defective as the result of disease. 

 On the other hand, much of the 

 timber felled by wind is defective. 



Annual loss caused by wind throw 

 was estimated at 80 million board 

 feet, approximately 0.1 percent of 

 the total stand of ponderosa pine. 



The annual drain on stands of 

 saw-timber size from all causes totals 

 nearly 2.6 billion board feet, log 

 scale, Scribner rule, and on all 

 stands in which the timber is 5.1 

 inches d. b. h. or more, approxi- 

 mately 527 million cubic feet (table 

 19). 



Assumed Future Depletion 



In estimating potential growth 



rates and future forest inventories 



the most uncertain and also one of 



the most important factors is drain. 



Although current drain can be 



closely approximated from available 



records of sawlog cutting, fire 



records, and insect damage surveys 



estimates of future drain must be 



based on assumptions. In the sur- 

 vey these assumptions were made 



after careful analysis of past and 



current rates of drain, present in- 

 ventory and ownership of timber 



resources, capacity of the wood- 



utilizing industry, and regional and 



Nation-wide lumber production 



trends. 

 Assumptions of depletion by 



cutting and fire were made for 

 each of three decades, 1936-45, 

 1946-55, and 1956-65. These are shown in 

 table 20 by survey unit. Depletion resulting from 

 the natural agencies of forest insects, disease, and 

 wind throw were not included, on the assumption 

 that losses attributed to these agencies are offset 

 by growth in old-growth stands and allowed for 

 in computing yield in second-growth stands. 



The estimate of annual cutting depletion for 

 the decade 1936-45 of 1,745 million board feet 



Figure 22. — A stand of lodgepole pine, type 26, killed by fire. The present condition of 

 this area creates a critical fire hazard. 



is approximately 39 percent higher than the aver- 

 age annual cut during the 12-year period 1925-36,. 

 but the trend of sawlog production, which is 

 about 87 percent of total production, was sharply up- 

 ward during the last 4 years of the period. Production 

 continued to increase in 1937, and the drop in 

 1938 was still well above the average for the period 

 1925-36 and more than balanced by the all-time 

 high in 1939. 



41 



