134 



FOREST INFLUENCES. 



taiice and lieiglit beyond tlie slielter. Unfortunately no accurate ex- 

 perimental data concerning this influence are at hand. According to 

 Becquerel, asiini)le hedge Ofeet in height will give j)rotection for a dis- 

 tance of 70 feet; and, according to Hardy, a belt of trees every 300 feet 

 will defend vegetation almost entirely against the action of the wind. 

 A.nother authority finds for every foot in height 1 rod in distance pro- 

 tected. (See also p. 14.9 of this bulletin.) 



From many reports received in this office on the effects of windbreaks 

 upon agricultural crops, we may quote one from a farmer in Illinois, 

 which expresses the observations generally made: 



My experience, is that now in cold and stormy winters wheat protected by timbet 

 belts yields full crops, while fields not protected yield only one-third of a crop. 

 Twenty-five or thirty years ago we never had any wheat killed by winter frost, and 

 every year a full crop of peaches, which is now very rare. At that time we had 

 plenty of timber around our fields and orchards, now cleared away. 



The damage done to crops by the cold, dry winter winds is mainly 

 due to rapid evaporation, and plants are liable to suffer as much by 

 winter as by summer drought. 



This is certain, that since summer and winter drought, i. e., rapid 

 evaporation, due to the continuous dry winds, is the bane of the farmer 

 on the plains, rationally disposed timber belts alone will do much to 

 increase available water supply by reducing evaporation. 



How the forest cover, and especially the litter of a well-kept forest, 

 may decrease the amount of evaporation within the forest to nearly 

 seven-eighths of that in the open has been discussed on page 99 of this 

 bulletin. The reason for this important influence of the forest is due 

 not only to the impeded air circulation, but also to the temperature and 

 moisture conditions of the forest air and forest soil. 



The stations of Prussia allow the following average for evaporation ; 

 the amount evaporated in the open fallow fleld being called 100: 



A balance calculation of the amounts of precipitation and the amounts 

 lost by evaporation for sixteen stations at varying elevations shows 

 that with increasing altitude the suri)lus of water remaining for the soil 

 increases, the mountain forest decreasing evaporation to its minimum 

 of 9 to 13 per cent, and leaving from 87 to 91 per cent to penetrate the 

 soil.* 



* This is a larger amount than that given on page 101, where the water running 

 off the trunks of trees is counted as lost. 



