64 PLANT SUCCESSION AND CROP PRODUCTION 



D. The Vegetation Centers Entering Ohio 



When the climatic features of a region are studied it is neces- 

 sary to correlate these features with the climatic conditions sur- 

 rounding it. We cannot understand the vegetation of Ohio as it 

 exists today without having something of the nature of the vege- 

 tation of Eastern North America in mind. 



Adams ('02) has pointed out the ideas of centers of flora and 

 fauna which have been accepted and amplified by ecologists- Since 

 plants migrate into and multiply in regions where the soil and 

 climatic conditions are favorable, we can regard the native vege- 

 tation as a criterion of the climatic and soil conditions. We must 

 not overlook the fact that after the plants are once established 

 they are themselves agents in altering climatic and soil conditions, 

 especially the latter, and they pave the way for the spread of other 

 species. From our point of view then, the plants are both passive 

 and active, are effect and cause. 



For the existing plant cover, it is not necessary to go back any 

 farther than post-glacial time following the last glaciation. The 

 vegetation in southeastern! Ohio is much older than that, however. 

 During glacial times southern Ohio became the refuge of northern 

 plants, retreating before the advance of the ice sheet. Along the 

 margin of the advancing ice was a series of bands which from 

 the ice outward were: (1) Arctic tundra, (2) a bog-shrub zone, 

 (3) northern evergreen forest. Beyond this the central deciduous 

 forest was little disturbed except for the advent of the northern 

 species. In the southeastern part of North America the southern 

 evergreen forest was not at all disturbed by the ice cap of the 

 northern part of the continent. These conditions were discussed 

 by Transeau ('03), Chamberlain and Salisbury ('06), and by 

 Clements ('16, p. 372). The climatic changes as recorded in the 

 peat bogs at that time have been discussed by Penhallow (1896, 

 '98, '00) and Harshberger (1911) and Dachnowski (1912, p. 209). 



There were several advances and retreats of the ice sheet, and 

 according to Leverett ('02) the glacial boundary is a combination 

 of four invasions, but when the ice finally made the retreat which 

 carries us into the present time, there was a gradual migration of 

 the vegetation northward. The shrub-bog zone advanced into the 

 tundra, the northern evergreen forest into the shrub, and the de- 

 ciduous forest regained its former terrain. There is no definite in- 

 formation as to whether the movement is still going on or has come 



