FOREST TYPES IN CENTRAL ROCKY MOCNTAINS. 27 



The time at which the first growth of a given tree may be noted in 

 successive years is so variable as to make the fixing of even a mean 

 date almost impossible. When the variations between different sites 

 and elevations and the different species on the same site are brought 

 into the problem it becomes so involved that its significance is likely 

 to be obscured. 



For all of the above reasons it has been decided to consider the 

 "growing season" as synonymous with "summer," or the months of 

 June, July, and August, and the first decade 4 in September. The 

 three months stand (mite apart from the remainder ox the year. At 

 the low elevations, of course, much vegetative activity begins before 

 June 1; at very high elevations deep banks of snow remain until 

 after July 1. But at all elevations in or very close to the high moun- 

 tain ranges the cooling effect of a general snow covering disappears 

 soon after June 1, and this causes a very abrupt change in the tem- 



Eerature conditions. The first decade of September is included 

 ecause it is nearly always as warm as if not warmer than the much 

 cloudier decade preceding. Thus a comparison of temperatures for 

 these 10 decades may be quite as valuable as a comparison of mean 

 temperatures or temperature sums for growing seasons of variable 

 length, as we have practically no knowledge of what temperatures 

 are growing temperatures for the plants under consideration. It 

 may be said quite safely for the limited latitudinal range covered by 

 this discussion that, if the summer temperature of a given site is 

 found to be relatively high, the season during which it has favorable 

 temperatures for growth must also be relatively long. It is there- 

 fore thought worth while in this bulletin to give summer or " growing 

 season" means for all data, as well as annual means. Any greater 

 refinement of the distinction between periods of growth and rest 

 does not appear justified. 



ABSOLUTE AND COMPARATIVE CONDITIONS IN THE VARIOUS 



FOREST TYPES. 



AIR TEMPERATURES. 



In taking up the study of atmospheric conditions which may 

 cause the differentiation of mountain forests into types dominated 

 by different species it is natural to turn first to the subject of air 

 temperatures for two reasons, viz : 



(1) The decrease in temperature with an increase in altitude is 

 the most obvious change which occurs between plains and mountain 

 tops, and hence has the appearance, at least, of being a primary 

 cause for the zonal distribution of each species of the forest. The 

 difference in the warmth of north and south exposures is equally 

 perceptible and suggests a primary cause for the difference in forest 

 cover which usually characterizes opposing slopes at the same 

 elevation. 



(2) Air temperatures are more readily measured, perhaps, than 

 any other conditions, and this has given rise to more records of air 

 temperatures than of other conditions except possibly precipitation. 

 This fact makes it possible to draw upon several localities besides 



* Throughout this bulletin the term "decade" is used to denote a third of a month, ordinarily :i period 

 of 10 days, but varying from 8 to 11 days when the last decade of a inoni h is referred to. 



