Native Vegetation 
i3 
its associates, are to be found on the cooler and moister north ex¬ 
posures. 
Plants most abundantly distributed at lower elevations find 
their uppermost altitudinal limits on warm, south exposures, while 
plants normally distributed, at high elevations reach their lower¬ 
most altitudinal limits on cool, moist, north exposures. For ex¬ 
ample, lodgepole commonly ranges from 8,000 to 10,000 feet, but 
on southwest exposures may reach 11,000 feet, as it does on Bor¬ 
eas Pass, and quite frequently goes down to 7,500 on cool, north 
exposures. Timber-line is frequently 1,000 feet, or more, higher 
on slopes facing south than on those facing north. 
The effect of exposure and slope is so marked at high alti¬ 
tudes that great care needs to be exercised in the selection of sites 
for crop purposes. Alter* emphasizes the significance of slope and 
exposure in this way: “Likewise, a. 5 0 slope to the south in south¬ 
ern Idaho is in the same solar climate as is a level field in the 
latitude of southern Utah, 305 miles nearer the Equator.” 
Throughout the state there are many locally “warm spots”, 
and “warm strips” surrounded by cooler districts. Such locally 
warm areas owe their higher temperature to topography. 
In narrow valleys, shaded slopes may have sometimes an ad¬ 
vantage over the sunny ones in the placement of an orchard. Slopes 
along the south side of narrow valleys that trend east and west 
may be shaded one or two hours longer each day than the neigh¬ 
boring slope across the stream. As a result of this shadow pro¬ 
tection, there may be a delay in the time of blooming, postponing 
it until the frost danger period is past. On the other hand, on the 
sunny slope, buds are hastened into activity, possibly to be injured 
later by frost. 
Length of Frostless Season .—“Length of frostless season” as 
used here means the number of days between the last frost in the 
spring and the first frost in the fall. From the climatological rec¬ 
ords it is evident that when the temperature goes to 32°, the day 
is reported as experiencing a frost. From Tables VII and VIII 
it is seen that our common fruits are not seriously injured by a 
temperature of 32° in any stage of their development. However, 
it must be held in mind that a recorded temperature of 32°, for ex¬ 
ample, is that of the air 6 feet from the ground, while the air tem¬ 
perature 6 inches from the ground surface is usually 3° to 8° 
lower. The maps showing length of frostless season and average 
date of last spring frost enable us to make valuable comparisons, 
and to better judge of the different localities in the state. Fig. 5 
gives the average length of the frostless season for the different 
*Crop Safety on Mountain Slopes, U. S. Dept. Agr. Yearbook, pp. 309-318, 1912. 
