216 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



consociations in each. Other important species which they have in common 

 are Pinus monticola, P. ponderosa, and P. contorta, while such subclimax 

 species as Arbutus menziesii and Quercus calif ornica occur in both. By far the 

 most significant fact, however, is the association of 5 coastal dominants with 

 4 from the montane forest to constitute the transition community. 



The chief contact of the Coast climax is with the montane forest. They 

 are in touch with each other from northern California to central Oregon, and 

 then as outposts through northeastern Oregon to Idaho and Montana. This 

 contact continues through British Columbia to the sixtieth parallel. Here the 

 montane and subalpine forests give way to the boreal forest of Picea mariana, 

 P. alba, and Pinus divaricata, which covers the interior of Yukon and Alaska 

 behind the coastal strip of Picea sitchensis and Tsuga heterophylla. On the 

 mountain ranges of the central area the Coast forest meets the subalpine 

 climax at altitudes of 5,000 to 7,000 feet. They mingle over a wide mountain 

 ecotone, and in the transition association, Picea engelmanni and Abies lasio- 

 carpa form subclimax communities at exceptionally low levels. 



Associations. — The chief reasons for recognizing two associations in the 

 Coast formation have already been touched upon. It may be well to state 

 them explicitly here, as this involves a readjustment of the current views. 

 The first and most important of these reasons is the change of dominance 

 from the western to the eastern portion. Picea sitchensis drops out before the 

 Cascade Mountains are crossed, while Tsuga and Thuja change from primary 

 to secondary rank. Pseudotsuga continues to be of the first importance, but 

 shares this with several other dominants. A second reason of almost equal 

 significance is that Pinus monticola and Larix occidentalis reach their best 

 development and maximum dominance in the mountains of northern Idaho 

 and the adjacent region. The behavior of Picea engelmanni and, to a less 

 extent, of Abies lasiocarpa in descending from the subalpine forest to play 

 an important role in valleys and on north slopes, is also significant. Further- 

 more, the subdominants of the transition forest and its subclimax stages are 

 largely Rocky Mountain in relationship, especially in Idaho and Montana. 

 Finally the differences in the vegetation are correlated with a similar differ- 

 entiation of the climate. Over the region of the coastal community, the rain- 

 fall ranges generally from 50 to 80 inches, with a maximum in the Olympic 

 peninsula of more than 100 inches. Over most of the transition forest the 

 rainfall is only 20 to 35 inches, with a maximum of 40 inches only in the 

 Bitter Root range. There is likewise a marked difference in the annual dis- 

 tribution for the two regions. In the case of the eastern area, 30 to 60 per 

 cent of the precipitation occurs between April 1 and September 30, while 

 in the western but 10 to 30 per cent— i. e., 70 to 90 per cent of the rainfall 

 takes place during the winter months. The significant difference in tempera- 

 ture relations is indicated by a mean temperature of 45° to 52° and a minimum 

 one of 14° to —4° for western Washington, and of 38° to 45° and —25° 

 to —49° for western Montana. 



The two associations may be almost equally well designated on the basis 

 of location and composition. The latter seems to afford the more clear-cut 

 and convenient distinction. The western or coastal portion is hence termed 

 the cedar-hemlock forest or Thuja-Tsuga association and the eastern is called 

 the larch-pine or Larix-Pinus association. 



