INJURY CAUSED BY FOREST FIRES 251 



material. On account of the depth of the humus (often one 

 to over two feet in thickness) ground fires apparently burn 

 underground in the soil itself. Actually only the vegetable 

 matter is consumed and the fire is burning above the soil or 

 in pockets formed by decaying roots. Such fires are sometimes 

 termed " duff " fires. 9 



When the surface is wet by a light shower, but the duff 

 beneath is dry, a ground fire may burn the lower layer of 

 organic material, leaving the surface intact supported by a 

 network of plant roots. When the fire is burning under- 

 ground, in this way, its exact location is difficult to deter- 

 mine. Cases are on record where ground fires burned for 

 several months in swamps. Such fires are said to have lasted 

 through an entire winter under a blanket of snow. 



The feeding roots of the trees extend throughout the thick 

 humus layer and are easily killed by the ground fire. Such a 

 fire ordinarily kills all the trees on the area burned over. 



Deep deposits of humus are formed only under conditions 

 which retard its normal decomposition. In swamps and low- 

 lands, with excess of water, the organic matter does not 

 readily decay. At high altitudes and in northern latitudes, on 

 account of the cold damp climate even upland sites may 

 develop thick beds of duff. This is particularly the case with 

 coniferous stands. 



Ground fires are relatively rare as compared with other kinds 

 of fires because the material on which they feed does not 

 dry out except in seasons of drought, and sites on which they 

 may occur form a small percentage of the total forest area. 



Surface fires are those which burn on or near the ground in 

 the humus, litter, ground cover and underbrush. They are 

 the commonest kind of fire and occur in all parts of the 

 country. 



Crown fires burn in the crowns of the trees. In tall timber 



