172 THREATENED ARIDITY ON THE PACIFIC SLOPE 

 THE HUMID REGIONS 



From the subhumid tracts we come to the humid ones. These 

 are chiefly limited to the mountain regions. When they extend 

 into the plains or into areas of lower humidity, they do so only 

 in the bottoms or on the northern slopes of deep canyons or the 

 northern slopes of ridges. On the other hand, it is everywhere 

 noticeable that the subhumid areas send long, strong lobes and 

 extensions into the humid tracts, carrying their characteristic 

 trees with them and indicating the coming ascendanc} 7- of drier 

 climatic conditions. 



The trees which compose the forests in this zone group them- 

 selves into three divisions, according to their altitudinal range. 

 The first group, occupying the higher elevations, contains the 

 following species : 



LarLv lyallii, Lyall larch ; 



Piniis albicaults, White-bark pine ; 



Tsuga pattonii, Mountain hemlock. 



The second group contains species which most generally oc- 

 cupy areas at the lowest elevations in the zone. They are : 



Thuja plicata, Pacific arbor- vitee; 

 Larix occidenlalis, Western larch ; 

 Tsuga mertensiana, Western hemlock ; 

 Plnus monticola, Mountain white pine. 



The third group contains species which- range indiscriminately 

 from the upper to the lower areas of the humid zone and are as 

 follows : 



Abies lasiocarpa, Alpine fir; 



Picea engelmanni, Engelmann spruce ; 



Pinus murrayana, Lodgepole pine. 



Of the species included in this group, the Alpine fir possesses 

 the least power of adaptability, the lodgepole pine the highest. 



In addition to the species enumerated, there are the following 

 whose behavior as to altitudinal extensions and limitations are 

 not very thoroughly known. They are : 



Libocedrus decurrens, Incense cedar ; 



Chamsecyparis nootkatensis, Yellow cedar ; 



Abies amabilis, Amabilis fir; 



Abies nobilis, Noble fir; 



Abies shastensis, Shasta fir ; 



Pinus flex'dis, Limber pine; 



Pinus lambertiana, Sugar pine. 



