16 Canadian Forestry Journal. 



ing effect. The amount of influence thus exerted depends, of 

 course, upon the character and the closeness of the tree-crowns, 

 — a thick stand of forest having a greater quantitative effect, in 

 the direction indicated, than a thin stand; while an evergreen 

 forest has less effect in summer, and greater effect in winter, than 

 a deciduous forest. 



Control of Winds and Storms by the Forest. 



One of the most important influences of the forest is due to 

 its action as a wind-break. On its windward side, the effect of 

 the forest is slight. But on the leeward side, the checking of the 

 velocity of winds results in partial or complete stagnation of the 

 air, with the phenomena attendant upon stagnation, namely, 

 increased humidity of the air, decreased evaporation from the 

 ground and from plants, higher temperature during the day, and 

 lower temperature at night. Hence the effect of the windbreak 

 is occasionally injurious, in favoring night frosts. But this is 

 only occasional. The ratio between the width of the area protected 

 and the height of the windbreak, has been variously stated by 

 different observers, but a moderate estimate is, that for every 

 foot of height of the windbreak, an area of ten feet in width is pro- 

 tected. Thus a belt of trees 30 feet high will protect from cold, 

 drying winds, objects on its leeward side at a distance of 300 feet 

 back from the belt. 



Protection from lightning is, in all probability, afforded to 

 buildings by single trees, to a greater extent by clumps and belts 

 of trees, and it is equally probable that extensive forests, while 

 they may increase the frequency, reduce the intensity and de- 

 structiveness of thunderstorms over their own areas and beyond. 



Influence of the Forest on Evaporation and Humidity . 



In the forest, evaporation from three sources is to be con- 

 sidered: (1) Evaporation from the soil; (2) transpiration from 

 the leaves; (3) evaporation from rainfall intercepted by 

 the leaves, branches and trunks of trees. German experiment- 

 ers have established for these quantities the following values: As 

 compared with the amount evaporated from a free water surface 

 in the open (1) the evaporation from soil under forest litter 

 and within the forest, is 13 per cent; (2) transpiration from the 

 leaves, 77 per cent; (3) evaporation of intercepted rainfall, 61 per 

 cent; total, 151 per cent. On the same basis of comparison, the 

 amount of moisture added to the air over cereals on the average 

 is 173 per cent; over sod, 192 per cent; over bare soil, 60 per cent. 



The forest, therefore, is more conservative of moisture than 

 sod or cereals crops, but returns to the air 2\ times as much mois- 

 ture as does the bare soil. 



