174 THREATENED ARIDITY ON THE PACIFIC SLOPE 



mistakably progressive semi-aridity into the subhnmid tracts, 

 subhumid extensions into the humid areas. 



In the semi-arid belt we noted the occurrence of detached 

 aggregates and scattered individuals of its forest growth sepa- 

 rated from the main body by deforested lanes and wide stretches. 

 They were taken to represent the effects of a gradual invasion of 

 the adjacent arid conditions, creating a sort of fringe or frayed 

 edge of timber growth along the edge of the forest. If our ideas 

 of progressively drier conditions extending throughout the dif- 

 ferent belts of humidity are in accordance with facts, we have a 

 right to expect analogous phenomena in the humid and sub- 

 humid areas. That is exactly what we find, but they differ from 

 those which exist in the arid and semi-arid region in this way : 

 that the edge of the advancing semi-aridity into the subhumid 

 tracts and the front of the subhumidity where it penetrates into 

 the humid areas are not typically marked by deforested open- 

 ings. Instead, they present detached groups of the species, which 

 belong to the upper and more humid tracts of each of the zones, 

 entirely surrounded by heavy bodies of the kinds which belong 

 to the lower zones and which are capable of withstanding greater 

 dryness. 



In examining the phenomena of forest growth in the humid 

 areas, as changed or in process of modification by the shifting 

 climatic conditions, we can find no localities within these regions 

 that present the various phases so clearly and indisputably as 

 does the west slope of the Bitter Root mountains. This area 

 is truly a debatable ground. Its forest growth is subject to great 

 and extensive stress — on the east from the arid conditions of the 

 Rocky Mountain regions, on the west from those which prevail 

 on the treeless plains of the Columbia River plateau. It is 

 seamed, furrowed, and crossed in various places by extensions 

 from those two great tracts. At the same time it contains very 

 large areas of extremely humid slopes, where the drying effects 

 of the changing climate are as yet scarcely felt, if at all. These 

 conditions provide numerous transition grounds for the study of 

 the forest modifications. 



Beginning with the group of summit trees, as they might be 

 called, we have three species which are in the Pacific northwest 

 true timber-line trees. Nowhere, however, in the Bitter Roots 

 do these species form a timber-line zone, for no peak in the 

 range is high enough to reach it. As summits exist 10,150 feet 

 in height above sea-level, it follows that the absolute timber-line 



