ieee 
Sad pe ee ts ete ’ * iy 
._ Minnesota State HorricvuLtturaL Socrery. 83 
of the wooded surfaces by means of shade from excessive evaporation and thus 
prevent the drying of springs, and ensuring a more equable flow of water in the 
streams. 
Effects of Shelter Belts on Temperature, Winds and Moisture. 
The soil of rugged surfaces is usually thin and of inferior quality and only 
adapted to the growth of certain species of trees that naturally flourish in poor 
soils, and in such exposed situations, to which the Scotch pine is especially 
adapted and is recommended as one of the best evergreens for profitable plant- 
ing. 4 
In the so-called Western States we have fortunately but limited areas of lands 
not suited for general agricultural uses, hence the principal portion of forest 
planting must necessarily be done upon tillable soils; and as the extent to which 
evergreens can be planted with profit, depends upon the system or form adopted. 
It requires, therefore, a limited statement of the advantages to be derived from 
the general adoption of the shelter-belt system. 
The soil of forests—as has been determined by a series of European observa- 
tions*—hbeing cooler in summer, and warmer im winter, than the soil of open 
fields, it is apparent, therefore, that heated air currents passing over forests must 
necessarily be reduced in temperature, and their capacity for saturation by mois- 
ture be correspondingly reduced and consequently a lessened evaporation of 
moisture from the soil, hence, with a properly systematized distribution of forests 
in the form of shelter-belts would tend to lower the summer temperature and 
increase the winter temperature of the surface air currents moving in any direc- 
tion and thereby lessening the evaporation of moisture in the summer and 
reduce the radiation of heat in the winter. 
Forests in the form of shelter-belts retard the velocity of surface air currents. 
The volume of evaporation of moisture from the soil, is proportioned to the 
velocity of thé winds as well as to the volume of atmospheric moisture. If by 
means of a system of shelter-belts, the velocity of surface winds is reduced 
one-half, a reduction in evaporation of moisture from the soil is effected, and 
the accumulation of moisture in the soil by rain fall in thefautumn, winter and 
spring, in excess of evaporation, is thus held in reserve for the support of vege- 
tation in July and August, when the evaporation is in excess of the rain fall, 
and thus ensues a more constant supply of moisture in the soil and be the means 
ensuring a larger yield of many kinds of farm crops. 
Whatever effect forests may have upon climate, it is evident that, as the ever-. 
greens retain their foliage throughout the year, they must necessarily effect a 
greater modification in the winter than the deciduous trees,zwhich have only 
their naked stems and branches to offer as a resistance to the winds. The degree 
of the modification of the winter temperature produced by evergreen trees would, 
aside from other climatic considerations, depend upon the form and extent of the 
forests. : 
It is a well known law that friction of any substances, liquids, gases or atmo- 
spheric air, generates heat. If it be supposed that there are two areas of land 
surface, each area to be two hundred miles square, having the same climatic con- 
ditions, and that the average velocity ot the winter winds is twenty miles per 
hour; one of these tracts of land to be planted with shelter-belts of evergreens 
around each forty acres, and that when the belts attained a-height of forty feet, 
*Prof. F. B. Hough’s Forestry Report, 1879. 
