88 MAN ON THE LANDSCAPE 



A severe forest fire creates an unbalance, to put it mildly. One 

 distant result may be that ten years later a truck falls through a 

 bridge 30 miles away because the tax income for proper maintenance 

 of the bridge went with the forest land value. 



Let us follow the trail of the unbalance more closely. After the 

 fire, the soil is exposed because the leaf litter and a large part of the 

 topsoil humus have been burned and nitrogen returned to the air. 

 When rain comes the ashes on the land become a paste which seals the 

 surface. This results in a high per cent of runoff, which, here and 

 there in depressions, concentrates large quantities of flowing surface 

 water. This inevitably starts gully cutting, and is the forerunner of 

 floods. As the ashes are gradually dissolved and washed off by subse- 

 quent rains, both sheet and gully erosion proceed more rapidly. In 

 the meantime the strongly alkaline ashes render small streams unfit 

 for aquatic plant and animal life; fish are either killed outright or 

 driven out of the area. (During intense forest fire, fish have actually 

 been boiled alive.) 



Fish may not return for several years, because silt will continue 

 to pollute and cloud the water, interfering seriously with aquatic 

 plant growth by reducing photosynthesis. The destruction of vegeta- 

 tion along the stream eliminates shade and the water temperature may 

 rise. (It may, for example, pass that critical 72 degrees which trout 

 cannot endure.) Gone too are the insects, both adult and larval, which 

 once dropped from the shore vegetation to help feed the fish below. 



The destruction of organic matter by heat oxidation on and in the 

 soil, and erosion and compaction by rain of the soil itself, mean that 

 its fertility and its ability to absorb water have been markedly de- 

 creased. The soil's ability to sustain life has been diminished in pro- 

 portion to the amount of damage done before the complex process of 

 destruction is stopped. In relation to time, this destruction is very 

 significant to humans, since it is estimated by soil scientists that from 

 200 to 1,000 years is required by nature to produce an inch of topsoil. 



With the soil go the organisms, the small and microscopic plants 

 and animals which are essential to maintaining a fertile topsoil. The 

 reduced shade increases soil temperature and speeds up oxidation of 

 any remaining humus. The larger forms of animal life are driven out 

 of the burned area; some of them doubtless were killed by the fire. 

 They cannot come back immediately because their food and shelter are 

 gone. 



By such a fire and its erosional aftermath the entire climax com- 

 munity has been destroyed (See Fig. 21.) The area has been rendered 

 bare and succession will set in with the invasion of such pioneer plants 

 and animals as can exist in the denuded area. For a considerable 

 period, the amount of photosynthesis will be small. When enough 

 herbaceous material is present, animals such as rabbits and field mice 

 will come into the territory. They will eat many of the seedling trees 

 and shrubs, and seeds which are blown in. Hawks, foxes, and owls 

 will come to feed on the rodents. The long and arduous trail back 

 to climax begins. 



