May 31,1924 
859 
Cell Sap Density and Environmental Conditions 
is exposed to the full sweep of the prevailing southwest winds, the velocity of 
which Sampson (113) has shown to average notably greater than in the lower 
types. Because of the prevailingly high temperatures, the longer growing season, 
and the higher rates of evaporation in the lower types it follows that the mean 
annual precipitation is less effective in these types. Even in the oak-brush zone 
the rainfall is insufficient to leach the lime from the soil. This complex of 
adverse factors, therefore, has a profound influence on the type of vegetation at 
the lower elevations. 12 
Table V .—Soil moisture and evaporation data on six different sites at elevations 
of 7,500 to 7,700 feet, Big Cottonwood watershed, Wasatch National Forest, Utah, 
season of 1919 
Site 
Northern aspect, 
brushy. 
Western aspect, 
Ceanothus-oak. 
Northeastern aspec 
open, grassy. 
Northeastern aspect, 
aspen. 
Canyon Bottom, 
scattered aspen. 
Southern aspect, 
sage-Ceanothus- 
oak-aspen. 
Depth of 
soil 
sample 
Inches 
0-6 
6-12 
Average.. 
0-6 
6-12 
Average- 
0-6 
6-12 
Average. 
0-6 
6-12 
Average- 
0-6 
6-12 
Average- 
0-6 
6-12 
Average- 
Total soil moisture percentages at different dates 
of sampling 
July 
2 
13.2 
12.4 
12.8 
4.4 
7.5 
6.0 
7.9 
9.8 
8.8 
14.9 
15.5 
15.2 
10.0 
8.8 
9.4 
3.0 
7.7 
5.4 
July 
12 
July 
29 
10.5 
12.8 
11.6 
5.0 
8.1 
6.6 
4.8 
9.9 
7.4 
15.1 
16.7 
15.9 
13.0 
12.7 
12.8 
3.1 
6.2 
4.6 
5.2 
7.6 
6.4 
2.7 
5.4 
4.0 
3.6 
8.5 
6.0 
7.6 
8.6 
8.1 
7.8 
9.2 
'8.5 
4.0 
7.0 
5.5 
Au¬ 
gust 
9 
9.8 
10.9 
10.4 
19.8 
17.3 
18.6 
21.0 
14.0 
17.5 
16.0 
20.8 
18.4 
24.4 
19.3 
21.8 
20.4 
20.6 
20.5 
Au¬ 
gust 
20 
7.8 
9.5 
8.6 
5.8 
6.6 
6.2 
8.3 
9.6 
9.0 
10.4 
10.2 
10.3 
9.2 
11.2 
10.2 
6.1 
8.5 
7.3 
Au¬ 
gust 
30 
7.4 
8.6 
8.0 
3.0 
4.6 
3.8 
5.6 
8.8 
7.2 
9.2 
10.6 
9.9 
8.4 
8.7 
8.6 
2.6 
5.8 
4.2 
Sep¬ 
tem¬ 
ber 
22 
23.4 
18.5 
21.0 
11.3 
12.2 
11.8 
30.4 
24.6 
27.5 
19.6 
16.6 
18.1 
19.8 
16.6 
18.2 
8.4 
11.4 
Sea¬ 
sonal 
aver- 
11.0 
11.5 
11.2 
7.4 
8.8 
8.1 
11.7 
12.2 
12.0 
13.3 
14.1 
13.7 
13.2 
12.4 
12.8 
6.8 
9.6 
8.2 
Wilt¬ 
ing 
co¬ 
effi¬ 
cient** 
Per 
cent 
13.6 
11.3 
12.4 
► 10.9 
17.9 
14.5 
16.2 
14.7 
12.6 
13.6 
16.2 
13.5 
14.8 
}l2.4 
Mean 
daily 
evap¬ 
ora¬ 
tion 
S. M. 
c. c. 
21.29 
33. 46 
30.07 
( c ) 
24.45 
38.27 
E.* 
1.85 
3.80 
2.46 
1.97 
3.99 
a Determined in the Biophysical Laboratory, Bureau of Plant Industry, United States Department of 
seasonafmSstur 0 ^Content of the soil at a depth of 6-12 inch**. 
C 52822? TObrokenby a be™ early in the season. In S 
mnishirp oontpnt the evaDoration was considered equal to that on the brushy northern aspect, i ms is 
shown to be S-^ervatTve by the 1920 records which show the evaporation to be lower on the aspen- 
covered northeastern aspect. 
12 Studies conducted in the Wasatch Mountains by Sampson (US) and Baker and Korstian (unpublished) 
agree in the main with the more detailed studies of Pearson (99) in indicating that the upper limits of the 
distribution of a species or forest type are determined primarily by low temperature as related to photo¬ 
synthetic activity and that the lower limits are determined primarily by a deficient moisture supply. 
Deficient moisture rather than high temperature is regarded as the determinant at the lower levels, since 
the observations by Baker and Korstian in this region, as well as those of Pearson in the Southwest, indi¬ 
cate that practically all of the species, when adequately supplied with moisture through irrigation or when 
growing along stream courses, some 1,500 feet below their natural range, are capable of enduring high tem¬ 
peratures far in excess of those to which they are subjected at the lower limits of their natural range. 
