406 PKACTICAL BOTANY 



374. Influence of forests on climate and water supply. The 



temperature and rainfall in and near forests are thought to 

 have slightly different values from those obtained in treeless 

 areas, but the amount of the differences is not fully settled. 

 The effect of woods, or even belts of trees, as windbreaks is 

 familiar enough to every one who has traveled along a road 

 through land partly wooded and partly open on a windy 

 winter day. Evergreen conifers are, of course, much more 

 serviceable than deciduous trees as a protection against winter 

 storms, and they are much planted for this purpose in belts 

 in the more northerly plains and prairie states. 



The greatest service, however, which is rendered by wood- 

 lands, next perhaps to their use as sources of timber, is in 

 preventing the water which falls as rain or snow from running 

 off at once into streams, thus causing floods which are fol- 

 lowed by long periods of low water. The run-off, even in a 

 grassy prairie country, during and after rains is very rapid, 

 so that streams may be overflowing their banks in a few hours 

 after the beginning of a heavy rain. In a region where the 

 land is mostly tilled, with little woodland, the run-off is still 

 more rapid, so that a great deal of the water precipitated on 

 a hundred-acre field may have found its way into ditches or 

 watercourses outside of its limits within much less than an 

 hour after it fell. In treeless regions, as every prairie boy 

 knows, many stream beds in summer become merely a suc- 

 cession of pools or are dry throughout their length. On the 

 other hand, the forest floor is often moss-carpeted, strewn 

 with leaves and covered with underbrush, and overlies a rich 

 black soil containing much partially decayed animal and vege- 

 table matter known as humus, which is penetrated to a depth 

 of many feet by an intricate network of rootlets. This soil 

 retains the water from melting snows, often for weeks, and 

 holds the heaviest rains for long periods. The water gradu- 

 ally drains off along the surface or travels slowly through the 

 deep, porous soil, gradually finding its way into the streams. 

 It is therefore of the highest importance that such regions as 



