THREATENED ARIDITY ON THE PACIFIC SLOPE 171 



of the Cascades ; but in the region under consideration it be- 

 longs to the subhumid areas, and, as before remarked, it is here 

 deficient in seed production. A factor enters here to be consid- 

 ered later. This is temperature conditions. It is evident that 

 unless a certain ratio of increase in the mean annual tempera- 

 ture accompanies the aridity, there is a limit of tolerance beyond 

 which certain species cannot be forced. When this limit is 

 reached the species must succumb, and this is probably the 

 reason why the red fir does not push far into the humid areas 

 in these regions. 



The lodgepole pine possesses the highest power of adaptability 

 among the subhumid group of trees. It ranges from the humid 

 down through the subhumid and well into the lower edge of the 

 semi-arid belts. While not a plentiful producer of perfected 

 seeds, most of the ovules aborting, it amply makes up for this 

 deficiency by its multitude of cones and the early age at which 

 it begins to produce them. If the present vigor of the species 

 continues, it promises to become the dominant one on all sub- 

 humid and humid areas in our region. In the subhumid forests 

 of eastern Oregon, along the lower slopes of the Cascades, three 

 species enter which are lacking farther north. They are : 



Abies concolor, White fir ; 

 Libocedrus decurrens, Incense cedar ; 

 Pinus lambertiana, Sugar pine. 



The white fir, perhaps not specifically distinct from the great 

 silver fir, occupies the same general place in the subhumid group 

 of trees on the more southern areas that the latter does on the 

 northern. We might even suppose that the great silver fir is a 

 modification of the white fir evolved to meet changing tempera- 

 ture and humidity conditions. It is evident from the relative 

 position which the white fir occupies that its limits of endurance 

 to increased temperatures and lower humidity are far higher than 

 those of the great silver fir. 



The incense cedar and sugar pine come into the middle areas 

 of the subhumid belts. Their distribution or retreat northward, 

 or into the humid areas, is limited by temperature considera- 

 tions. As they show no adaptability to meet them, their exten- 

 sion northward is precluded and their extinction will be rapid, 

 compared with other species in this region. The sugar pine is a 

 free cone and seed producer, while the incense cedar appears to 

 be deficient in this respect. 



