ArTICLE XVI—dAn Ecological Survey of the Prairie Vegetation of 
Illinois.* By Homer C. Sampson. 
INTRODUCTION + 
The purpose of this survey was to determine as far as possible 
the composition and ecological relation of the prairie vegetation of 
Illinois. The prairie area of this state is an eastern arm of the prairie 
region proper which forms a distinct formation between the grass-lands 
of the Great Plains on the west and the deciduous hardwood forest on 
the east. While the origin of this prairie dates back to the changes in 
climate following the uplift of the Rocky Mountains and subsequent 
changes during and following the glacial period, the factors limiting its 
present distribution are to be found in the present climatic and edaphic 
conditions together with the biotic factors which affect succession from 
prairie to forest associations. 
'. The occurrence of all the great prairie regions of the world in 
temperate climates indicates that temperature plays a role, either directly 
or indirectly, in determining the general boundaries of distribution of 
the prairies. For the prairie under consideration, Transeau’s map (26), 
comparing the ratios of rainfall to evaporation from a free water sur- 
face in the three great vegetational areas noted above, shows rather 
clearly that definite ratios of these two important climatic factors, with 
which temperature is indirectly involved, coincide closely with the present 
limits of distribution. The western boundary of the prairie is marked 
by a rainfall equal to about 60 per cent. of the evaporation from a free 
water surface, while on the eastern boundary of the prairie the ratio of 
rainfall to evaporation is about 80 per cent. Similarly the rainfall-evapo- 
ration ratios marking the boundaries of the Great Plains are 20 and 60 
per cent., while those of the eastern forest range from 80 to 130 per cent. 
The prairie therefore lies in a region in which the ratio of rainfall to 
evaporation ranges from 60 to 80 per cent. (See map on next page.) 
The presence of local areas of forest together with man’s ability to 
carry on silviculture throughout the greater part of this range is suff- 
cient evidence that the occurrence of prairie vegetation is not limited 
by climate alone. Edaphic factors and the difficulty with which a forest 
vegetation invades and succeeds that of the prairie must also be taken into 
account. As the complex of climatic factors approaches the limiting 
conditions for forest development, the weight of the edaphic factors be- 
* Papers from the Department of Botany, Ohio State University. No. 126. 
j For non-technical summary, see page 569. 
