218 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



per cent of the stand, with Douglas fir most abundant except in damp places, 

 and more or less Abies grandis, Tsuga, and Thuja. On river flats along the 

 coast, scattered Picea, Chamaecyparis lawsoniana, Tsuga, and Abies occur in 

 it. Abies amabilis and A. nobilis occur more or less commonly through the 

 groupings of the four major dominants, but usually form only a small fraction 

 of the stand, as is true also of Chamaecyparis nootkatensis. 



Tsuga heterophylla and Picea sitchensis constitute the coastal forest of 

 Alaska. Sometimes they form pure stands, but they usually occur in mixture, 

 one or the other being dominant, Picea preferring the vicinity of the coast. 

 In southern Alaska they are more or less mixed with Thuja plicata, Abies 

 grandis and A. amabilis, and Chamaecyparis nootkatensis. 



Factor and serai relations. — The cedar-hemlock forest occupies a region of 

 excessive rainfall and frequent or constant fog, with consequent low evapora- 

 tion. Over much of it the annual rainfall is in excess of 80 inches, the range 

 being 50 to 120 inches. In the United States 10 to 30 per cent of this falls 

 during the six winter months, and much the same conditions obtain to Sitka 

 and beyond. The temperatures are generally equable except at the higher 

 altitudes. The absolute minimum as far north as Sitka is but —4°. 



Quantitative studies of the water and light relations of the dominants are 

 still few (cf. Cooper, 1917 : 179), but they are in general agreement with the 

 topographic and serai relations. The following table of tolerance, based upon 

 successional relations, agrees fairly well with the conclusions of foresters. 

 The sequence is from the least to the most tolerant. 



Tolerance of Dominants. 



Pseudotsuga mucronata. Abies amabilis. Picea sitchensis. 



Abies nobilis. Sequoia sempervirens. Thuja plicata. 



Abies grandis. Chamaecyparis nootkatensis. Tsuga heterophylla. 



The last five are unusually close in their tolerance, and the order given here 

 is not infrequently changed by soil, water, or temperature relations. In a 

 region of such excessive precipitation, the water relations are less clear and 

 are much influenced by temperature. The general relation to these com- 

 bined factors is indicated by the altitudinal range, though this is not in full 

 accord with that in latitude. The typical fog-belt trees are Picea sitchensis, 

 Sequoia sempervirens, and Chamaecyparis lawsoniana. These represent the 

 maximum conditions as to water-content and humidity. They are followed 

 closely by Thuja plicata, and this by Tsuga heterophylla and Abies grandis. 

 The ability of Abies amabilis, A. nobilis, and Chamaecyparis nootkatensis to 

 endure more xeroid conditions is indicated by the fact that they occur in the 

 subalpine zone, where the first is frequent at timber-line. Pseudotsuga is the 

 most xeroid of all the dominants, a fact in complete accord with its dominance 

 in burns and its importance in the montane forest. 



SOCIETIES. 



The development of shrubby societies often reaches a maximum in the 

 cedar-hemlock forest, though the actual number of species is few. As a con- 

 sequence, the light at the ground level is greatly reduced, and the herba- 

 ceous societies as a result are poorly developed. 



