SUMMARY 173 



when conditions were more favorable, or more recently by way 

 of the Okanogan country and the Selkirk Mountains, and enter- 

 ing Montana mostly by way of the Coeur d'Alene gateway, form 

 a large part of the plant covering on the western slope of the 

 divide. Most of the woody species represented bear either wind- 

 blown seeds or succulent fruits. Few heavy seeded species have 

 entered the region from any source. The eastern contingent is 

 less numerous among the woody forms. Some of these apparent- 

 ly have followed the streams across the plains, but the larger 

 number have made their way across the continent from the 

 Great Lakes to Alaska and have followed the Rocky Mountains 

 southward. 



5. The mountain ranges act as barriers to eastward and 

 westward migrations. The continental crest in itself is not the 

 efficient barrier, but the elevation of a large land mass, con- 

 sisting of the main chain and its parallel ranges east and west, 

 so affect the climatic conditions over a wide space as to in- 

 fluence the movements of plants. The Bitter Root and Cab- 

 inet Ranges form a secondary barrier west of the main divide. 



6. About 17 species of Gymosperoms make up the bulk of 

 the Rocky Mountain forest in Montana and Idaho. The number 

 of these is greatest along the western border of the area. Two 

 reasons are assigned for this. First the western part is nearer 

 the source whence most of the species probably came and, sec- 

 ond, the climatic conditions are there most favorable. From 

 the western side eastward this number of coniferous species 

 diminishes to three or four on the eastern side. 



7. Over much of the western part, of the State forest dis- 

 tribution at lower elevations conforms to topographic influences 

 in response to moisture, temperature or light. Above 5,000 feet 

 the forests become continuous over most of the mountain region, 

 clothing the mountains to their summits, except in some cases 

 where desiccating influences render the peaks untenable for 

 woody species. There is no summer snow line on the mountains 

 of this region and no upper timber line that is constant or clearly 

 defined or subject to a particular elevation. 



8. The life zones, according to Merriam, are represented in- 

 distinctly. The foothill region of yellow pine and Douglas spruce 

 represents largely the Arid Transition intergrading through the 



