FOREST TYPES IX CENTRAL ROCKY MOUNTAINS. 35 



The probability of freezing temperatures in the first decade of 

 June has already been mentioned in the discussion of the "growing 

 season." Nevertheless, it is difficult to see how the factor of frosts 

 at this time can have much effect on the problem of what causes the 

 survival of one species and the failure of another. New seedlings 

 have rarely appeared at this time, and those from previous seasons 

 which have survived the winter should hardly be susceptible to 

 injury at so late a date. New growth on older trees will barely have 

 started by June 10, and shows no injury from the snows which may 

 occur at this time. Even young aspen leaves just emerging from the 

 bud withstand temperatures considerably below 32° F. without 

 apparent injury. Observation at a more advanced stage has not 

 yet been possible. The great certainty of freedom from frost in the 

 first decade of September fully justifies extending the growing season 

 over this period. The second decade is equally certain to record a 

 thorough freezing. 



The lowest temperatures of the year are almost certain to be 

 recorded in January, notwithstanding the fact that the lowest mean 

 temperatures occur in February. This is made possible by the fact 

 that the absolute minima fall in periods of calm, when the protection 

 (if such it may be called) of the westerly winds is temporarily with- 

 drawn. It will later be seen that the high, rather than the low, 

 temperatures of January have the greater significance. 



Classification of days according to temperature. — The computation 

 of mean temperatures even for so short a period as a decade tends to 

 hide the fluctuations and smooth over the extremes. In a biological 

 study it may be of interest to know, for example, that in a January 

 having a mean temperature of 17.7°, whose warmest decade was 23.7°, 

 there might be two days having mean temperatures above 41°, and 

 to consider whatever possibilities this may present in the matter of 

 the photosynthetic activity of trees, and also the more important 

 probability of damaging transpiration. 



Therefore, to present most strikingly the temperature conditions 

 at the control station, Table 5 has been prepared, showing for each 

 month up to March, 1918, the number of days of the following 

 arbitrary classes, based on mean temperatures: 



No thawing: Mean and maximum, 32.0° F. or less. 

 Freezing: Mean below 32°, and maximum above 32°. 

 Cold: Mean temperature, 32.1° to 41°. 

 Cool: Mean temperature, 41.1° to 50°. 

 Moderate: Mean temperature, 50.1° to 60°. 

 AYarm: Mean temperature, 60.1° to 72.° 



It should, of course, be realized that thawing or melting of snow 

 is not precluded on days whose maximum air temperatures are 

 below 32 degrees. In bright sunlight, thawing may take place with 

 temperatures as low as 0° F. 



