32 BULLETIN 722, U. S. DEPARTMENT OF AGRICULTUEE. 



Its moisture requirements seem to be the principal limiting factor 

 of its distribution, since it is found growing on a variety of soils. 

 Its altitudinal range extends from sea level in Alaska, British Colum- 

 bia, Washington, and Oregon to an altitude of 6,000 feet, and in 

 Idaho and western Montana it is found at a maximum altitude of 

 5,000 feet. It is always at its best in cool moist draws or north slopes. 

 Regions with a relatively high humidity favor its development, 

 although at some of the higher altitudes the humidity is much less 

 than in the bottom-land sites. 



A very tolerant species, it is found to thrive in Idaho and Montana 

 in the white-pine type, generally in a mixed stand. Referring to its 

 tolerance, Sudworth ' says it is "very tolerant of shade throughout 

 life, especially in seedling stages. In later life vertical light is neces- 

 sary for best growth. Allowed overhead light, it recovers remarkably 

 well from long suppression and renews rate of growth. Prolonged 

 suppression in dense shade greatly checks growth. It thrives in cool, 

 open, humid places with abundant soil moisture." 



No natural thinning takes place under normal conditions, and in 

 mixed stands the pruning of the lower branches is a slow and imper- 

 fect process. Shade causes the lower branches of the crown to die, 

 and these remain on the trunk until broken by wind, windfalls, or 

 other causes. This condition leaves the tree with a large number of 

 branch stubs open to infection by fungous spores. 



Hemlock in its green condition contains 40 to 60 per cent of its own 

 dry weight of moisture,- a relatively large amount compared to the 

 other trees of the region. This fact has a direct bearing upon the 

 action of the fungus in the heartwood and accounts for the water- 

 logged condition of the base of the tree which is often encountered in 

 stands growing on poorly drained soils. 



In summing up the points brought out by this study the most pro- 

 nounced results are found in the variations in the action of the decay 

 in the two types studied. A glance at the plat descriptions given for 

 each type (Table I) will show the variation in slope and exposure as 

 well as the marked difference in soil and atmospheric moisture. The 

 river-bottom type, growing as a dense suppressed stand on a heavy 

 undrained soil in close proximity to the river and to its numerous 

 sloughs, presupposes its greater susceptibility to the attack of the 

 fungus. Xn the absence of trees of an older age class it can only be . 

 assumed that the rot percentage would increase with age. The fact 

 that the rot percentage in the older age class was lower does not 

 invalidate this assumption in view of the small number of trees it 



' Sudworth, O. B. Forest trees of the Pacific- Slope, |). (»">. lOOs. I'liiilishcd liy U. S. l>ei)artment of 

 Agriculture, Forest Service. 



'■' llanzlik, E. J., and Oakleaf, JI. 15. U'e.slcru licinlock; its forest characterislics, properties, and uses. 

 In Timberraan, v. 15, no. 12, I9I4, p. 25-3:5, tab. .5. 



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