Annals of the Association of American Geographers 

 Volume XII, pp. 39-85 



THE VEGETATIONAL HISTORY OF THE MIDDLE WEST 

 HENRY ALLAN GLEASON 



CONTENTS Page 



Part. I. Introduction 



The Dynamic Viewpoint 40 



General Control of 'Distribution 40 



Vegetational History and Human History 40 



Outline 41 



Part II. The General Nature of Plant Migrations. 



Invasion, Migration, Ecesis 42 



Efficiency of Plant Dispersal 42 



Migration and Succession 43 



Rate of Migration 44 



The Time Factor and Migration 45 



Significance of the Margin of Range 47 



Advancing and Retreating Migrations in the Middle West 47 



Effects of Changes in Physiological Requirements 47 



Migrations as Related to Continuity of Suitable Habitats 48 



The Interpretation of Isolation 49 



Adjustments in Distribution 50 



General Results of Continued Migrations 50 



Summary of Part II 51 



Part III. The Evidence of Plant Migrations. 



Historical Evidence of Migrations 52 



Succession as Evidence of Migration 52 



Determining Migration Centers 53 



The Five Elements in the Flora of the Middle West 54 



Glacial vs. Modern Climate 54 



Relic Colonies 55 



Summary of Part III 56 



Part IV" The Development and History of Vegetation in the Middle West. 



Preglacial 57 



Early Glacial Stages 59 



The Mississippi Embayment 59 



Northern Limits of Southern Plants 60 



The Migration of the Conifers 61 



The Migration of Deciduous Forests 61 



Wisconsin Glacial Period 62 



Distribution of Plant Life During the Wisconsin 63 



Climate in the Early Post-Wisconsin 66 



Tundra and Coniferous Forest Migration 67 



Early Migration of the Prairie Flora 70 



Early Migrations of the Deciduous Forests 73 



The Southern Migration 74 



The Northern Migration 74 



The Combined Effects 75 



Migration and Evolution 75 



Prairie and Boreal Relics 76 



Development of the Prairie-Grass Formation 77 



Extent of Forest Migration in the Prehistoric Period 78 



Second Period of Prairie Dominance 80 



Second Period of Forest Advance 82 



Coastal Flora of the Great Lakes 83 



Summary of Part IV and Conclusion 84 



' 39 



