WHEN TREES GROW 



BY PROF. J. S. ILLICK 



CHIEF, BUREAU OF SILVICULTURE, PENNSYLVANIA DEPARTMENT OF FORESTRY 



NOTHING about the numerous processes of trees is 

 more readily comprehended than that they grow, 

 for the results of growth are so obvious, and in 

 some cases striking, particularly in temperate regions 

 where annually a period of vegetative rest alternates 

 with a period of vegetative 

 activity. 



The belief is prevalent 

 that trees grow throughout 

 the general growing or 

 vegetative seasons, which 

 embraces the middle states 

 from 150 to 200 days, and 

 extends from the last kill- 

 ing frost in the spring, that 

 is, when the leaves of the 

 larches, birches, cherries, 

 and maples appear, to the 

 first killing frost in the 

 autumn when the leaves ex- 

 hibit their autumnal colo- 

 ration. This, however, is 



a mere supposition, for most of the native and introduced 

 forest trees in the vicinity of Mont Alto, Pennsylvania, 

 make ninety per cent of their height growth in less than 

 forty days. . , 



The following tabulation, based on data obtained in 

 Pennsylvania, lists five representative species of forest 

 trees, gives the date in spring when the growth of each 



DO YOU KNOW THAT 



Trees make nine-tenths of their height growth 

 in less than forty days? 



Most trees start growing in April and stop 

 growing in May or June? 



Trees grow twice as much at night as during 

 the day? 



Some trees grow steadily during the growing 

 time and others rest for days and then continue 

 their growth? 



That two rings may sometimes represent only 

 one year's growth ? 



starts, indicates the percentage of the total growth of 

 the season opposite specified dates, and schedules the 

 progress, duration, and cessation of growth. 



Not all forest trees begin to grow at the same time. 

 Some start early in spring while others begin rather 



late. The Wild Black. 

 Cherry, Primus serotina is 

 the first forest tree in the 

 vicinity of Mont Alto to be- 

 gin height growth. The 

 elongation of its twigs 

 starts about the fourth of 

 April. The Domestic 

 Cherry, Prunus avium, be- 

 gins its growth about four 

 days later than the native 

 Wild Black Cherry. The 

 Sweet Buckeye, Aescuhts 

 octandra, begins about 

 April 6, White Pine, Pin us 

 Strobus, about April 18, 

 Tulip Tree, Liriodendron 

 Tulipfera, about April 25, and Norway Spruce, Picea 

 Abies, about May 6. The date when the different species 

 start the elongation of their twigs depends upon the in- 

 herent tendency of the species and the factors of the 

 environment. The late opening of the buds of Norway 

 Spruce is not a local characteristic, but an inherent ten- 

 dency, for records from Germany show that they usually 



EUROPEAN LARCH IN FULL FOLIAGE 



A coniferous tree which sheds all its foliage 

 each autumn. Lower buds begin to swell early, 

 leaves emerge rapidly, but elongation of shoot 

 does not begin until about the middle of May. 



1386 



TERMINAL SPRAY 

 PINE 



OF PITCH 



I. ATE SPRING 



AWAKENING 



SPRUCE 



OF NORWAY 



Showing the original and the second 

 growth of the season. Fictitious rings 

 are regularly formed when, a prolonged 

 resting period occurs within the grow- 

 ing season. 



During early May the buds usually begin to 

 swell. Elongation of the twigs begins at the base 

 of the trees and proceeds upwards. 



