneration in differ- 
ent sites, 2,500 sprouts per acre at the end of the third year being 
considered satisfactory density. 
Table 9. — Percentages of sprouts safe from destruction by sheep and cattle 
grazing at the end of first to fifth years 
Height class 
Percentage of total sprouts in each height class at 
end of year specified 
First 
Second 
Third 
Fourth 
Fifth 
1 to 40 inches, not safe from sheep or cattle 
100 



79 
21 
18 
3 
50 
50 
38 
12 
33 
67 
46 
21 
20 
Over 40 inches, safe from sheep 
80 
40 to 60 inches, safe from sheep, not from cattle 
Over 60 inches, safe from sheep and cattle 
36 
44 
Gophers are sometimes responsible for the destruction of many 
sprouts and superficial roots, and on areas where they are extremely 
numerous regeneration may be decidedly unsatisfactory. Such sit- 
uations are not frequent, and are of slight practical importance. 
Mice, rabbits, and gophers sometimes indulge in wholesale girdling 
in sprout stands and cause entire destruction of trees over limited 
areas, much of this work being done in winter under the snow. 
On the whole, aspen suffers very much less' from damage by ro- 
dents than do the conifers associated with it, although the conifers 
are much more resistant to other forms of animal damage. Sheep 
grazing, the worst enemy of aspen, can be readily controlled, but 
the rodent damage to conifers can not be generally limited at the 
present time. Most of the damage is done by snowshoe rabbits which 
eat off the conifer terminals repeatedly as long as the tree top re- 
mains below the level of the deepest snow of the winter, which is 
3 to 8 feet in most of the aspen zone. Trees are rarely killed out- 
right by this damage, but height growth is greatly retarded, the 
trees developing into flat, spreading bushes', unable to reach a height 
of 60 inches (the top of the snow) for possibly as long as 30 years. 
Very frequently several laterals succeed in getting above the maxi- 
mum snow level simultaneously, forming a misshapen tree with sev- 
eral trunks'. As a tree so attacked increases in diameter the injury 
becomes less visible, but when the butt logs from such trees are sawed 
the lumber at the lower end is of very poor quality, full of pin knots 
and large loose knots from old dead leader stubs. 
No species of conifer is immune from these attacks, although the 
degree of damage varies widely with location and species ; and there 
is a certain degree of safety in numbers', conifers thinly scattered 
through the aspen being the most seriously attacked. Where groups 
of mature conifers are extending their borders into the surrounding 
aspen with multitudes of seedlings, the reproduction suffers much 
less. Of the important conifers, Douglas fir is usually eaten and 
damaged most severely, with white fir next. Alpine fir is badly 
attacked in the Wasatch National Forest, but practically not at all 
in the neighborhood of the Great Bas'in Experiment Station. Engel- 
mann spruce is not an active invader of aspen sites, and isolated 
seedlings are rare, so that its relative immunity may be due as much 
