TREES OF THE UNITED STATES IMPORTANT IN FORESTRY. 51 



In addition there are probably nowhere to be seen such extensive fields of distribution of single 

 species. 



These two facts are probably explained by the north-and-south direction of the mountain 

 ranges, which permitted a reestablishment after the Ice age of many species farther northward, 

 while in Europe and the main part of Asia the east-west direction of the mountains offered an 

 effectual barrier to such reestablishment, and reduced the number of species and their field of 

 distribution; nor are the climatic differences of different latitudes in North America as great as in 

 Europe, which again predicates greater extent in the fields of distribution north and south. On 

 the other hand, the differences east and west in floral composition of the American forest are 

 greater than if an ocean had separated the two parts instead of the prairie and plains. This fact 

 would militate against our theory that the intermediate forestless region was or would be eventu- 

 ally forested with species from both the established forest regions, if we did not find some species 

 represented in both regions and a junction of the two floras in* the very region of the forestless 

 areas. In the sand hills which traverse Nebraska from east to west there are now found in eastern 

 counties the sand-drowned trunks of the western bull pine, and the same pine belonging to the 

 Pacific flora is found associated with the black walnut of the eastern region along the Mobrara 

 River. 



Of the many species which in each of the forest regions compose the forest, only a limited 

 number can be classed as economically valuable, although the question as to what is valuable is 

 not one readily answered, since many trees which appeared valueless at first have proved their 

 usefulness when better known or when the more serviceable timbers became scarcer. The trade 

 papers quote at best only 35 to 40 kinds, of which only 10 to 12 are regular staples. In addition, 

 some species possess value to the forester in his silvicultural operations, as nurses, soil cover, etc., 

 which to the wood consumer appear only as tree-weeds. Finally, some species, like the lodgepole 

 pine of the Northern Eocky Mountains, are most valuable from the national economic point of 

 view, because covering large areas of mountain slopes, thereby not only furnishing wood supplies, 

 albeit of an inferior character, to the resident population, but covering the watersheds and favorably 

 influencing soil and water conditions. 



The selection of species to be considered economically valuable, therefore, must be, to some 

 extent, arbitrary, and be guided by a variety of considerations in which those of the present may 

 vary from those of the future. The relative value of those selected may also change from time to 

 time and from locality to locality. 



Thus for the present we can dismiss from consideration the 60 to 70 species of tropical origin? 

 importations from the West Indies, found along the coast and keys of Florida in small quantities, 

 as economically of little consequence on account of the small area which they do and can occupy. 



Another similar exclusion may be made of some species which overlap from the Mexican flora, 

 some 26 or 27, with but a confined distribution in the United States. There remain then about 360 

 species which call for a discriminating classification, and if we exclude all species which, as a rule, 

 do not exceed 1 foot in diameter, we decrease this number again to, say, 235 species, which, possibly, 

 may enter into the consideration of forest management and are of economic value. 



A full checklist of the entire arborescent flora is to be found (besides the magnificent work, 

 the Silva, by Prof. O. S. Sargent, which describes this flora fully by word and picture) in Bulletin 

 14 of the Division of Forestry, and a more condensed statement in Bulletin 18. For the present 

 report, which is to consider economic questions mainly, the list given in the next few pages, being 

 reproduced from the Annual Eeport of the Division for 1886, somewhat revised, may suffice. 



Trees of the United States Important in Forestry. 



[The relative value of the different species here enumerated is indicated in three classes Tby difference in type, as iollows: First grade, 

 WHITE PINE ; second grade, JEFFREY PINE; third grade, PITCH PIiVE. 



The size stated refers to averages of mature trees; the + sign denoting that larger dimensions are not uncommon.] 



A. CONIFERS. 



(Evergreen and needle-leafed trees, with a few exceptions.) 



The most valuable forest trees, as well on account of their usefulness as for their effects in forestry, due to the 

 evergreen foliage of most of them persistent through several years; most capable of covering extensive areas exclu- 

 sively, and with deciduous trees most excellent aids in forestry on account of their habit of growth and their soil- 



