78 BULLETIN 12%, U. S. DEPARTMENT OF AGRICULTURE 
values. But perhaps the strongest evidence of the superior economy 
of this theory is that with every fire or fire-threatened area handled 
on the basis of insuring minimum damage, the risk of subsequent fires 
is lessened and future losses are reduced. The destructive processes 
of acceleration and attrition which this bulletin has endeavored to 
bring out in their true guise can be checkmated only by fire protection 
planned on such a basis. 
SUMMARY 
Throughout every section of the detailed examination of the réle 
of fire in the forests of the California pine region there have appeared 
two principal phases which may fairly be regarded as major conclu- 
sions of the entire investigation. 
_ 1. Fire in the virgin forests, in restocking brush fields, and on cut- 
' over lands is important not only in the loss of timber resources it 
causes, but also because each fire paves the way for greater and more 
serious losses from subsequent fires. This process of acceleration has 
characterized the action of fires through past centuries and is of out- 
standing importance in the fire problem to-day. As a result of this 
process, each fire, by allowing the invasion of inflammable brush 
species, and adding fuel in other forms, makes future protection more 
costly, more difficult, and more uncertain. 
2. Fires in the virgin forests of the California pine region rarely 
are catastrophes, for they do not wipe out at one stroke the entire 
stand over a large area. Indeed, they are generally distinguished 
by the fact that much of the damage is relatively inconspicuous and 
not immediately evident. But a study of the fires of the past and 
those of the present shows unmistakably that attrition is the inevi- 
table concomittant of repeated fires. This wearing down of the for- 
est is remarkably exhibited in all its varied stages in the California 
pile region to-day, from the well-stocked areas of mature timber to 
the nontimber-producing chaparral. The fire-scarred virgin forest; 
the broken, patchy timber stand of no present merchantability; the 
brush fields with scattered, isolated trees, and small groups of trees; 
the continuous brush fields occupying potential timberland and re- 
stocking only slowly; and finally, pure brush or chaparral, the end 
product, are but the different chapters of the story of attrition. 
The rapidity with which the processes of acceleration and attrition . 
_ operate to reduce the virgin forest to a nontimber-producing chap- 
_arral area varies widely, depending on a large range of factors, the 
‘most important of which is site quality. Kven the lightest spring 
‘surface fires, such as have been used in an attempt to reduce fire 
hazard, exhibit the same destructive tendency toward quality and 
quantity reduction as do the more devastating summer fires. In the 
virgin forest the initial steps of attrition and acceleration are slow; 
but in brush fields and cut-over lands of the present day we find 
these processes in their most destructive and consequential phases, 
since here even a single fire ordinarily accomplishes the annihilation 
of the new forest. 
Through site deterioration effected by centuries of acceleration of 
fire damage and attrition from fire injury, the forest of to-day has 
assumed a definite character very different from what it is popularly 
supposed to be. The general public viewpoint that the national 
