24 BULLETIN 152, U. S. DEPARTMENT OF AGRICULTURE. 



probable that the best conditions for the development of hemlock 

 mycorrhiza exist where the soil is sweet or only slightly acid and 

 where a good crown cover is maintained. 1 



Hemlock reproduction is rarely found in clearings, a condition for 

 which fire is chiefly responsible, though other causes, such as intense 

 sunlight and evaporation, no doubt play a part. Fire, however, 

 while actually promoting the reproduction of many species by exposing 

 the mineral soil, may at the same time entirely prevent that of hem- 

 lock by destroying the organic constituents of the forest soil. In the 

 relatively few hemlock regions from which fires have been kept out 

 after logging remarkably thrifty stands of second growth have often 

 developed. Such second-growth hemlock in the Tionesta Valley and 

 elsewhere in the northern Alleghenies is undoubtedly due to the 

 absence of fires in these localities in the past, while the entire absence 

 of hemlock in other localities as favorable to its growth can be 

 attributed to the burning of seedlings and soil. 



Even-aged stands of hemlock second growth are very rare. Small 

 groups occur in protected valley bottoms and lower slopes in the 

 Allegheny and Catskill Mountains. One of these, which occupied a 

 few square rods in a ravine bottom, was 40 years old and contained 

 about 12 thrifty trees per square rod, the dominant ones 30 feet high 

 and 3 inches in diameter. The stand was very dense, and there were 

 many small dead trees which had been killed by the shade. 



RATE OF GROWTH. 



Under the shade of the mature forest the growth of the average 

 hemlock is extremely slow. The period of suppression commonly 

 lasts from 30 to 70 years, but if the shade remains dense it may con- 

 tinue for more than 200 years. Even at an advanced age, however, 

 a suppressed tree will respond to an increase in its fight supply by a 

 proportionate increase in its height growth. If it ultimately attains 

 a dominant position in the stand with plenty of light, it will grow 

 fairly rapidly in diameter and volume. 



Individual hemlocks show a wide variation in rate of growth, 

 according to the amount of light they receive. Trees of the same 

 diameter in the same stand may differ in age by more than a century. 

 The average growth of hemlock obtained from measurements of 

 many individual trees therefore represents many different degrees of 

 suppression and does not indicate what a tree would do if given full 

 light. The maximum growth, similarly obtained, more closely 

 resembles the growth of a tree in the open, though even here the 

 retarding influence of suppression is felt to some extent. 



1 Cf. " Roots of the Hemlock," by S. H. Harlow, in Jour. N. Y. Bot. Gard., July, 1900, Vol. I, No. 7, 



100-101. 



