Fig. 104. Slash, Left in the Woods, and Ready to Catch Fire. U. S. Forest Service.
Opportunities for fire. There are a number of facts that make the forest peculiarly liable to fire. Especially in the fall there are great quantities of inflammable material, such as dry leaves, twigs, and duff lying loose ready for ignition. The bark of some trees, as "paper birch," and the leaves of others, as conifers, are very inflammable. It follows that fires are more common in coniferous than in deciduous forests. After lumbering or windfalls, the accumulated "slash" burns easily and furiously, Fig. 104. Moreover a region once burned over, is particularly liable to burn again, on account of the accumulation of dry trunks and branches. See [Fig. 107].
Long dry seasons and high wind furnish particularly favorable conditions for fire. On the other hand, the wind by changing in direction may extinguish the fire by turning it back upon its track. Indeed the destructive power of fires depends largely upon the wind.
Fig. 105. Forest Fire. U. S. Forest Service.
Causes of fire. Forest fires are due to all sorts of causes, accidental and intentional. Dropped matches, smouldering tobacco, neglected camp fires and brush fires, locomotive sparks, may all be accidental causes that under favorable conditions entail tremendous loss. There is good reason to believe that many forest fires are set intentionally. The fact that grass and berry bushes will soon spring up after a fire, leads sheep men, cattle and pig owners, and berry pickers to set fires. Vast areas are annually burned over in the United States for these reasons. Most fires run only along the surface of the ground, doing little harm to the big timber, and if left alone will even go out of themselves; but if the duff is dry, the fire may smoulder in it a long time, ready to break out into flame when it reaches good fuel or when it is fanned by the wind, Fig. 105. Even these ground fires do incalculable damage to seeds and seedlings, and the safest plan is to put out every fire no matter how small.
Fig. 106. Burned Forest of Engelmann Spruce. Foreground, Lodgepole Pine Coming in. U. S. Forest Service.
Altho it is true that the loss of a forest is not irremediable because vegetation usually begins again at once, Fig. 106, yet the actual damage is almost incalculable. The tract may lie year after year, covered with only worthless weeds and bushes, and if hilly, the region at once begins to be eroded by the rains.
After the fire, may come high winds that blow down the trunks of the trees, preparing material for another fire, Fig. 107.