MICHIGAN ACADEMY OF SCIENCE. 
77 
INVADING SPECIES. 
Abies balsamea (seedlings). 
A ccr i 'i i b r um ( s eed 1 in gs ) . 
Acer saccharum (seedlings). 
Tilia americana (seedlings). 
Fagus grcindifolia (seedlings) 
Picea canadensis ( seed 1 i n gs ) . 
Thuja occidentalis (seedlings) 
Sambucus canadensis. 
D iervilla lonicera. 
Counts circinata. 
RELIC SPECIES. 
Erechtites hieracifolia. Epilobium august if olium. 
The Aspen (Pop alus) Association. 
In addition to the fireweeds, as the herbaceous type of vegetation 
developing after a fire, and the brambles, as a thicket type, there re- 
mains an arboreal post-igneous type of vegetation, the aspens. The 
aspen association usually secures greater prominence because it is com- 
posed of trees which impress the eye to a greater degree and because 
it is so much longer lived than the other fire associations. At the present 
day in this region there is no other association so wide-spred in extent 
nor occupying so many different edapliic conditions as the aspen asso- 
sociation. Following the numerous smaller and larger fires of the 
past ten and more years there are wide variations in the stations in this 
association depending upon its age. In the younger stages the general 
character is that of a dense shrubby growth composed of small trees 
of the dominant species as shown in Plate 1Gb. Later as these trees 
grow higher the association assumes the tree type which gives it a 
different appearance to an observer near at hand. For tree associa- 
tions this association is relatively short-lived, under ordinary conditions 
less than 35 years. Frequent fires, however, continue aspen possession 
for a much longer time. 
The association is composed of a large representation of individuals 
of Populus tremuloides, Populus grandidentata, B etui a alba pa pgr if era 
and Primus pennsylvanica , with a scattering and usually meager repre- 
sentation of several other tree species, a very few of which are relics 
which escapt the fire and most of which are invading species of the suc- 
ceding associations. Among the dominant trees of this association there 
are certain tendencies of distribution which were constantly noticed, 
altho no hard and fast lines could be drawn. Of the two species of 
aspens, which comprize about 80% of the average arboreal growth, 
Populus tremuloides prefers the moister sandy soils while Populus 
grandidentata is much more likely to be found on the drier or more 
xerophvtic situations. Birch, Betula alba papyrifera , a very common 
dominant species, not only prefers the moister situations but almost 
demands better soil. Consequently it usually found in depressions or 
hollows and along the edges of the lakes. On original hardwood land 
the percentage of birch to aspen is much larger than on pine land, very 
likely on account of the better soil. Primus pennsylvanica, is not so 
abundant in this region and here is a small tree, developing better on 
hardwood land than on pine land. 
The undergrowth of the aspen association is very characteristic. Usually 
from 85 to 95% of the ground is occupied by Pteris aquilina and Dier- 
