20 BULLETIN 1291, TJ. S. DEPARTMENT OF AGRICULTURE 
to the settlement of the country. After the mountains were invaded 
by the logger and stockman, there was a period of frequent and large 
fires, after which fires became fewer and fewer, and now virtually 
none occur. 
A very light fire will kill aspen, as the bark is thin and green, 
with no protecting corky layers; while basal scars, which lead to 
destructive heart rot, will be made on good-sized trees by the lightest 
of fires. In such surface fires, however, the temperatures in the 
ground never become very high and root suckering is not impaired. 
Conifers are, of course, more resistant to fire when past the sapling 
stage, but once destroyed they seed in slowly. A 50-year fire rota- 
tion would probably keep conifers entirely out of all the aspen 
type, except on north slopes or 'in moist localities favorable to the 
rapid development of the coniferous trees, although aspen would 
flourish under such conditions. But under present conditions, fire 
is not a factor to be reckoned with in forest management in the 
aspen zone. 
CLIMATIC FACTORS 
Injurious climatic factors are not of sufficient importance either 
to aspen or the associated conifers to affect management. Wind- 
fall in aspen is usually clearly due to excessive butt or root rot 
and is seldom extensive over large areas, being usually confined to 
single large trees or groups of trees. In conifers associated with 
aspen, windfall is occasionally widespread; but nowhere- is it of 
enough importance to cause any modification of management in 
favor of aspen. 
Frost frequently retards aspen seriously, particularly the tender 
sprouts. When late spring frosts occur, not only the leaves but 
also the twigs of the last year's growth may be frozen, and the 
second crop from older dormant buds may not become well de- 
veloped until well into July. 
These frosts, or more properly freezes, may occasionally be so 
severe that entire full-grown trees are killed outright. This oc- 
curred in many parts of Utah and southern Idaho, especially within 
the Engelmann spruce zone, during the freeze of May 31, 1919. 
Temperatures probably fell as low as 12° to 15° F. at this elevation, 
at a time when the buds were just bursting. In the Douglas fir 
zone, where the minimum temperatures probably ran from 17° 
to 20° F., injury was rare (7). 
Early fall frosts have no bad effect except killing back sprouts 
that have developed tardily, since growth ceases and hardening of 
the tissues normally begins during the last week in July at middle 
elevations (8,500 feet) in Utah. 
REPRODUCTION 
As there is practically no production of seed by aspen, at least 
not in Utah, this method of reproduction is of no importance in 
the management of aspen stands in that State. Suckering from the 
roots is the method by which regeneration takes place. (PL V, fig. 
1.) It is almost inconceivable that this was always the case, as the 
aspen could hardly have spread by sprouts alone so widely or into 
so many small, isolated areas, even in times of more favorable climatic 
