EANGE PRESERVATION AND EROSION CONTROL. 17 
The above summations show clearly thai the maximum velocity of 
wind in the spruce-fir type exceeds by about 200 per cent that in the 
aspen type at certain periods. These gales on the* elevated plateaus, 
especially where the ground cover is sparse, have a marked effect on 
the movement of the soil and without .doubt are an important factor 
in causing erosion when the surface soil is dry and exposed (PI. II). 
It is especially important, therefore, that the vegetable cover on 
these elevated lands be maintained in a maximum state of clenshVy in 
order to bind the soil firmly. 
VEGETATIVE COVER. 
AYhile the foregoing data indicate that the extent of surface run- 
off and erosion are determined by the combined action of a number 
of factors, the vegetative cover is the most important single control- 
lable factor under the conditions in question. Man has little control 
over climate and topography, and improvement in soil conditions 
most favorable to the control of erosion on the range lands under 
discussion must be accomplished chiefly through the improvement 
in the vegetative cover. Even this possibility of control is limited 
primarily to what can be accomplished by management of the lands 
so as to favor the development of the native vegetation to the great- 
est possible extent because western range conditions in general are 
not favorable to the planting of cultivated species. This importance 
of the native vegetative cover in maintaining conditions unfavorable 
to erosion may be considered both a drawback and an advantage, for, 
on the one hand, certain precautions must be taken in harvesting the 
forage crop in order to preserve and maintain the vegetation ; but on 
the other hand, there are relatively few lands which, under proper 
management, can not be revegetated enough so that serious erosion 
and destructive floods may be prevented. 
Anybody on a virgin or completely vegetated range during a 
heavy rainstorm can not fail to notice to what a great extent the 
vegetation, whether grass, weed, browse, or timber, protects the soil 
and increases its power of soaking up the water. Instead of the 
entire force of the rain falling on an unprotected and exposed soil 
surface, as in the absence of vegetation, the rain is intercepted more 
or less by the vegetation, so that by the time the water reaches the 
soil surface its original force is broken. There are several reasons 
why a well vegetated surface offers the best condition for absorp- 
tion and underground storage of water. The foliage and stems of 
the vegetation form a storage place from which water drips slowly 
to the ground for considerable time after each rain ; and the leaves 
and stems, in a more or less advanced stage of decay, absorb mois- 
46360°— IS— Bull. 675 3 
