PME RRS Tem MES CRS TREE Ny MY OUT Cale ANCE ET A 
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126 ILLINOIS ACADEMY OF SCIENCH — 
the influence of topography in control or modification of vege- 
tation-distribution and vegetation-development. They show 
that while forest invasion of prairie is general, it is by no 
means universal. 
INVASION OF FOREST LAND BY PRAIRIE ALONG 
RAILROADS 
ArTHUR G. VesTaL, EasterN ILLinois State Normat ScHoon, 
CHARLESTON 
The Big Four, Clover Leaf, and other railroads in central 
Illinois, in crossing the numerous and generally forested stream 
valleys, make treeless paths through many wooded areas. The 
rights of way in these cleared areas become vegetated from 
nearby plant populations, which are of three types, forest, 
prairie, and ruderal. The forest immediately adjoins these 
cleared areas, and would in most cases quickly reproduce itself 
if the railroad companies would allow seedling trees to reach 
tree size, which they do not. However, many forest herbs and 
certain forest shrubs, including sumac, blackberry, hazel, and 
Symphoricarpos, make up the dominant vegetation in such 
places, especially on the east side of streams, where the forests 
are more extensive, having there been well protected from 
former prairie fires, as shown by Gleason. 
Certain other cleared parts of the rights of way are not far 
from areas of prairie vegetation, and have received abundantly 
the seeds of prairie as well as forest plants. In numerous 
small areas one finds mixed communities of prairie and forest 
herbs. It is probable that these mixed growths may in places 
be relatively enduring. The new growths of mixed forest and 
prairie plants which must have followed the original felling of 
the trees were probably unstable, as many of them now are. 
Their development either into forest growth minus the trees or 
else into typical prairie, probably depended, as it does now, 
upon a number of factors. Among those which favor the 
development into prairie are: coarse well-drained soil; consid- 
erable exposure to wind and sun, as presented by certain topo- 
graphic situations; deficiency of rainfall during one critical or 
several successive growing seasons; and the destructive effects 
of burning or of mowing, both of which are common on rights 
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