136 



hence if "cut clean" or completely dissipated it takes many years to 

 build it up again to its full productiveness. 



The annual yield of these wood-lots as indicated by the farm ques- 

 toinnaire was 43.045 cubic feet per acre. When we compare this yield 

 with the data shown in Table on page 135, indicating the productiveness 

 of dififerent classes of soil, it is evident that average grades of forest 

 soils will grow this much wood annually, hence it is possible to grow 

 timber fast enough to maintain the present rate of cutting forever, and 

 it may be possible to double it. But the probability is that, instead, 

 this yield will continue to decrease. There are two reasons for this. 

 First, the maintenance of an annual cut requires the reservation of a 

 certain quantity of wood capital in the form of young trees partly grown. 

 Depending on the products desired and the period required to produce 

 them, the growing stock required may be roughly calculated by taking 

 one half of the total growth or final yield of the crop. For example, if 

 the yield is 40 cubic feet per year and can be harvested at 50 years, the 

 crop yields 2000 cubic feet. Then 1000 cubic feet per acre is a normal 

 forest capital ; actually somewhat less than this quantity will suffice. But 

 if the stock or capital is exhausted by clear-cutting for cord wood or 

 niine timbers, it drops way below this required minimum, and when this 

 happens it becomes impossible to maintain the same rate of yield until 

 the gradual processes of growth have again built up the stock. This 

 scarcity occurs first for the larger and more valuable products, such as 

 saw-timber or veneer logs. The smaller the material harvested the 

 sooner it can be replaced and the greater the probability of sustaining 

 the yield. The second factor is far more serious, and consists of destroy- 

 ing the reproduction which would naturally take the place of the timber 

 removed. When this happens for any cause, such as fire or excessive 

 grazing, there is no hope of ever restoring the yield to its former volume 

 until such cause is removed and the reproduction re-established, perhaps 

 by expensive measures followed by the long wait for the new crop to 

 mature. 



The average acre of woodland in Illinois now contains but 635 cubic 

 feet, which, for a low average yield of 43 cubic feet per year, would 

 indicate that this yield can be maintained only on the basis of cutting 

 the material at an age of 30 years. This period is insufficient to pro- 

 duce saw-timber or even cross-ties and will yield only posts, mine, tim- 

 bers, and cordwood. Either the future yields of Illinois wood-lots 

 will be confined to these products after the residue of the larger trees 



