MICHIGAN ACADEMY OF SCIENCE. 
79 
that the oaks in this region merely indicate a step in succession be- 
tween aspen and pine which is better seen in regions farther south. 
There need be no such step in the genetic series in this region but its 
presence indicates rather definitely that a given station is tending toward 
pine rather than hardwood. 
In the case of the average hardwood burn, the aspen association is 
quite frequently present but represented by a large percentage of Betu-la 
alba pcipi/riferw and the large numbers of seedling and small hardwoods 
readily indicate the approaching hardwood. In case the burn is so 
severe as to burn out the humus, the normal aspen association takes 
possession and the station tends towards pine rather than hardwood. 
In the burns and clearing in the Picea-Abies association, the bramble 
association usually obtains dominance and by its shade precludes the 
aspen association. Occasionally it maV obtain a weak start but the 
ground is so suitable for the spruce and balsam that they very quickly 
succede it. 
A cleared bog normally returns to bog without any intervening step 
but a burnt bog may or may not do so. If the burn is not too severe 
there is a brief dominance of fireweed followed by the bog species. In 
case of a severe burn, as shown in Plates 20a, 20b and 21a, the fireweed 
is followed by aspens — especially Populus tremuloicles — and the aspen 
association obtains a temporary dominance on account of the lack of 
available food materials for bog species in the soil rather than for any 
other reason. The soil is wet, too much so for good development of 
aspen, and the growth is continually killed off at the margins by the in- 
vading bog species, which in a few years regain their dominance. 
With a rising of the water table the bogs can easily invade the terri- 
tory now occupied by the aspens but with a lowering of the water table 
the aspens cannot invade the bog, on account of the dense shade, until 
the bog trees die and fall. 
Some of the land east of Pellston shown in PI. 16b, which was very 
severely burnt a few years ago and has since come up to aspen, particular- 
ly Populus grandidentatci with a mixtur of Prunus pennsylvamca and a 
ground cover of Poa pratensis, Diervilla lonioera , Pteris aquilina, and 
Gncipli aliuni decurrens , with hardly another species of plant for miles, 
shows indications of becoming a “Pine barren” by the occasional pres- 
ence of seedlings of Pinus banksiana. These seedlings are healthy and 
appear to be developing quite rapidly. In case this association 
does become developt before white or red pine should get in, it would 
be an extension of the pine barren country into this region where, upon 
the advent of the lumberman, it was not present. 
List of the Species of the Aspen Association : 
A. Trees. 
' DOMINANT SPECIES. 
Populus tremuloidcs f 
Populus grandidentatn . > 06%. 
Betula alba papyrifera. ) 
Prunus pennsylvanieu. 
