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. 
i Cf. " Roots of the Hemlock," by S. H. Harlow, in Jour. N. Y. Bot. Gard., July, 1900, Vol. I, No. 7, 
100-101. 
