162 THREATENED ARIDITY ON THE PACIFIC SLOPE 



nize differences such as that of texture and color of wood, varia- 

 tions in bark characteristics, or in the general port of the various 

 individuals of a species upon any given area, but the differences 

 are not such as to indicate that they constitute a definite and 

 sharply determined trend in adaptability. They rather convey 

 the impression that they are a series of expiring gasps of a type 

 of vegetation which reached its culminating point of develop- 

 ment immensely far back in time, and is now on the road 

 toward complete extinction. 



The forest areas in this region which have been more closely 

 examined than any other in relation to the effects of increasing 

 aridity are the tracts adjacent to and encircling the Columbia 

 watershed in Idaho, eastern Oregon, and eastern Washington. 

 We shall first examine the tracts lying within these limits, thence 

 passing to others elsewhere, not so well known. 



When the coniferous flora of the region is investigated it is 

 found that certain species have a far higher ratio of endurance 

 to conditions of aridity than have others. This might be taken 

 to indicate a certain degree of adaptability, but the strongly 

 marked characters which separate the species were acquired 

 ages ago, and, with the exception of one or two species, do not 

 in our region in the present age show any marked evolutionary 

 tendencies. 



The minor effects of the encroachment of aridity upon the 

 forested areas are many, but comparatively unimportant. 



The greater effects are contained in one general phase, which 

 strikes at the very foundation of the species' existence. It con- 

 sists in a gradual loss of reproductive power in the individual 

 trees, and hence in the species as a unit, and is marked by two 

 periods. In the first we have a gradual crowding back to more 

 humid tracts of such species as require a considerable degree of 

 soil and atmospheric moisture for their growth. They are re- 

 placed by others capable of enduring subhumid or distinctly 

 semi-arid environments. In the second period we have a gradual 

 crowding out or a complete extinction of the species of replace- 

 ment, hastened or caused in the latter, as in the former, case by 

 a loss of reproductive vigor, and a final complete deforestation 

 of the particular area and the creation of a treeless region. 



There are three general types of climatic conditions to which 

 the term arid will apply. They are semi-arid, arid, and desert. 

 As here employed, the semi-arid are regions not necessarily de- 

 forested, but which support a tree growth of peculiar species in 



