557 
THE RELATION OF PRAIRIE AND Forest IN ILLINOIS. 
The general relations of prairie and forest in the state have already 
been noted, and some of the invading forest species were listed with 
the secondary species of the prairie associations. Apparently any of the 
prairie associations may be invaded by forest under the present con- 
ditions. Four methods of invasion were noted: (1) forest invasion 
accompanying erosion by streams, (2) growth of tree seedlings along 
forest borders where the grasses are checked by the shade of over- 
hanging branches, (3) the occasional establishment of isolated seedlings 
farther out on the prairie, and (4) the growth of adventitious branch- 
buds from the roots of certain trees and shrubs extending short distances 
into the prairie. See Plates LXXVI and LXXVII. 
Many of the early writers claim that wherever the prairie was 
protected from annual fires the forest gradually encroached upon the 
grass-land. A quotation from Gerhard is interesting in this connection: 
“The first efforts to convert prairies into forest land were usually made 
on the part of the prairie adjoining the timber. A range of farms, which 
girded the entire prairie along its circumference having been established, 
three furrows were plowed all around the settlements in order to stop 
the burning of the prairie for the whole distance of the circuit in the 
neighborhood of these farms; whereupon the timber quickly grows up 
spontaneously on all the parts not burned, the groves and forests com- 
mencing a gradual encroachment on the adjoining prairies, so that one 
after another concentric circle springs up inside of the preceding, and 
thus the entire prairie is steadily narrowed from all sides until it is 
finally occupied, forming a vast region covered with timbers and farms.” 
Whether all of the prairie area of Illinois would have ultimately 
become forested under natural conditions is a matter of speculation. 
Within a few decades man has drained the sloughs and destroyed the 
prairie turf through cultivation with the result that trees may now be 
grown throughout the state. The development of natural drainage would 
no doubt in time have reduced the area of sloughs, destroyed a limited 
amount of prairie turf, and led to an increase of forested area. A basis 
of fact for this conjecture is found in the older glaciated regions with 
their older and better-developed drainage systems where forest occupies 
a greater proportion of the area than on the youthful topography of the 
more recently glaciated region. But forest invasion on the better-drained 
Andropogon prairies is exceedingly slow, and the existence of rather 
extensive Andropogon prairies on the older glaciated regions of the state 
shows that prairie vegetation may dominate for many thousands of 
years. 
When the Deciduous Forest Formation comes into competition with 
the Prairie Formation, the Andropogon furcatus Association may be suc- 
ceeded by several of the associations of the Deciduous Forest Forma- 
tion. This is not to be interpreted as saying that the Prairie Formation 
is a transition between Plains Grass-land and Forest; neither is it to be 
