SECOND PERIOD OF PEAIKIE DOMINANCE 81 



ful if they penetrated far into the forest, but by destroying the under- 

 growth and killing the more susceptible species, they gradually reduced 

 the forest to the open parklike condition known as oak openings. Here 

 the herbaceous vegetation was composed largely of prairie grasses, which 

 furnished additional material for fires and led to the annual extension 

 of the prairie through the eventual death of the remaining trees. The 

 forest was thus slowly pushed back to the river bluffs, where the greater 

 topographic diversity favored its persistence, or, where the bluffs were 

 low and with gradual slopes, was completely destroyed on the west side 

 of the streams. Fires even crossed the smaller streams, attacked the 

 forests of the eastern side, and by burning through them isolated areas 

 of forest toward the headwaters of the stream from the main body below. 

 This condition is shown in several places on the original land surveys 

 of central Illinois, and examination of them indicates that the separa- 

 tion was always in places of low relief, leaving forests isolated where the 

 bluffs were steeper or higher. 



On the eastern side of the forest, protected against fires driven by 

 the prevailing winds, encroachment by the prairie was less rapid and 

 doubtless in many cases negligible. It is probable that the forest 

 advance was completely stopped in some places, but may have continued 

 slowly in others. 



Along the moraines, which are generally characterized by a gently 

 rolling topography, the forests were destroyed completely, except in 

 those few favored situations where ponds or swamps on the west side 

 acted as an effective barrier to the fires. 



Another effect of prairie fires was the production of the so-called 

 barrens, areas characterized by a sparse growth of "scrub" oak 

 (apparently Quercus velutina in most cases), hazel, and wild plum. 

 The exact cause of the barrens is unknown, but descriptions from early 

 literature and accounts of personal observers in Illinois, Indiana, and 

 Iowa, agree that these three species of plants were the principal, if not 

 the only, woody species present, and that they seldom exceeded four or 

 five feet in height. They seem to represent a late stage in the degenera- 

 tion of the forest, where seedlings were established and the young plants 

 burned to the ground frequently without completely destroying the root 

 system. This developed further each year, reached a large size, and 

 sent up sprouts annually which persisted a year or two longer before the 

 next fire again killed them to the ground. 



During this same period, a few new isolated groves were established 

 on the prairies, but always in areas well protected from the fires. Two 

 such have been examined in Champaign County, Illinois, are mentioned 

 before in this paper, and described in more detail elsewhere 50 . They 



