270 Gleason: Unsolved problems of the prairies 



are one or more definite orders of succession, leading from pioneer 

 to climax associations. The steps in the succession follow each 

 other in a regular series and constitute what may be called a nor- 

 mal succession. The normal succession for prairie associations 

 has not yet been fully investigated. Cowles has described a por- 

 tion of it in the pond-swamp-prairie series in the Chicago area,* 

 and I have discussed some of the successions on prairie-covered 

 dunes, t but this by no means exhausts the subject, 



8. It is a matter of common knowledge that in this region the 

 forest is everywhere pushing out upon the prairie. This matter 

 has been investigated by but few^ and we are still ignorant, if we 

 may depend upon the published accounts of direct local observa- 

 tion, of the factors which tend to retard or accelerate the advance 

 of the forest, or of the nature of the tension zone between the 

 two associations, or of those particular species which may be 

 called the pioneers in the forest advance. § It may be said at this 

 point that the forest undoubtedly invaded the prairie along two 

 distinct lines, a hydrophytic to mesophytic advance longitudinally 

 along the water courses, probably led by cottonwood, elm, maple, 

 and ash, culminating in the climax flood-plain forest, and a xero- 

 phytic lateral advance along the bluffs, led by oak and hickory. 

 Likewise we do not know what species, normally of the forest, 

 migrated independently into the prairie and there, constituting the 

 derived element of the prairie flora, possibly aided in resisting the 

 encroachments of the forest. Neither do we know why the 

 forest in central and southern Illinois invaded along the river 

 courses, while in northern Illinois it advanced also along the 

 morainic hills, nor what was the effect of prairie fires in checking 

 the spread of the forest. Each of the points is still open to direct 

 observation, to some extent at least ; and by a careful study of the 

 habitats of various forest species along country roadsides, at the 

 margins of woods, along small streams, and in woodland pastures, 

 we should still be able to get some idea of the structure of the 

 original forest margin, and with it of the pioneer species in the 

 forest advance. ' 



* Loc, ciL f55, 156. 



•\ Loc. cit. 158- 171. 



I Harvey [loc. cit,) gives a good general summary of this subject 



