FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 



45 



For the most part, the variations from the control station, what- 

 ever the condition measured, progress rather regularly through the 

 four stations, that is, become greater or smaller, as the case may be, 

 with increase in density of the cover. The influence of cover may 

 then be directly measured by comparing Station 9 with 7-8. In 

 July, the complete canopy reduces the maxima 2.2° and raises the 

 minima 2.5°; in January the effect on the minima is similar, but, as 

 almost no January sunlight reaches the groimd under the canopy, 

 the maxima also are reduced about 3.1°. 



Table 10. — Actual temperatures at the control station and departures therefrom 

 on north-slope Douglas fir sites, with different weights of cover. 



Station. 



Control, Oct., 1915, to 

 Dec, 1917. 



F-7-8, open 



F-14, part cover. 

 F-15, part cover. 

 F-9, virgin stand 



Datum. 



I Moan maximum. . . 

 Mean minimum 

 Daily range 

 Absolute minimum 

 I Mean maximum. . . 

 Mean minimum 

 Daily range 

 Absolute minimum 

 !Mean maximum . . . 

 Mean mimmum 

 Daily range 

 Absolute minimum 

 I Mean maximum . . . 

 Mean minimum 

 Daily range 

 Absolute minimum 

 I Mean maximum . . . 

 Mean minimum 

 Daily range 

 Absolute minimum 



Record in degrees Fahrenheit. 



January. April 



33.86 



11.46 



22.40 



-15.65 



-1.9 



- 1.4 



- .5 



- 1.0 



- 4.0 



- .3 



- 3.7 



- .2 

 -3.8 

 + .6 

 -4.4 

 + .2 



- 5.0 

 + .7 



- 5. 7 

 + 3.4 



42. 



22. 



20. 

 4. 

 +2. 

 

 + 2. 

 -3. 

 + • 

 + . 



+l! 



+ • 

 +1. 



-1. 

 + 2. 

 -1. 

 + • 

 -1. 



July. October. 



70.14 

 47.52 

 22.62 

 41.0 

 +6.3 

 -1.8 

 +8.1 

 -1.3 

 +5.0 



- .6 

 +5.6 



- .6 

 +4.9 

 + .2 

 +4.7 



o 



+4il 



+ .7 

 +3.3 



- .2 



52.67 

 28.15 

 24.51 

 5.80 

 + 1.5 

 -1.0 

 +2.5 

 -1.7 



- .1 

 + .2 



- .2 

 -1.0 

 —1.0 

 + 1.2 

 -2.2 





 -3.3 

 + 1.5 

 -4.8 

 + 1.0 



Mean 

 annual. 



27. 30 

 22. 57 

 8.96 

 +2.2 

 -1.0 

 +3.2 

 -1.9 

 + .3 





 + -4 



- .1 

 



+ .9 



- .8 

 + .6 

 -1.4 

 + .9 

 -2.2 

 + 1.2 



i See note, p. 29. 



October is doubtless the most important month with respect to 

 the fate of new, poorly developed seedlings. Its temperatures are 

 similar to those of September, when the first frosts are to be expected. 

 In October the complete canopy has the effect of raising the mean 

 minima 2.5° and the absolute minima 2.7°. This is sufficient to 

 postpone freezing under the canopy for several days, the mean rate 

 of cooling, figuring from September to October, being 1° every 3 

 days. Frost may, then, "normally" be postponed a week through 

 the protection of the canopy. This should be of some slight benefit 

 to seedlings. However, it should tend to favor the less hardy 

 species, Douglas fir, as against the species, Engelmann spruce, which 

 in many of its habitats is subject to frost every month in the year. 

 This it does not do. The canopy tends to favor spruce, but fir and 

 limber pine flourish more abundantly in the open. It may, therefore, 

 be conceived that the influence of the canopy in reducing extremely 

 high temperatures, or mean temperatures for the year as a whole, 

 has more importance in selecting the successful species. 



Other air temperatures at more distant points. — The mean air tem- 

 peratures for a number of stations in the central Rocky Mountain 

 region, believed to be representative of its different localities are 

 shown on a comparative basis in Table 11. With a few exceptions 

 the records of these stations cover the period during which the 

 control station has been operated, up to March, 1918, and all the 

 comparisons are made between identical months of the eight years, 

 or parts thereof. 



