346 FOREST INDICATORS. 



crease of rainfall from woodland to subalpine forest, while the Coast forest 

 has the highest rainfall of all. In similar fashion, woodland, montane, and 

 subalpine forest indicate a progressive decrease in the length of season and the 

 temperature values, though the Coast forest marks the longest growing season 

 and the most equable temperatures. The rainfall and temperature relations 

 of the several formations have already been suggested in Chapter IV and need 

 not be repeated here. The associations indicate subdivisions or subclimates 

 of the formational climates. In general, the Petran associations are drier and 

 colder than the Sierran associations of the montane and subalpine climaxes. 

 For the three woodland associations, the total rainfall varies less than its 

 seasonal distribution, and the temperature relations seem more decisive than 

 the rainfall. The pinon-cedar indicates the coldest climate with much of the 

 precipitation as snow, the oak-cedar the warmest, and the pine-oak the most 

 equable. The first two have from 40 to 70 per cent of their rainfall in the 

 summer, and the latter about 20 per cent. The two associations of the Coast 

 forest show two subclimates strikingly different in both rainfall and tem- 

 perature. 



The consociations serve to indicate still finer climatic divisions, both as to 

 altitude and latitude, though in general their indications are merged in those 

 of the association or formation to which they belong. This is well illustrated 

 by the montane forest, in which Pinus ponderosa indicates drier and warmer 

 climatic conditions than Pseudotsuga taxifolia, while Abies concolor is more or 

 less intermediate. Consociations also indicate potential climates, with 

 especial reference to the wet phase of the climatic cycle, where they form 

 savannah, as in the case of Pinus ponderosa in the grassland climax, or Juni- 

 perus in the sagebrush. The varied groupings of consociations throughout 

 an association also have some climatic indications, but these are often obscured 

 by edaphic indications of more importance. 



Two outstanding investigations have been made of the physical factors of 

 climatic types. The first is that of Bates, Notestein, and Keplinger (1914: 78), 

 the second, that of Sampson (1918). The former deals with yellow pine, 

 Douglas fir, and Engelmann spruce groupings of the central Rocky Mountains. 

 The factors of the air and soil were measured during 1910-1911, and the fol- 

 lowing conclusions were reached as to the differences of the several types : 



"There are wide differences in the heat requirements of the species and in 

 the temperatures of the types. The types vary somewhat in air temperatures, 

 but much more distinctly in soil temperatures. The length of the growing 

 season as determined from soil temperatures is a fairly accurate basis for 

 determining what tree should be grown on the site. It is possible that after 

 a series of careful observations a rule may be laid down by which the growing 

 season may be determined from a very few soil-temperature measurements, or 

 a direct relationship between the degree of solar radiation at any time and the 

 length of growing season may be established. This last, of course, will simply 

 be a scientific method for 'sizing up' the combined effects of slopes, aspect, 

 and altitude — a thing which is done roughly by the forester every day. 



"The soil moisture of the types varies greatly, the spruce requiring the 

 most and the pine the least soil moisture ; but the soil-moisture percentage is 

 not a good basis for comparing types except in the same immediate vicinity, 

 where it is known that the physical properties of the soils are uniform. In any 

 locality the spruce type probably always receives a greater amount of pre- 



