THE NATURAL FOREST. it 
oration there would be ina calm.” In the spring, however, the veloc- 
ity 1s considerably greater, and an average speed of 15 or 16 miles 
per. hour may be maintained for a month at a time. The prevailing 
direction is northwestefly in winter and south to southwest in summer. 
As a direct result of the dry atmosphere and the high winds there 
is a great evaporation from every surface capable of giving off 
moisture, whether it be water, soil, or leaf. Since this evaporation 
increases from east to west, while the rainfall decreases, there is a 
further lessening of the amount of moisture available for plant life 
from this cause. 
The result of all these adverse climatic factors is that the common 
hardwood trees are confined closely to the water courses or to com- 
paratively wet situations. Even the permanent subterranean water 
is not sufficient for all species; the excessive evaporation also limits 
plant distribution. It is entirely possible for water to be taken from 
a large leaf surface faster than it can be supplied by the roots, even 
when the latter are in soil which contains much moisture. The 
western farmer knows only too well that the “hot wind” may 
wither and kill his corn at times when the soil is comparatively moist. 
The blanched blades and tassels show plainly that the plants have 
proved unequal to the strain and have died because they could not 
take up water from below as fast as it was given off to the dry air. 
Trees have been killed in seasons of severe drought, and there is no 
doubt that climatic conditions are the limiting factors in the distri- 
bution of the species belonging to the valley type. It is evident that 
anything which checks the wind and diminishes the evaporation will 
render conditions more favorable for tree growth. The sides of can- 
yons which face the north, and are consequently less exposed to the 
summer winds and the intensity of the sun’s rays, are often covered 
with shrubs and small trees, when the opposite side of the same 
canyon, with identical conditions in every other respect, contains 
almost no woody growth of any kind. Throughout this whole sec-— 
tion north and northeast slopes are more favorable to every kind of 
tree growth than any others and than the open levels. 
THE NATURAL FOREST. 
TYPEs. 
The natural forest growth of Kansas and Nebraska 1s: limited to 
the river courses and to a few upland areas of pine and cedar. The 
greater part of the region is destitute of trees. Two sharply defined 
forest types, which may be appropriately designated the pine type 
and the valley type, are recognizable. The pine type is. that char- 
«See Monthly Weather Review, September, 1888. 
