Apr. 14,1933 
Physiological Requirements of Rocky Mountain Trees 
117 
2. With bright diffuse light, such as might penetrate the canvas cur¬ 
tains on a sunny day, the transpiration varies from 21 per cent of standard 
for lodgepole to 32 per cent for yellow pine and Douglas fir. 
3 - With dull diffuse light, as on cloudy days, the percentages vary 
from 17 per cent to 23 per cent of standard. Though lodgepole shows at 
all stages the greatest depression from the lack of sunlight, it is hardly 
safe to say that this is a specific character. 
Table V—Response of various species to different conditions of light and air movement 
Species. 
Conditions. 
Average transpiration (cubic centimeters per 
day, standard), no ventilation, over 400 
minutes sunshine. 
Proportionate transpiration: 
No ventilation, less than 400 minutes 
simshine. 
No ventilation, bright diffuse light 
No ventilation, dull diffuse light. 
Some ventilation, over 600 minutes sim¬ 
shine, temperature over 75° F. 
Some ventilation, over 600 minutes sun¬ 
shine, temperature under 75 F. 
Some ventilation, 400 to 600 minutes sun¬ 
shine, tempCTature over 75® F. 
Some ventilation, 400 to 600 minutes sun¬ 
shine, temperature under 75° F. 
Some ventilation, less than 400 minutes 
sunshine, temperature over 70® F. 
Some ventilation, less than 400 minutes 
sunshine, temperature under 70“ F. 
Some ventilation, diffuse light. 
Num¬ 
ber of 
days. 
Yellow 
pine. 
Doug¬ 
las fir. 
Lodge¬ 
pole. 
Engel- 
mann. 
Limber 
pine. 
, Bristle- 
cone 
pine. 
13 
21.82 
® 13 - 59 
11.86 
19.84 
9. 62 
13- 22 
II 
. 62 
. 62 
•54 
. 62 
.62 
•59 
19 
•32 
•32 
• 21 
. 26 
• 31 
• 25 
7 
• 23 
. 20 
•17 
. 22 
. 18 
.x8 
4 
1.05 
1.08 
1-35 
.98 
1.18 
1.18 
2 
.91 
H 
0 
I. 12 
.86 
1. II 
1.10 
7 
1.27 
1.12 
I. 40 
1.08 
1.41 
I. 23 
13 
•99 
1.06 
I. 19 
•99 
.94 
1.02 
10 
1. 00 
I. os 
I. 06 
.90 
1.00 
•99 
8 
•59 
.58 
.64 
. 61 
•63 
•57 
5 
. 21 
• 28 
.28 
■31 
.28 
• 29 
fl Only tree No. 4 used, account seasonal change in fJo. 3. 
4. With ventilation, the transpiration of all species is increased over 
that without ventilation, other conditions being about the same. The 
amount of ventilation in the greenhouse was not sufficient to produce 
striking changes. The exposure of the trees outdoors for one day did not 
materially increase the transpiration rate, temperatures being considered. 
Finally, since the specific responses are subject to the seasonal changes 
already noted, we may examine the results expressed by the total tran¬ 
spiration of the 12 trees during each day or longer period. 
An attempt has been made to relate this to the vapor deficit, or 
differential between the atmospheric vapor pressure and the saturation 
pressure conceived to exist within the leaf, as determined by mean tem¬ 
peratures. For the latter there is available either the air thermograph 
record or the “sun’’ thermograph (blackened bulb) record. The latter 
seems preferable in theory, but the record is not very trustworthy because 
of very large corrections in the instrument as used during most of the 
season. This, together with the fact that atmospheric vapor pressures 
were determined only in the early part of each period (9 a. m.) and at 
its end (7 a. m.), makes the computation of vapor deficits the roughest 
approximation. In addition, it becomes evident that the leaf tempera¬ 
tures (and saturation pressures) should not be considered as equal to 
the black-bulb temperatures, but more nearly equal to what the wet 
bulb of the psychrometer would show synchronously. As the psychro- 
metric data are not sufficient even to approximate the mean wet-bulb 
temperatures, the results of “sun temperature” computations alone will 
be shown. Recognizing that transpiration at night, because of the lack 
of sunlight, is in a different category from that during the day and 
