14 * BULLETIN 1496, U. S. DEPARTMENT OF AGRICULTURE 
at the same time. Once aspen stands are thoroughly established, the 
invasion of such conifers as white pine or spruce is exceedingly slow. 
The impression that the pine and spruce appear in the aspen stands 
as late comers is due to their slower growth when young. This 
makes them seem to have come up under the shade of the aspen. 
Balsam fir, however, does come into aspen and birch stands long 
after they are fully established. 
The reason for the slowness of the conversion of aspen stands into 
conifers may be the lack of available seed supply, dense shade under 
the aspen, and the ravages of rabbits. Rabbits, in years when they 
are abundant, are particularly destructive to white pine, and less so 
to spruce and balsam fir. Whatever the cause may be, once aspen 
and birch take possession of the cut-over area, white pine and spruce 
get a foothold only with great difficulty. The conditions at the 
time of cutting or burning determine, therefore, to a large extent 
whether the aspen stands will in one generation be succeeded by 
conifers. 
A survey of aspen stands throughout the region shows that not 
more than from 15 to 20 per cent of them have enough white pine or 
spruce to replace them in the course of 50 or 60 years when the 
aspen begins to deteriorate. Since aspen stands cover close to two- 
thirds or even three-fourths of the entire cut-over area of the orig- 
inal hardwood and white-pine forest, some 21,000,000 acres of aspen 
will remain for all practical purposes nothing but poor aspen—birch 
stands, succeeded here and there by balsam fir, unless converted with 
the aid of man into the more valuable forests of white pine and 
spruce. 
SWAMP FORESTS LITTLE CHANGED BY LOGGING 
There are some 9,000,000 acres of swamps and swamp forests in 
the Lake States region. A great deal of the swamp land is practi- 
cally waste, since the growth in a typical swamp is not over one- 
sixth of a cord per acre per year and often is less. It is doubtful 
whether the swamp forests at their present rate of growth lend 
themselves to profitable forest management by private owners. Yet 
the swamp forests are the only extensive virgin forests remaining 
in this region, and they are the source of pulpwood, posts, and poles— 
material that is getting increasingly scarce. 
The swamp forests are rarely cut clean, except when pure black 
spruce is cut for pulpwood, or when the land is drained and cleared 
for other use. If cutting is not followed by fire the original type will 
follow, except that where cedar was present in the old forest it will 
probably predominate in second growth. Cedar is the most aggres- 
sive in reproduction of all species found in northern swamps. It 
will often make up 75 to 90 per cent of reproduction. If cutting is 
followed by a light fire—that is, if the fire occurs when slash is fairly 
dry but the ground still moist—the same type will follow but with 
greater or less mixture of aspen, birch, red maple, black ash, balsam, 
poplar, and alder (Alnus incana). If, however, the peat is dry and 
the fire burns up the surface soil, the original type will reestablish 
itself only slowly, if at all, and alder, willows, and similar species 
will occupy the ground for indefinite periods, 
