ASPEN IN THE CENTRAL ROCKY MOUNTAIN REGION 21 
conditions. Rocky outcrops, swamps, and alkali areas are effective 
barriers to root extension. It seems more probable that at the close 
of Pleistocene times aspen came in by seed, occupying an ecological 
place similar to that which it holds to-day in the forests of Canada, 
spreading throughout the conifer types. The same drying and 
warming of the climate which reduced Lake Bonneville to the pres- 
ent Salt Lake and nfelted the glaciers also made aspen reproduction 
by seed more and more difficult, particularly as June is the driest 
month of the year. The temperatures, however, were not unfavor- 
able to the vegetative growth of the established trees. The progress 
of drying, furthermore, tended to hinder the reproduction of the 
conifers and to make them grow in open stands, thus helping to 
prevent the exclusion of aspen by suppression. 
The form of present-day stands, as well as the charcoal in the 
soil, proves that fires were prevalent in the aspen type during the 
last few hundred years at least. The impression is very strong also 
that the fire agency, coupled with the dryness of the climate, has 
for a very long time restricted the conifers, while it has tended to 
encourage the persistence of aspen stands. Nothing but the destruc- 
tion of all sprouts in three successive years, or suppression by the 
close growth of conifers, appears to be able to displace aspen from 
areas it now occupies. On the other hand, it is now unable to invade 
areas made favorable by fires or logging, from which it was once 
excluded, except through slow marginal extension by roots. (PL 
V, fig. 2.). 
The existence of islands of aspen which are clearly more than 
holding their own in the surrounding areas of sagebrush or chaparral 
suggests very strongly that these are the last relics of an old timber 
type, wiped out by surface fires, in which the aspen existed only as 
a scattered tree here and there. The progressive drying of the 
climate also has played its part in the formation of aspen islands in 
other types, leaving them in places where subsoil moisture is plenti- 
ful but the surface practically as dry as that on surrounding sites. 
These islands are common in mountains of sedimentary origin, 
where long, horizontal patches of aspen on dry hillsides mark the 
presence of subterranean outcrops of water-bearing strata. 
At the present time, natural stands under undisturbed conditions 
appear to advance very slowly into adjacent openings. (PL VI, 
fig. 1.) This may be due largely to the destruction of the scattered 
sprouts by grazing. In such stands, fires not severe enough to kill 
the trees have the effect of immediately extending the type boun- 
daries, as the roots sprout vigorously to their fullest extent, making 
on the margin of the old stand a new belt of aspen type. As a 
rule, this is about 15 feet wide, although sometimes it may be as 
wide as 50 feet. This is a very slow advance, however, since the 
process can probably not be repeated with success more often than 
once in 20 years, and actually takes place very infrequently. 
Sprouting occurs in aspen stands continually and even in virgin 
stands, but owing to the intolerance of the sprouts and the small 
amount of growth energy they have as long as the parent trees re- 
main alive, they are weak and inconspicuous and rarely live longer 
than a few years. Such sprouts are rare in young stands, but in- 
crease from year to year; and as the parent stand breaks up they 
become very numerous, forming an understory, if light is sufficient. 
