700 A NATIONAL PLAN FOR AMERICAN FORESTRY 



stocking. Fully stocked stands are exceptions rather than the rule 

 in this country. For example, in the Pacific Northwest the average 

 stand of immature Douglas fir is stocked from only 82 to 83 percent 

 of board-foot capacity, while for other species in other regions the 

 percentage of understocking seems generally higher. Although the 

 character of this loss is apparent, its exact evaluation is difficult, and 

 it is particularly hard to accurately gage the effect of each of the 

 casual factors of this understocking when more than one is operative, 

 as is usually the case. However, parasitic fungi and dwarf mistletoes 

 of conifers play an important part in reducing yield, both by killing 

 trees, thus causing understocking, and by lessening growth rate, thus 

 requiring a longer time to produce a given quantity of timber. Dis- 

 eases of the foliage, while rarely causing the death of enough trees to 

 affect stocking, by killing many leaves do interfere with food produc- 

 tion to such an extent that rate of growth is reduced. A particularly 

 notable case is the brown-spot needle disease of longleaf pine already 

 mentioned in connection with plantations, which so weakens young 

 trees in natural reproduction in parts of the South that it not only 

 adds a number of years to the time required to grow merchantable 

 timber but on the poorer sites it may kill so many seedlings as to 

 entirely prevent the development of a properly stocked stand. Root 

 rots and stem-inhabiting fungi, particularly rusts, also kill many 

 young trees. These parasites rarely become suddenly epidemic in 

 such a way as to kill trees over extensive areas in a short time; they 

 more commonly take a steady annual toll from infected stands. 

 Here again if the productive capacity of the stand is not reduced, the 

 losses are of no consequence, but if enough trees are killed either to 

 cause understocking or in addition in a mixed stand partial elimina- 

 tion of the more valuable species, then significant damage results. 



The most serious diseases reducing growth rate and also killing 

 trees outright are those caused by dwarf mistletoes of conifers. These 

 dwarf mistletoes are very different from the holiday mistletoe known 

 in the East ; they have no leaves and are without any sentimental or 

 esthetic value. In the West there are few conifers free from their 

 attack. The valuable stands of ponderosa pine in particular suffer 

 severely. Over extensive areas throughout the range of this pine 

 many trees are so deformed as to be worthless, some are killed, and 

 many more are infected sufficiently so that their rate of growth is 

 considerably lessened. A normally stocked stand of 40- to-50-y ear-old 

 ponderosa pine in northern California, moderately infected with 

 dwarf mistletoe, was found to have one third of the trees infected and 

 14 percent ruined by stem infections. On another area where the 

 attack was heavier every tree was infected and over 80 percent were 

 ruined by stem infections. In northern Arizona the average volume 

 increment over a 5-year period was found to be 4.53 cubic feet for 

 healthy trees and 2.23 cubic feet for heavily infected trees. Stands 

 of ponderosa pine over limited areas are so severely attacked that 

 they are worthless for lumber. 



The aggregate loss from diseases in immature stands destined for 

 timber production is high and probably approaches or exceeds the 

 losses caused by decay in mature stands. In the future, as mature 

 stands are even more depleted, the damage caused by these diseases 

 in immature stands will increase greatly in relative importance. 



Native diseases of immature stands must be largely controlled by 

 measures applied when the timber is cut. It is axiomatic that direct 



