44 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE. 



whore the minima are relatively low after the long period of radiation, 

 and possibly somewhat affected by air drainage in the slight depression. 



The temperature record of the local limber pine type (F-G) is a 

 wholly dependable one, and is, perhaps, significant of the require- 

 ments of this species. It is especially noteworthy that the winter 

 temperatures, in spite of the fact that they represent ground condi- 

 tions, are slightly higher than those of the control station, and there 

 can be little doubt that this is due to direct exposure to west winds. 

 As the slope is a northwest one, the high summer temperatures may 

 be accounted for only by the fact that such insolation as is received 

 locally, coming late in the day when the general air temperatures are 

 already high, causes high and late maxima and hence high minima. 



The record of the higher limber pine type (F-13) does not seem 

 to bear out the assumption that this species demands a great deal of 

 heat. Although the mean temperatures are always considerably 

 lower than for the control station, it is, however, possible to conceive 

 that during the summer months, when the difference is rather slight, 

 this site experiences a few hours of high temperatures satisfactory 

 to a heat-demanding tree, and that the growth of limber pine is re- 

 stricted to such short periods. Even so, the maxima are considerably 

 lower than for the control station, especially during the winter 

 months. The maxima for the growing season are 1.7° lower than at 

 the control station, the minima 1.8° higher, and the mean daily 

 ranges 3.5° less. For this period the station would compare favor- 

 ably with ground temperatures at the control station. The daily 

 ranges at the higher station are relatively the least during the winter 

 months. The decade minima are usually higher during the coldest 

 months when they are most affected by wind. For the growing 

 season they are 0.8° lower, on the average, and only very rarely 

 higher than at the control station. 



On the whole, no plausible explanation is found for the occurrence 

 of limber pine in burns of the spruce zone except on the assumption 

 that ground temperatures during the growing season may be about 

 the same as at lower elevations, at least giving short periods of fav- 

 orable temperature. Having no competition, the limber pine is 

 able to survive even with a very small annual accretion. 



The temperatures of the four north-slope stations that represent 

 different weights of cover in the Douglas fir type, are worthy of note. 

 The mean temperature in the fully open position is slightly in excess 

 of that of the control station — about as much in excess as the ground 

 temperatures at the control station might be expected to be. The 

 two stations under partial cover are almost identical with the control 

 station for the year as a whole; however, they are decidedly cooler in 

 winter and warmer in summer. These high temperatures on a north 

 slope are to be accounted for only by air stagnation, the wind move- 

 ment being much less here than at the control station, even if veloci- 

 ties at the latter point an 4 reduced to ground conditions. The whole- 

 cover station (EM)) is appreciably cooler than the others during mosl 

 of the year. 



The mean temperatures, however, do not tell the story for these 

 four Btations. In Table 10 the extremes and daily ranges for the 

 control station and the four north-slope stations are shown. The 

 absolute figures given for the control station are for thelonger period cor- 

 responding to Station 7-S, namely, October, 1915, to December, L917. 



