8 BULLETIN 1200, U. S. DEPARTMENT OF AGRICULTURE. 
TABLE 2.—E/ffect of vegetative cover on air and soil temperature. 
Average maxima—Degrees Fahrenheit. 
| 
May. June. July. | August. | September.} October. 
(2 et sil 
Air temperature:! 
| 
| 
| 
| 
NaturakcOver ssc. s see = See al 60.8 59.2 70.7 86.6 | 72.0 55.7 
: aa Bl ow See Nhe w ce BeOS | 72.2 64.4 84.7 | 102.8 | 76.2 60.0 
oil temperature: 
Natural cover, surface............. 55.6 56.0 62.7 73.4 | 63.5 56.2 
Natural cover, 6 inches deep-..-.... 52.2 55.2 60.5 68. 4 61.0 54.5 
Nature Cone. 12 inches deep..-... | es aot he oe 60.6 54.5 
enu Surface £056 Ss. fos ht - ’ . 124. 89.2 (64.5 
Denuded, 6 inches deep.....-..... | 528 62.0 68.0 78.7 67.0 54.2 
Denuded, 12 inches deep.......... | 36.3 61.5 67.0 74.4 66.2 56.2 
1 Air temp2rature taken at the crown of 1-year-old seedlings. 
Table 3 shows clearly the effect of evaporation from surface soil. 
In August the moisture in the denuded soil was 1 per cent, as com- 
pared with 10.5 per cent under the natural cover. The percentage 
of soil moisture at the 6-inch and 12-inch depths, however, was 
greater during the dry season on the denuded area than on the area 
having the natural cover of vegetation, because, where this cover was 
present, the moisture from the soil was absorbed by the roots of the 
plants. 
TABLE 3.—Effect of vegetative cover on soil-moisture content. 
Average percentages of soil moisture. 
May. | June. July. BERS ‘September. October. 
| | | 
Natural cover: 
Gariacel tit /f_ S008s) ee. TSE § 33.3 32.4 23.11 10.5 | 33.4 | 36.5 
G@anchesdeen..- +2). 5.2 te 21.3 26.7 20.0 | 12.9 | 29.5 | 26.5 
i2 inches deep LF 1.5. 2h. 23.9 20.5 18.6 | 15.4 | 28.4 | 27.7 
Denuded: | | 
Stig Cas ile ted ae > RR ae Fea aia a 11.0 10.2 4.1] 1.0 | 12.7) 18.4 
6 inches deep:.........-......2-2-- | 26.7 24.1 | 22: 51 17.5 | 24.2) 29.0 
i 2332 25.7 | 21.1 19.8 | 27.2 | 29.2 
' j 
EZHRCRES ACE D ss. < Masses eo eee | 
The extreme maximum temperature of the surface soil of 124.2° F. 
in the denuded area, as compared with 73.4° F. at the same time 
under the natural cover, shows clearly to what rigorous conditions 
the seedlings are exposed when growing in the open on these severe 
slopes. The 12.9 per cent of soil moisture at the 6-inch depth in the 
natural cover, and the 17.5 per cent at the 6-inch depth in the de- 
nuded area, is evidence that seedlings with root systems 6 inches 
and deeper have moisture available even during extreme droughts. 
The deciding factor here, however, is the amount of drying result- 
ing from the exposure of the plant. A plant in the open is under 
a much more severe test than a plant under natural cover, for a 
plant exposed to the sun becomes considerably warmer than the sur- 
rounding air, whereas shaded plants become colder than the air. 
Even if the soil moisture is equal in two localities, the soil texture 
may have a decided influence on the availability of the moisture to 
the plants, as expressed by a marked difference in the wilting coefii- 
cients. This difference would not influence the types if the soil mois- 
