162 OUR NATIONAL FORESTS 



The next important step is to prevent the infection 

 and infestation of sound trees by getting rid of all 

 diseased and insect-infested living and dying trees. 

 By means of timber sales and free use, Forest offi- 

 cers very materially help in establishing healthy con- 

 ditions on the National Forests. There is a clause 

 in most timber sale contracts which requires the cut- 

 ting by the purchaser of all snags and other un- 

 healthy trees on the area. This measure not only 

 eliminates undesirable trees from a hygienic stand- 

 point, but it also makes it possible to utilize the 

 merchantable timber left in undesirable trees, which 

 would otherwise go to waste. On timber sales, For- 

 est officers who do the marking leave for reproduc- 

 tion only such trees as are perfectly sound and 

 healthy. Mistletoe infested trees, especially, are 

 marked for cutting, for neither in plant nor in ani- 

 mal life can healthy offspring be expected to de- 

 velop under unhealthful conditions. 



Water Supply. Undoubtedly the greatest value 

 of the mountain forests of the West, most of which 

 are within the National Forests, lies in their influ- 

 ence upon the regularity of the water supply. In 

 many States these mountains afford the only water 

 supply for domestic use, for irrigation, and for the 



