MICHIGAN ACADEMY OF SCIENCE. 73 



CLIMATIC CENTEES AND CENTERS OF PLANT DISTRIBUTION. 



E. N. TRANSEAU. 



Many attempts have been made to divide the continent of North Amer- 

 ica into zones or life areas which sliould represent natural divisions of 

 the flora and fauna, and aid in the investigation of the geographic dis- 

 tribution of plants and animals. The earlier writers divided the con- 

 tinent into several transcontinental zones by means of isotherms. These 

 zones were supposed in the main to correspond with the actual distribu- 

 tion of many ecologically or floristically related forms. Other authors, 

 laying special emphasis on the relation between vegetation and rainfall, 

 have recognized divisions having a generally north and south trend. 

 Schimper, following Sargent, bases the climatic formations of North 

 America on moisture mainly. Recently transcontinental zones have 

 been worked out by Merriam, which are based on "sums of positive tem- 

 peratures" during the growing season. These zones have been mapped in 

 considerable detail and correlated with the distribution of plants, animals 

 and crops. Still more recently, Adams has pointed out that both animals 

 and plants are distributed about certain centers and do n'ot follow trans- 

 continental lines. Certainly in the case of the greater plant formations 

 this is true, and transcontinental zones cannot be drawn which will in- 

 clude throughout plants of either the same ecological or floristic type. 

 Taking, for example, the Transition zone of Merriam, there are brought 

 together the forests of New England, the treeless plains of eastern Mon- 

 tana, the foothill forests of eastern Colorado, the pine lands of the Ari- 

 zona plateau, the wooded slopes of the southern Coast Ranges and the 

 dense conifer forests of the Puget Sound region. A greater mixture of 

 vegetation tjpes, viewed from either a floristic or ecological standpoint, 

 is almost impossible. While such zones may be of very great value for 

 an agricultural basis, they do not aid materially in pointing out forest 

 relationships. In fact in so far as the forests are concerned there is a 

 closer relationship between Merriam's Canadian, Transition and Upper 

 Austral zones in eastern North America than between the Alleghanian, 

 Arid and Pacific areas of the Transition zone. 



Investigation shows that forests, grasslands and deserts are arranged 

 about certain centers, which owe their positions on the continent mainly 

 to climatic causes. That such centers cannot be correlated with the dis- 

 tribution of heat or rainfall alone is evidenced by an examination of the 

 monthly, seasonal and annual distribution of these climatic elements. 



The fact that so large a part of the adaptations shown by plants are 

 more or less directly connected with transpiration, led the writer to con- 

 struct a map combining the figures for rainfall and evaporation. The 

 amount of evaporation depends upon the temperature of the evaporating 

 surface, the relative humiditv of the air and the velocitv of the wind. 

 Therefore if we combine the figures for rainfall and evaporation we have 

 a number which will represent at least four climatic factors, that must 

 powerfully influence the water relations and distribution of plants. 

 10 



