INSECTS AND DISEASES THAT DESTROY FORESTS 71 



younger and healthier trees to grow. Usually 

 the trees that are blown down have shallow roots 

 or else are situated in marshy, wet spots so that 

 their root-hold in the soil is not secure. Trees 

 that have been exposed to fire are often weakened 

 and blown down easily. 



Where excessive livestock grazing is per- 

 mitted in young forests considerable damage 

 may result. Goats, cattle and sheep injure young 

 seedlings by browsing. They eat the tender 

 shoots of the trees. The trampling of sheep, espe- 

 cially on steep hills, damages the very young 

 trees. On mountain sides the trampling of sheep 

 frequently breaks up the forest floor of sponge- 

 like grass and debris and thus aids freshets and 

 floods. In the Alps of France sheep grazing de- 

 stroyed the mountain forests and, later on, the 

 grass which replaced the woods. Destructive 

 floods resulted. It has cost the French people 

 many millions of dollars to repair the damage 

 done by the sheep. 



The Federal Government does its best to keep 

 foreign tree diseases out of the United States. 

 As soon as any serious disease is discovered in 

 foreign countries the Secretary of Agriculture 

 puts in force a quarantine against that country. 



