20 



BULLETIN 722, U. S. DEPARTMENT OF AGRICULTURE. 



as in semiswamp, bottom sites, decay is prevalent and a retardation 

 of growth also results. No relation between the composition of the 

 soil and the prevalence of the fungus has been observed. Hemlock 

 growing on lime, sand, or clay soils as a base for the regular humus 

 layer cxliibits no greater proclivity to fungus attack than do other 

 species. Of the total answers received, 41 stated definitely that the 

 least defective hemlock was found at the upper elevations and slopes 

 and upon well-drained soils. The most defective stands were en- 



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Fig. 11. — I^iagram showing the relation of various factors to the age classes of western hemlock on plats 



of the river-bottom type. 



countered at lower elevations, on fiats and bottom sites, and upon 

 poorly drained soils. 



As the tree grows older it reaches a certain period in its life at which 

 its vigor seems to have rached its maximum, after which time the 

 vitality of the tree ebbs. This is often spoken of merely as old age and 

 is the resultant lowering of vigor due to the increased unfavorable 

 environment of its surroundings. Many factors enter into this rela- 

 tionship, competition with younger and sturdier trees for light, water, 

 and food being the principal ones. Not the least of these factors is 

 the effect of cumulative injuries received throughout its life. Many 

 writers on forest pathology have expressed this opinion, and the data 

 following (Table IV) convey a like conclusion. Von Schrenk' states 

 that "it has been pointed out that as trees grow older they become 



1 Schrcnk, Hermann von. Some diseases of New England conifers: A preliminary report. 

 Agr., Div. Veg. Phys. and Path. Bui. 25, p. 51. 1900. 



U. S. Dept. 



