About all that can be done to protect the forest against fungi is to keep it clean, that is, to clear out fallen timber and slash, and in some cases to dig trenches around affected trees to prevent spreading or to cut them out and destroy them. Such methods have heretofore been too expensive to employ in any ordinary American forest, but the time is at hand when such action will prove profitable in many localities. For the preservation of cut timber from decay, several methods are used. Fungi need heat, air, moisture and food. If any one of these is lacking the fungus cannot grow. Air and heat are hard to exclude from wood, but moisture and food can be kept from fungi. The removal of moisture is called seasoning, and the poisoning of the food of fungi is a process of impregnating wood with certain chemicals. Both these processes are described in _Handwork in Wood_, Chapter III. ANIMAL ENEMIES. The larger animals working damage to our forests are chiefly rodents and grazing animals. Beavers gnaw the bark, while mice and squirrels rob the forest of seed and consequently of new trees. The acorns of white oak are particularly liable to be devoured because of their sweetness, while those of red and black oak, which afford timber of comparatively little value, are allowed to sprout, and thus come to possess the land. Hogs annually consume enormous quantities of "mast," _i.e._, acorns or other nuts, by pasturing in oak and other forests. They, together with goats and sheep, Figs. 87 and 88, deer and cattle, work harm by trampling and browsing. Browsing destroys the tender shoots, especially of deciduous trees, but trampling entirely kills out the seedlings. The cutting up of the soil by the sharp cleft hoofs injures the forest floor, by pulverizing it and allowing it to be readily washed away by storms until deforestation may result, as was the case in France after the Revolution. It has cost the French people from thirty to forty million dollars to repair the damage begun by the sheep. In this country, this matter has become a very serious one on the Pacific Coast, where there are enormous flocks of sheep, and therefore the government is trying to regulate the grazing on public lands there, especially on steep slopes, where erosion takes place rapidly.[1] [Footnote 1: The evils of grazing are increased by the fact that fires are sometimes started intentionally in order to increase the area of grazing land.] [Illustration: Fig. 87. Goats Eating Foliage, New Mexico. _U.S. Forest Service._] [Illustration: Fig. 88. Sheep Grazing in Forest, Idaho. _U.S. Forest Service._] The most destructive animal enemies of the forest are the insects. The average annual loss of trees in the United States from this cause alone has been estimated to be one hundred million dollars.