Apr. 14, 1933 
Physiological Requirements of Rocky Mountain Ttees 1 53 
of 160® F. It is, therefore, readily seen that in soils exposed to sun¬ 
light the injury resulting from drought at the surface may be indis¬ 
tinguishable from that due to superheating. Under ordinary circum¬ 
stances the two injurious conditions will be inseparable. 
The difficulty of determining the heat tolerance of seedlings at the 
point where they are commonly injured by heat is, because of the in¬ 
fluence of moisture, very great. We have not been able to conceive 
a test of heat tolerance in the normal sense, except through the employ¬ 
ment of sunlight or some other more powerful radiant energy. Every 
other possible plan of exposure to heat seems to have the objectionable 
result of injuring the foliage first, which rarely happens in nature, or 
of preventing normal evaporation with whatever protection that may 
afford. 
Therefore, the only test ® that has been made to determine the 
relative tolerance of heat by forest-tree seedlings has been on this basis 
of obtaining as high temperatures as possible in sunlight, with the air 
to some extent artificially warmed. The actual temperatures attained 
were measured only so far as was possible by placing mercurial ther¬ 
mometers directly above the soil surface. The seedlings of each species 
were developed in several pans, each of which represented a different 
moisture content. Because of the fact that the largest amounts of 
soil moisture permitted almost no injury, the moisture contents were 
in several cases lowered before the test was completed, so that the 
record is considerably confused. From the data secured, however, the 
following conclusions, admittedly tentative, may be drawn: 
1. Injury to seedlings from excessive heat is plainly greatest when 
the seedlings are youngest. This introduces a complicating factor in 
the test, because exposures to high temperatures were begun before 
germination was entirely completed and when, therefore, there was 
the most marked difference in ages. Engelmann spruce ordinarily 
germinates most promptly and spontaneously. Consequently, while 
there was marked early damage to this species, the fact that there were 
few, later germinations left the species then immune for some time. 
Lodgepole pine exhibits just the opposite characteristics and effects. 
2. &edlings which survive a certain degree of exposure are not likely 
to be injured until the conditions become considerably more severe. 
3. The ease with which any species may be injured increases very 
markedly as the moisture content of the soil decreases. With the lowest 
content, 3 per cent, which in this soil was appreciably above the wilting 
coefficient, it may be questioned whether the injury was not due to 
drought almost wholly, since between waterings the wilting coefficient 
of the soil was reached. 
4. At all times the nature of the wilting was indistinguishable from 
that which occurs with similar seedlings when no excessive heat is in¬ 
volved. Consequently, it appears that wilting may be due as much 
to inability to supply transpiration losses as to the direct effects of the 
temperatures. The fact that no wilting was secured with 14 per cent 
moisture appears to bear out this idea, yet it must be remembered that 
this free moisture may have greatly reduced the temperature extremes 
of the surface soil. The fact that temperatures recorded just above 
the soil were not in excess of 135® F. further suggests that wilting was 
the result of transpiration losses rather than a direct temperature efifect 
on the protoplasm. 
• Credit for the conduct of this test should be given to Forest Asisstant J. Roeser, jr. 
