National Geographic : 1899 May
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.