FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 33 



From the average values it is seen that the daily range is slightly 

 greater during the growing season than for the year as a whole. 

 However, only the year 1910 shows this character in a marked degree. 

 If this year were eliminated, the growing season and annual means 

 would be almost identical. The peculiarity of the season of 1010 

 deserves consideration. The very high mean temperatures for that 

 year, as well as the high daily ranges, it was at first conceived, might 

 have been due to some condition in the exposure of thermometers 

 which was later altered. However, no such factor could be recalled 

 except that during 1910, and 1911 as well, extra observations were 

 taken at 1 p. m., and the opening of the shelter at that hour very fre- 

 quently resulted in a slight jump of the thermometers and thermo- 

 graph. Allowance w T as always made for this when the maximum at 

 this moment exceeded that of any other hour, which it rarely did. 

 On examining all the conditions for 1910, however, it is found that 

 that year was one of the driest on record, both as to precipitation and 

 atmospheric humidity, and had a very high percentage of sunshine. 

 As these conditions are associated with great daily ranges for short 

 periods, so they may be seen to account for the general character of 

 the year 1910, and for the somewhat high ranges of 1917. 



Absolute minimum, air temperatures. — The Pikes Peak region is not 

 subject to extremely low temperatures in winter, even for short 

 periods, nor is it evident that where very great extremes do occur, 

 in other parts of the central Rockies, they have any but a beneficial 

 effect on established forests. However, a discussion of minimum 

 temperatures (Table 4) is of interest in connection with forest repro- 

 duction, especially in a locality where late germination is induced by 

 especially favorable conditions in July and August. Pearson (16) 

 concluded that young reproduction of yellow pine is extremely sus- 

 ceptible to frost injury, and indeed there is evidence to indicate that 

 lodgepole pine, spruce, and Douglas fir seedlings succumb to fall frosts 

 if they are not fully matured. The forester is in the habit of saying 

 that the seedling must be "lignified,'' that is, its tissues must be 

 solidified, before it can withstand freezing. It would probably be 

 more correct to say that it is necessary for the young plant to carry 

 on photosynthesis and by the accumulation of soluble carbohydrates 

 mate the sap so dense that it will not freeze with the first frost or 

 freeze firmly at any temperature. Lignification would naturally be 

 an immediate sequel to this accumulation of building material. 



In Table 4 the lowest temperature for each decade of the record 

 has been set down, and these temperatures have been averaged at 

 the foot of the table in such a way as to show the probable (not the 

 possible) minimum for any period of the year, in so far as a record 

 of this length may indicate probabilities. Because of very unusual 

 occurrences since March, 1918, it has seemed desirable to extend this 

 record to April, 1921. The data recorded since May, 1920, are for a 

 site where trie probable minimum appears to be 1° to 2° higher than 

 at the control station. However, this will not affect the averages 

 more than 0.1° or 0.2°, and the additional record assists in eliminating 

 irregularities. 



73045°— 24 3 



