METHODS OF DETERMINING TOLERANCE. 45 
Minimum 
Species. Region. Elevation. light 
intensity. 
Populusiaibaseesssssecee eee iWitahvandeldah ory aye oie te epee wena 3,000-4,000 | 1/ 8 -1/10 
P. tremuloides............---.-- Yellowstone Park, Wyo........-.---.-.--- 6,000 | 1/ 4.2-1/6 
12) PGI ooh eoeod deeded ivsMNestOM UNION tapas eee eee ee 4,500 | 1/8 -1/9.1 
IPA deltoides aaa eee Sas eee Colorado Springs, Colo..............-..... 6, 000 1/4 
DD YON eset ey UN ee teanD Me nS IB UMS EMO mG ys ys aoe ee ae 3, 100 1/9 
ebalsamliterays - a hee a eee Colorado Springs, Colo.......-----.....--- 6, 000 1/16 
1B, lovee NICE) So Ak ooo osoosooue SaltabakerC@ityewWitalae see ee = eee ae 4, 250 1/21 
Betula fontinalis............--- Mammoth Hot Springs, Wyo........-..-- 6, 000 1/14 
Symphoricarpus oreophilus. ...-|...-- CONG RP Nae OR OE Ie aos NE od 6, 000 1/20 
Shepherdia argentea .........-. Yellowstone region near Livingston....... 4, 500 1/14 
Vaccinium myrtillus........-.. Yellowstone region. ......0.5220. 222222222. ep ae halraea tee a eyahae 1/16.6 
CLEMENTS’ INVESTIGATIONS. 
Clements determined, by the aid of his photometer, the light 
requirements of a number of forest trees in Colorado in the lodge- 
pole pine zone of the Rocky Mountains. He found that the lght 
values in mature lodgepole pine forests range from 0.12 to 0.05 
(1/8.3 to 1/20). The values most frequently found were 0.08 
(1/12.5) and 0.07 (1/14.8), which he accepts as the normal. At 
intensities of 0.20 to 0.14 (1/5 to 1/7.1), germination and growth 
of lodgepole pine were found to be fairly good, though much below 
that in sunshine. No vigorous seedlings were found in forests with 
hght values from 0.08 to 0.05 (1/12.5 to 1/20). These light values 
tend to show that the minimum light intensity for lodgepole pine is 
very low, much lower than the values obtained by Wiesner. Wiesner 
determined the minimum light intensity for lodgepole pine as 1/6 
of full daylight, while the light intensities under which lodgepole 
pine seedlings still persist, according to Clements, may be even as 
low as 1/20. This difference is due partly to the difference in the 
methods of measuring light, partly to the difference in the accuracy 
of the two instruments, but probably chiefly to the difference in the 
standard with which the light intensities were compared. Clem- 
ents’ measurements of the light requirements of limber pine (Pinus 
flexilis) gave him values approximately the same as those for lodge- 
pole pine (Pinus contorta). This again does not agree with the 
results obtained in Yellowstone Park by Wiesner, who found the 
minimum light intensity for limber pine to be from 1/8 to 1/9. For 
Douglas fir the minimum light intensity, according to Clements, 
apparently hes below 0.05 (1/20). In Estes Park he found that 
Douglas fir very rarely thrives in light intensities below 0.05 (1/20), 
though it grows fairly well in light of 0.02 (1/50) at Pikes Peak. 
Wiesner determined the minimum light intensity for Douglas fir 
in the Rocky Mountains also as 1/20. Engelmann spruce (Picea 
engelmannt) and alpine fir (Abzes lasiocarpa) were found by Clem- 
ents to be almost identical in their tolerance, and no forest light 
measured was too weak for fair reproduction of both species. 
