170 THREATENED ARIDITY ON THE PACIFIC SLOPE 



than one and a half to two inches in length. A most conspic- 

 uous example of this phenomenon are the yellow-pine forests on 

 the northern slopes of the Paulina mountains, where thousands 

 of individuals bear cones but a trifle longer than those of the 

 lodgepole pine, and the cone dimensions of the entire growth are 

 far below the normal. As this tract of forest is separated only 

 by a narrow strip of semi-aridity from areas of intense aridity, it 

 is naturally under a high stress of the latter condition of climate, 

 and the inference is fully warranted that the decrease in cone 

 dimensions is a forerunner to general sterility in the species upon 

 these particular areas. 



Going farther northward there are seen thousands of localities 

 throughout the yellow-pine areas which are absolutely deforested 

 or contain a few, very old, lone individuals. Some of these tracts 

 consist of south-facing hillsides, which receive the full force of 

 the desiccating rays of the sun. If they, in addition, possess a 

 high angle of slope, causing too rapid drainage, soil aridity is 

 likely to result, with consequent deforestation. But many bare 

 tracts exist when soil aridity is not a factor, and the influence 

 which prevents the spread of the adjacent forest into such areas 

 .must be climatic, so far as can be determined. 



The middle and upper portions of the subhumid belts carry, 

 in addition to the yellow pine, the other species enumerated. 

 Two of them exhibit clear traces of yielding to the effects of 

 semi-aridity. They are the great silver fir and the red fir. The 

 former is exceedingly deficient in cone production, but yields a 

 high percentage of seeds with germinating power; the latter is 

 a free cone producer, but matures an insignificant proportion of 

 its ovules. In this respect it acts exactly similarly to the west- 

 ern juniper. The great silver fir possesses small powers of adapt- 

 ability. On the western spurs of the Bitter Roots it has devel- 

 oped a type of tree low, small in diameter, soft and sappy in its 

 wood, short-lived, and with extremely scanty cone production. 

 This form takes a lower place in the subhumid zone — that is to 

 say, nearer to the line of semi-aridity — than does the larger and 

 more fruitful type. The adaptability of the red fir is of a much 

 higher type than the foregoing. Notwithstanding its deficientseed 

 production/there is no evidence that it is not maintaining the in- 

 tegrity of its stands throughout our region. It is not confined to 

 subhumid areas exclusively, thriving and developing its largest 

 dimensions in extremely humid situations on the west slope 



