THREATENED ARIDITY ON THE PACIFIC SLOPE 169 



The western yellow pine occurs generally throughout the en- 

 tire suhhumicl area in this region. In course of time it has 

 succeeded in establishing a high degree of adaptability to the 

 desiccating climatic changes, and it therefore forms the extreme 

 rear of the coniferous growth in the subhumid belt, receiving 

 the full force of the oncoming semi-aridity. While the tree thus 

 shows its drought-resisting power, it is erroneous to suppose that 

 it has reached a stage of adaptation where it absolutely requires 

 dry regions for its development. The heaviest stands of western 

 yellow pine that have come under my notice, varying from 30,000 

 to 50,000 feet B. M. per acre, occur in small patches in the Selway 

 basin of the Bitter Root forest reserve, where the precipitation 

 probably is not less than 70 or 80 inches per annum. Where 

 the species is found under such conditions, it is fair to assume 

 that it represents the more ancient form, capable of enduring 

 more humid environments than the forms which now make up 

 the bulk of the species in these regions. As a rule, however, the 

 tree occupies the lower areas of the subhumid regions, and is 

 mostly of open or scattered growth. 



Coming now to the effects of semi-aridity upon the growth of 

 this species, we may observe that as a rule it has not progressed 

 far enough to seriously affect its reproductive capacity over any 

 very large area. We find, however, where the species borders 

 the semi-arid tracts of greatest intensity or where here and there 

 long narrow tongues, lobes, or thin lines of it project several 

 miles from the main body of growth into them, that in such 

 localities the reproductive capacity of the tree is exceedingly 

 limited or altogether wanting. In other places, especially in 

 eastern Oregon, where a few small groves or single trees are found 

 crowning some isolated eminence entirely cut off from all direct 

 connection with the species elsewhere, the same condition is 

 noticeable; and, precisely as is the case with the western juni- 

 per, the ovules are generally unfertilized, or, if fertilized, most 

 of them abort, and those that are fertile and develop into perfect 

 seeds fail to germinate. In consequence, seedlings are rare or 

 altogether lacking in such localities. One of the phenomena 

 noticeable in this species, when much exposed to the desiccat- 

 ing influences proceeding from contiguous arid tracts, is a re- 

 markable dwindling in its cone dimensions. Normally, in these 

 regions, the mature cones are from three to four inches in length ; 

 but where the species occurs in proximity to the deforested areas 

 on the eastern Oregon plains, the cones are frequentl} 7, not more 



