Dec. i 5 ,1925 Yield Capacities of Yellow Pine Type in California 1133 
DETERMINATION OF ROTATION 
Various methods have been used for determination of rotation age 
and it is not the intention here to discuss these further. As a matter 
of pure silviculture the culmination of mean annual increment is 
probably the most generally recognized criterion. An examination of 
Figure 9 will show that mean annual increment reaches its highest 
point at an age of 120 to 130 years, after which it remains at 
practically the same level for 20 to 30 years before beginning to fall. 
The only excuse for extending the cutting age much beyond the culmi¬ 
nation would be on the ground of quality increment, but where this is 
not important a rotation of 120 years at the outside is ample. 
Figure 10, showing the board-foot cubic-foot ratio for the entire 
stand, also points to the conclusion that 120 years is about the^proper 
cutting age. Up to that period in the stand’s development on most 
of the sites the yield in board feet per cubic foot increases rapidly; 
beyond that point the yield is either practically constant or rises 
but slowly. 
FACTORS OF CONSERVATISM 
In the tables there are several elements which may be called factors 
of conservatism, which deserve discussion. The first of these is the 
allowance of 20 years for seedlings to reach breast height. As has 
already been explained, this will no doubt be lowered 5 to 10 years 
provided cutting is heavy, which will very likely result in a somewhat 
shortened rotation or a higher yield per acre on the same rotation 
of 120 years. 
The second factor is the undoubted influence of the mature timber 
on the growth and yield of the plots. There is no way at present of 
measuring the influence of this factor, since practically no plots of 
the 50 and 60 year age classes, were found within the virgin timber, 
whereas all of the true second growth measured was of those age 
classes. At any fate, the elimination of this factor will tend not so 
much to reduce the rotation as to increase the yield. 
The third factor is the density of present stands. Within the next 
century thinning will undoubtedly come into effect and will tend to 
increase the yield. Still another element which may be very im¬ 
portant is the fact that many stands which are now pure yellow pine 
will in the future be a mixture of pine and white fir, with probably 
higher yields per acre than from the pure pine. Here again it is 
very difficult to forecast just how much additional yield per acre a 
change in composition will make, but whatever the change, yields will 
undoubtedly be increased and the use of the present table will therefore 
give conservative estimates of the productive capacity of the entire 
area. 
The application of the data in Table II to the stands obtained after 
cutting is a distinct problem which this study has not attempted to 
solve. The yields obtained on fully stocked small plots can not be 
expected over large areas, and reductions will be necessary in employ¬ 
ing the table. 
YIELD OF WHITE FIR STANDS 
Although the data do not permit any positive conclusions regarding 
the yields of white fir stands under different conditions of age and 
site, the indications are (fig. 11) that on a given area of land white 
78383—26t-3 
