30 BULLETIN 12%, U. S. DEPARTMENT OF AGRICULTURE 
~and by this process of attrition has been reduced in density, thereby 
increasing the amount of undergrowth. The latest fire on this area, 
which has caused tremendous damage, is the logical and inevitable 
result of the previous fire history. All such examples lead to the 
same conclusion: Fire in forest areas invariably breeds stil more 
Serious fires. 
Because of attrition, exerted in these and other ways, the present 
virgin forest, however splendidly complete and magnificent it may 
appear, is not fully stocked and represents but a part of the produc- 
tive power of the land. The volume per acre in virgin forests 
repeatedly subjected to fire is not comparable to the volume per acre 
in fully stocked second-growth forests on sites of the same quality, 
but from which fire has been excluded. One instructive example is 
illustrated in Table 17. From this table it is evident that for exten- 
sive areas of virgin forests the average yield to the acre of stands 200 
to 300 years old is very much less than the yield obtained from the 
more fully stocked stands of 50 or 60 years of age. (PI. V, fig. 1.) 
TABLE 17.—Yield of virgin forest and fully stocked second-growth forest, by sites 
[Lassen National Forest] 
| 
a Degree of 
Area of | Yield of Yield of normal 
Site| virgin virgin second- stocking 
growth |: Bera 
forest forest forest 1 | 12 Virgin 
forest 
Acres M od. ft. | M bd. ft.| Per cent 
2, 34. 74. 0 46 
1 , 149 2 
2 20, 277 24. 0 56. 0 43 
3 129, 767 16.7 40. 7 41 
4 118, 349 7.5 26. 0 29 
1 Second-growth yields are determined by analysis of 130 measured plots in fully stocked stands, second 
growth, 50 to 60 years old. 
It is unnecessary to insist either that the discrepancies shown 
above are due solely to fire or that the actual figures in this table 
are representative for the entire forest region. The inference is plain 
that even where the virgin forest has persisted fairly well in spite 
of repeated fires, the yield is naturally lower than if the area ae 
escaped from fires. A second illustration of the same truth is found 
in an estimate of the yields of virgin and fully stocked second-growth 
tracts in the Plumas National Forest, On second-quality sites the 
mature virgin stands show a present yield per acre of 28,800 board 
feet, estimating on a basis of 33,033 acres, while fully stocked second 
rowth aged 50 years, on similiar sites, shows already 27,130 board 
eet. 
From this it is seen that, for the mixed conifer forests of the west 
slope of the Sierras, the average present stand of virgin timber can 
not compare favorably in yield per acre with fully stocked young 
growth, which is already nearly equal in yield. In the rare patches 
of fully stocked virgin forest, carefully measured sample plots showed 
an average yield of 110,000 board feet to the acre, compared to 42,000 
board feet for even the average best of the typical virgin forest. These 
illustrations make it clear that the present yields of even our best vir- 
gin forests probably represent less than half/the timber-producing 
capacity of the ised. 
