274 

 Effects of IVindbreaks. 



A positive and more readily conceivable effect of a forest 

 growth on moisture conditions of the air is that which it has in 

 common, probably in increased degree, with the so-called wind- 

 break. The mechanical obstruction which a forest represents is 

 the principal effective element. By breaking the velocity of dry 

 winds and possibly enriching them somewhat with moisture, the 

 rate of evaporation over a neighboring field is considerably re- 

 duced, so that, in regions where winds are common, the protection 

 thows itself in increased crops on protected fields. A demonstra- 

 tion of this may be found along the North Hilo and Hamakua 

 coasts on the Island of Hawaii, where windbreaks, mainly of 

 ironwood trees, are planted at the edge of the cliffs overlooking 

 the ocean and are an effective means of protecting the cane to 

 leeward not only from the strong northeast trades, but from the 

 salt deposits which they carry with them from off the ocean. 



Regulation of Stream flow. 



It is the beneficial effect of forests on the rainfall, after the 

 rain has fallen, that interests us in a tangible manner, for there 

 is no influence of the forest that is of greater importance in the 

 distribution of water supplies than its effect in retarding the 

 runoff. To be sure, the topography and the geological structure 

 exert a powerful influence which a forest cover may either not 

 be sufficient or else is not needed to modify. It is noticeable, how- 

 ever, that the streams arising from a watershed that is well 

 forested -rise more slowly after a storm, remain in flood for a 

 longer period of time, and fall more slowly than similar streams 

 in non- wooded areas." As the rain will flow off the bare roof 

 of a house very rapidly, so will the runoff come away from a 

 watershed that has no retarding forest cover upon it. 



This influence of a forest cover in regulating the streamflow 

 is based on the principle that rain waters penetrate more readily 

 a forest-covered soil than one that is bared of a protective cover. 



This action is manifested in three ways : 



First, the mechanical obstruction which the foliage of a forest 

 offers reduces the amount of water that reaches the soil and 

 lengthens the time during which it can do so. The foliage, to- 

 gether with the loose litter of the forest floor, also reduces the 

 compacting effect of the raindrops and the drying eft'ect of sun 

 and wind and keeps the soil granular, so that the water can 

 easily percolate. 



Second, The mechanical obstruction which the litter, under- 

 brush, and trunks, and possibly here and there moss, offer to the 

 rapid surface drainage of waters, lengthens the time during which 

 this percolation may take place. 



Third, the network of deeply-penetrating roots, live and de- 



