42 ELEMENTARY FORESTRY. 



FOREST INFLUENCES ON WIND AND HAIL STORMS. 



We have seen that the wind-breaking power of the forest is 

 a very important factor in retarding evaporation, and in prevent- 

 ing the drifting of sandy soil and snow. In the forest the air 

 may be rather still while in the open a piercing gale may be 

 blowing; in consequence there are no blizzards in a wooded 

 country. Tornadoes of great force have occasionally broken 

 down wide areas of timber in this section, but instances are very 

 rare in which they have continued for long distances through 

 forests, and it is probably true that forests have a tendency to 

 prevent their formation and perhaps entirely break up those of 

 lesser violence. M. Becquerel is said to have found by careful 

 study that in some parts of central France hail storms show a 

 marked disinclination to enter forests, and yet occasionally they 

 do so, but nothing of this sort has been noticed here. 



FOREST INFLUENCES ON FOGS AND CLOUDS. 



The influence of forests on fogs and clouds has frequently 

 been mentioned. The fog seems to linger in the woods after 

 it has cleared off elsewhere. Trees act also as condensers and 

 gatherers of dew, hoar-frost and ice; the latter phenomenon is 

 especially remarkable in the so-called ice storms, where the 

 accumulation of ice on the trees is so great as to break them 

 down. The load of ice on some large trees is probably a ton 

 or more. In this case the tree acts simply as an inorganic body. 



IMPROVEMENT OF LAND ON WHICH TREES GROW. 



As has been shown, trees add large amounts of soluble min- 

 eral matters to the soil through the fall and decay of their 

 leaves. In the same way they add large amounts of humus to 

 the land, which helps to keep the soil porous and yet makes it 

 more retentive of moisture and gases. The roots of trees often 

 penetrate deep into the soil and bring up plant food that would 

 not be reached by agricultural crops. A part of this is returned 

 to the surface soil by the yearly fall of the leaves and in the 

 twigs and brandies that are left on the ground when the tree 

 is cut down. The roots deepen the soil, and by their decay fur- 

 nish plant food to the soil and leave channels through whidi 



