THREATENED ARIDITY ON THE PACIFIC SLOPE 175 



is here phenomenally high — a significant fact in connection with 

 the wide extensions of subhumid and semi-arid conditions into 

 the interior of this range, and a possible consequent rise of the 

 mean annual temperature. The absence of a timber-line even 

 at the highest peaks was noted and commented upon by the 

 various parties engaged in the first surveys for a northern trans- 

 continental railroad route, but was generally ascribed to the 

 effects produced by a current of warm air supposed to move 

 eastward from the plains of the Columbia in this latitude. 



None of the three species contained in the summit group of 

 trees possesses any marked power of adaptation. The Lyall 

 larch is wholly deficient in this respect. The white-bark pine 

 ranks slightly higher, and the mountain hemlock somewhat 

 above the latter, as shown by its occurrence within undoubted 

 subhumid conditions in some localities, as in the middle portion 

 of the Deschutes basin in Oregon. In the Bitter Roots we find 

 the Lyall larch along the high crests of the main range from a 

 point just north of Nez Perce pass to an as yet undetermined 

 northern point. However, it does not go very far be}^ond the 

 ridges which bound the north fork of Clearwater basin. It is 

 found on both the east and west slopes of the range, extending 

 three to four miles away from the crest on either side. The 

 western spurs of the range present one or two outlying small 

 groves of the species on the divide between the Lochsa and 

 Selway forks. Its habitats in the Bitter Root range are abso- 

 lutely cut off from all connection with others elsewhere by gaps 

 of low altitudes a hundred miles or more in width, which now 

 cannot possibly be spanned by the species. In these regions this 

 larch is clearly approaching extinction. Its cone and seed pro- 

 duction are extremely scanty. Its growth is excessively slow. 

 Most of the individuals which make up the stands are far ad- 

 vanced in age. Seedlings or saplings are rare and scattered. No 

 farther back than three centuries there must have been abundant 

 seed production, as a majority of the trees are approximately of 

 this age. Three centuries hence the stands, if existing at all, 

 will show great diversity of age, unless the cone-bearing periods 

 run in cycles, long intervals of barrenness being followed by 

 periods of fertility. Whatever rotation may exist in this respect 

 (and that some does occur admits of no doubt) it operates only 

 within narrow limits of time, producing what are called " off 

 years," and does not impress itself very strongly upon the stand 

 of the species as a whole. 



