68 H. A. GLEASON VEGETATIONAL HISTORY OF MIDDLE WEST 



the tundra persisted until destroyed by the warmer climate or by the 

 succession of the coniferous forests. In general, tundra withstands 

 a climate slightly warmer than its optimum better than the competition 

 of forest species, which quickly destroy it by shading. 22 As in all 

 such retreating migrations, relic colonies were isolated, surrounded by 

 the advancing forests, and persisted for a longer or shorter time until 

 finally completely overgrown by the spruces and firs. Naturally those 

 endured the longest which occupied habitats least favorable to forest 

 growth. We may presume that the sand dunes along the shores of 

 the postglacial lakes and such isolated rock outcrops and cliffs as 

 Starved Rock in Illinois and the Dells of the Wisconsin River were 

 for a long time occupied by tundra plants, just as they are now occu- 

 pied by relics of the coniferous forests. The xerothermic period and 

 ordinary successional processes have long since destroyed the last vestige 

 of tundra vegetation in these latitudes, and the southernmost species 

 are now found scarcely farther south than Lake Superior. On Isle 

 Royale 37 associations still exist which bear a general resemblance to 

 the arctic tundra and contain a few species of distinctly northern dis- 

 tribution. As the tundra migrated northward into cooler and moister 

 regions, and the cooling influence of the ice was felt farther beyond 

 its margin, the belt of tundra broadened, that is, it migrated toward 

 the north faster than it retreated from the south, until it now occupies 

 an area some hundreds of miles wide. 



The boreal coniferous forests which had occupied a narrow or inter- 

 rupted strip along the glacial margin during the Wisconsin stage also 

 migrated northward following the retreat of the glaciers, surrounding 

 the relic colonies of tundra and eventually replacing them by succession. 

 The pioneers in the movement were probably then as now the xerophytic 

 species advancing along the rock outcrops and till of the upland, and 

 the bog species proceeding along the drainage lines and the glacial 

 lakes. These forests also advanced toward the north faster than they 

 retreated from the south, and consequently occupied a strip of increas- 

 ing breadth. This led to their present dominance over a large area 

 north and east of the Great Lakes and to their temporary dominance, 

 which is still continued in many habitats, in northern Minnesota, Wis- 

 consin, and Michigan. Their northward movement brought the coni- 

 fers into climates progressively cooler and relatively moister. This led 

 not only to a greater number and size of individuals, but also per- 

 mitted a northwestward migration into northern Minnesota and 

 Manitoba. This northwestern prolongation was eventually extended, 



37 Gleason, H. A. The ecological relations of the invertebrate fauna of the Isle 

 ftoyale, Michigan. Eep. Mich. Geol. Survey 1908: 57-78. 1909. 



