FOEEST TYPES IN CENTRAL BOCK'S MOUNTAINS. 51 



To understand the limitations of the forest types, therefo] 

 seems essential that there should be knowledge of the maximum tem- 

 peratures to which seedlings are subjected, and by this is meant the 

 maxima at the surface of the soil, where injury to seedlings is first 

 apparent. These will not wholly tell the story, as the fatal effects 

 are probably produced by lack of moisture; but the maximum tem- 

 peratures would probably be a very fair criterion as to the degree of 

 drought produced if some allowance were made for the type of soil. 



Considering the matter in its broader aspects, it is fairly apparent, 

 even before the other conditions of the forest types have been con- 

 sidered, that temperatures are primarily and closely controlling in 

 the natural distribution of each species. It is to be expected that 

 each would show some range of requirements, and that, as indicated 

 by the data, spruce and yellow pine might show a somewhat greater 

 range than the other species, because, occupying the edges of the 

 stands, they are not so closely limited by competition. It may be, 

 of course, purely accidental that the data do show this quality. 

 And in considering the range of temoeratures under which any species 

 thrives it should be apparent that different temperatures, as they are 

 measured, may be physiologically alike. Thus, it may be said, 

 atmospheric humidity modifies the apparent temperatures for plants 

 as it does for animals. The plant temperature must be relatively 

 higher in a moist than in a dry atmosphere. On the contrary, in the 

 critical connection mentioned as affecting seedlings, the greater the 

 humidit} r , no doubt, the higher the temperature that may be tolerated. 



These considerations explain fully enough why the absolute tem- 

 peratures enjoyed may cover a wide range, and especially why the 

 temperature requirements in different regions may be markedly 

 different. In this connection, however, it should be remembered 

 that different races of the same species are now beinoj dealt with. 

 The behavior of the Montana and Arizona forms of yellow pine has 

 been briefly described in the earlier phvsiological paper. The func- 

 tioning of the Arizona form would lead one to expect that it would 

 tolerate higher temperatures than the Colorado variety. Pearson (10) 

 shows that the mean temperatures of the type in Arizona during the 

 growing season — June to September — range from 59.5° to 65.6° F., 

 the highest (Williams) being a little in excess of the Black Hills 

 temperature, and being accompanied by a daily range of 33°, and 

 with less precipitation than is characteristic of the Black Hills. 

 The Arizona forest, however, reproduces itself with the greatest 

 difficulty, while in the Black Hills reproduction is very abundant. 



The temperatures given by Larsen (13) for an even longer growing 

 season — May to September — are 61.2° for the Washington-Idaho 

 section, 59.8° for the intermontane region of Idaho and Monta 

 and G4.7° for eastern Montana. It is believed that the intermontane 

 region with the lowest temperature is the most favorable as rega 

 moisture. The relatively high value for the type in eastern Montana 

 may be determined by summer dryness, but there is a possibility, at 

 least, that the Chinook of that region may have some influence in 

 confining the type to warm sites where the danger of soil freezing is 

 least. 



Pearson's (10) Douglas fir type in the San Francisco Mountains, 

 at an elevation of 8,900 feet, has a growing-season temperature 

 almost identical with the average, but the annual mean is about 3° 

 higher than the average in Colorado. 



