PART VII — WATER RESOURCES, FORESTRY, AND AGRICULTURE 



Figure VII-5 — EFFECTS OF FOREST FIRES 



The upper photograph shows an orchard near Santa Barbara, California. The single 

 open storm drain was normally adequate to handle storm runoff. In 1941, however, 

 the region enclosed by the dotted line was burned out in a brush fire. The lower 

 photograph shows the debris deposited by the runoff of a single light rain after 

 the fire. 



pine forests prescribed fire is actually 

 a legacy from annual burning by 

 Indian populations, and several stud- 

 ies fail to show any deleterious con- 

 sequences of its repeated use. Known 

 or probable exceptions, however, are 

 some areas of new forests planted 

 on severely eroded lands, and some 

 steep and sensitive soils. Again, some 

 studies of "slash burning" for reduc- 

 tion of logging debris in the Douglas 

 fir region reveal that the soil cover is 



totally removed from only a small 

 percentage of the burned area so 

 that infiltration remains high and 

 sedimentation negligible. But greater 

 fire severity, or slopes on which mass 

 movement occurs, increases the likeli- 

 hood of soil movement into stream 

 channels. Generally feasible alterna- 

 tives to fire have not yet been 

 found, but several interests — includ- 

 ing smoke abatement, possible value 

 of logging wastes, and fish manage- 



ment, as well as water quality per 

 se — have encouraged such research. 



In some regions the predominance 

 of alder and some other nitrogen- 

 fixing shrub species can be increased 

 by fires, disturbance, or silvicultural 

 treatment. Stands of alders fix sig- 

 nificant quantities of atmospheric 

 nitrogen, and some fraction of this 

 addition enters streams. The extent 

 of such contributions and their even- 

 tual effect on stream concentrations 

 are unknown, except by order-of- 

 magnitude estimates. However, these 

 indicate that fixation per unit area 

 over a period of some years must 

 often exceed the nitrogen additions 

 considered in forest fertilization pro- 

 posals. Hence, consequences of these 

 natural additions are of very con- 

 siderable interest. 



Reduction of the forest cover by 

 fire, wind, insects, and clearcutting 

 causes an abrupt increase in surface 

 temperatures and in mineralization 

 of the organic matter. The resulting 

 nutrient release may be followed by 

 increased leaching of nitrates and as- 

 sociated cations into streams. These 

 effects are highly dependent on cli- 

 mate and the quantity of surface 

 organic matter, and on the rapidity 

 with which a new cover of vegetation 

 appears. The well-known studies at 

 Hubbard Brook (New Hampshire), 

 although artificial in some degree, 

 served to focus attention on the 

 maximum quantities of nutrients that 

 may thus enter streams. Several other 

 studies in regions of lesser organic 

 accumulation and where natural re- 

 vegetation is allowed, show only 

 minor increases. A considerable 

 number of experimental treatments 

 and monitoring to study this effect 

 further are now under way. 



Virtually no attention has been 

 given to other forest management 

 treatments which probably act in 

 the same direction although at lower 

 intensity. These are drainage of for- 

 ested wetlands, broadcast burning, 

 and site preparation by destroying 

 vegetation and disturbing the soil. 



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