34 
BULLETIN 14! 
U. S. DEPARTMENT OF AGRICULTURE 
drought was experienced. An otherwise similar fall planting of the 
same species in a more favorable season had TO per cent and 51 per 
cent, respectively, of the trees living. 
HIGH TEMPERATURES 
The periods of extremely high temperature which occur almost 
every year also cause heavy losses, although the effect of the heating 
is difficult to distinguish from that of drought, since the two condi- 
tions usually occur together. Air temperatures of over 90° F. result 
in temperatures at the surface of the soil in the sun of 110° to 130° F. 
Temperatures above 120° have been shown to be fatal to the life 
activities of protoplasm, and it is only the protection given by the 
bark of the trees that prevents complete loss under these conditions. 
Northern white pine has a very thin bark and therefore suffers most 
SPRING PLANTING 
FALL PLANTING 
WEATHER CONDITIONS: I, MOST UNFAVORABLE TO 5, MOST FAVORABLE 
Figure 4. — Survival in relation to weather of season of planting 
from the heat. Examples of disastrous losses from this cause are 
furnished by two northern white pine plantations in Michigan, where 
1-year seedlings were set out in May, 1916, on sandy soils. As a 
result of maximum temperatures of over 90° F. in the shade on 10 or 
more days in late July, only 14 to 16 per cent of the trees survived. 
This is the chief reason why northern white pine should not be 
planted in exposed soils. 
FROST. SNOW, ICE, AND WINTER KILLING 
The other effects of weather which sometimes cause serious losses 
are late spring frost, heavy wet snows or ice which bend over and 
break the trees, and what is usually called winter killing. Frost 
damage, as with all crops, occurs in years when a heavy frost comes 
after growth is well started. The damage varies locally and is much 
more severe on low ground near swamps or stream courses and in 
slight depressions, or even in openings in the natural tree growth 
on the uplands. The evidence of this variation is plain to anyone 
