178 THREATENED ARIDITY ON THE PACIFIC SLOPE 



ridges it exists as a small, knotty, branchy, undersized tree. The 

 lodgepole pine follows the same general rule. The reproduction 

 of these two species is excellent, and they are constantly occupy- 

 ing new ground to the exclusion of the other species. 



The forest fires which ravage the mountains show how closely 

 balanced are the majority of the humid species and how slim a 

 hold they possess on existence along the front line of the spread- 

 ing subhumidity. It is a fact patent to every one who studies 

 the after-effect of a forest fire in this region that the increased 

 evaporation from the denuded surfaces causes intense soil aridity. 

 This condition is not alike in all places. Some localities, by 

 reason of local topography or exposure, suffer more severely 

 than others. There are thus on south-facing hillsides, near the 

 larger valleys, numerous places where centuries ago the subalpine' 

 forest was destroyed by fire, and arid conditions set in to the 

 extent of absolutely preventing reforestation to this day ; but 

 in the majority of cases the first burning of the forest destroys 

 only the more tender species and favors the growth of those 

 which possess greater power of adaptability. This in the humid 

 areas means a preponderance of the lodgepole pine, because of 

 its wide limit of tolerance to different climatic conditions. Fires 

 in the humid growths hardly ever destroy the forest completely 

 over any very large area. Small patches are left untouched, 

 though surrounded by wide lanes of burned forest. The growth 

 of lodgepole pine which comes in after the fires, because better 

 fitted than any other species to endure soil aridity, follows the 

 denuded areas and often covers them with dense stands. In so 

 doing it cuts off these slices of unburned forest from all chance 

 of regaining their former connection with the main body of their 

 own type of growth, and gives rise to conditions which are some- 

 what analogous to those in the semi-arid belts, where deforested 

 areas, supporting t} 7 pes of vegetation peculiar to arid regions, 

 separate the outlying groves of forest. 



It is not alone in the region of the Columbia river watershed 

 that the increasing climatic aridity is modifying or disturbing 

 the forest types and their ancient balance. The same phenomena 

 are repeated in California, and are doubtless general throughout 

 the Rocky mountains and the areas collectively termed " The 

 Pacific Coast." A conspicuous example occurs in southern Cali- 

 fornia, in the behavior of the big cone fir, Pseudotstcga macrocarpa, 

 and the redwood, Sequoia sempervirens. The big cone fir is a 

 common species on the slopes of the mountain ranges in south- 



