INTRODUCTION 



THE following discussion of the distribution of forests is 

 concerned with that part of the Rocky Mountain region 

 and neighboring plains which is included within the 

 State of Montana and adjacent parts of Idaho. These boun- 

 daries furnish a convenient definition of an area which presents 

 certain aspects of forest vegetation and which by reason of its 

 extent and location exhibits features of special interest. Here 

 on both slopes of the main divide appears a wide overlapping 

 of the Atlantic and Pacific floras and, while the hemlocks, 

 larches and some other forest species of the north reach their 

 farthest southward extension in the Rocky Mountains, they too 

 overlap, though at different elevations, the most northward 

 reaching elements of the southern vegetation as represented by 

 cactus, yucca and other forms. Moreover, factors influencing 

 the local distribution of species more widely prevalent are mod- 

 ified at least in their intensity by latitude, altitude and other 

 causes. 



The complex interaction of climatic, edaphic, biotic and 

 other influences which guide and control the distribution of 

 species is of keen interest to the student of phytogeography, and 

 no treatment of the subject is fair which does not seek to give 

 full weight to the many factors involved, and especially to the 

 nature and requirements of the individual species. Similarly 

 the ecology of local areas must be studied closely from the 

 standpoint of the adaptability and demands of the species of 

 those areas, if lasting results of importance are to be obtained. 

 All questions concerning the dominance of some species and the 

 suppression of others are questions of the interaction of the 

 constituent species and their environment. 



Among such questions are the specific light, moisture and 

 temperature requirements at different stages of development and 

 under particular conditions, the rate of growth and the form 

 during youth and maturity, the form and extent of the root 

 system, its strength, functional activity and relation to the soils 

 of particular composition and structure, the reproductive ca- 



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