44 BULLETIN 1294, U. S. DEPARTMENT OF AGRICULTURE 
Recent studies have shown (23) that fires in brush are far more 
difficult to control than those in virgin forest, and attain a much 
larger average size. Once started, also, they are likely to sweep into 
adjoining timber stands with an intensity that results in wiping out 
the immediately adjacent timber belt, thus extending the brush fields 
themselves. 
_ Fires in brush fields are typically crown fires and partake of the 
nature of crown fires in timber. The conclusion already pointed out. 
that one fire paves the way for greater intensity and damage by suc- 
ceeding fires applies in the highest degree to firesin brush. Without 
exaggeration, it may be said that the ultimate productivity of the 
pine region and the success of systematic forest management in Calli- 
ornia depend in large measure on the reclamation of the brush fields. 
This is not merely because the productive capacity of the brush 
lands is essential for our timber requirements, but for the more 
important reason that it will be impossible to guarantee success in 
protecting either the virgin forests or cut-over lands as long as the 
threat of disastrous crown fires in brush exists. 
DAMAGE TO WATERSHEDS 
In some of the older countries, where the effect of destructive 
agencies 1s most thoroughly understood, the secondary or indirect 
influences of the forest are given as much consideration as its value 
in producing a wood crop. 
In California, Munns’s investigations (20) have shown clearly the 
influence of fire not only on the site itself, but on erosion and run-off, 
Fires seriously reduce the mechanical interference with erosion 
afforded by the forest or brush cover, and. also destroy the fertile 
vegetable mold or humus of the top layers of soil. This reduction 
is in itself a lowering of site quality, since the nitrogenous material 
derived from humus is essential for a vigorous growth of forests. It 
also adversely affects the moisture-holding capacity of the soil, so 
that less water is held per cubic foot of soil after the fire than before.” 
Experiments over a period of years show that run-off is more rapid on 
burned than on unburned areas, and that erosion is more likely to start 
and to reach more disastrous proportions, and that the flow during the 
dry period is much less in streams heading in burned watersheds than 
in those in the forested areas. 
In so far as a single fire is concerned, even a very intense or destruc- 
tive one, the period of heavy erosion does not. continue indefinitely ; 
but particularly on brush fields and cut-over areas, where fires are 
ordinarily severe, site deterioration and erosion after fire have been 
shown to follow most readily. These secondary forms of damage— 
site deterioration, erosion, and changes in stream flow—have proved 
to be very difficult to evaluate, since their effects are not so imme- 
diate or so readily discernible as direct damage to virgin timber. 
Serious as are the results of fire and subsequent erosion on the for- 
ested lands of the mountains, it is at least an open question whether 
the tributary valley lands are not in the long run affected equally. 
12 Cooper’s studies (7) show that the soil of the forest has more humus and consequently a greater 
moisture holding capacity than the soil of the brush field. 
