34 THE WHITE PINE. 



GROWTH IN THICKNESS. 



The growth in thickness, or diameter accretion, although remarkably regular in this species, is 

 much more variable, but it is also more persistent, than the height growth, as will appear from the 

 following comparisons: Thus, in five groups of trees from different sites, ninety-four to one 

 hundred and nine years old, the heights differ only by a little over 8 per cent, varying from 91 to 

 98} feet, while the diameters differed by almost 50 per cent, varying from 16 to 23.7 inches. Again 

 the persistence is illustrated by the comparison of the height growth of five groups from two 

 hundred and seven to two hundred and thirty-three years old, which showed an increase over the 

 group just mentioned of somewhat over 20 per cent, while the diameters were by 30 per cent 

 greater; and if the poorest groups of the two sets had been compared the difference would have 

 been still more striking, namely, 15 per cent for the height as against 37 per cent for the diameters. 



This is in part explained by the fact that, where the seedling springs up in the virgin forest, 

 it is very apt to be suppressed for a longer or shorter period by the large mother trees and the 

 host of deciduous and other forms which make up the forest cover. While the height growth is 

 by this shade also impeded, this is not so to the same degree as the diameter, which is a direct 

 function of the amount of foliage that is at work. 



The sapling may thus remain a slender pole for many years, and not until it is able to lift its 

 head above its crowding neighbors, or until light has been admitted to its branches, does it begin 

 to expand its crown and consequently thicken its stem. 



In managed forests, or in tracts where from any cause crowding has been prevented, the 

 growth in diameter progresses somewhat more in the manner of the height growth, namely, slowly 

 at first, then rapidly until the maximum is attained, when a slowly decreasing rate sets in. In 

 the seedling the diameter growth is exceedingly small, very rapid in the young trees, when the 

 annual ring is often one-sixth to one-half of an inch wide, but decreases with the slower rate of 

 height growth. When the tree is sixty to eighty years old, the yearly ring is commonly not more 

 than one-twelfth of an inch wide; it then gradually sinks to one fifteenth of an inch, which is then 

 maintained throughout life, rarely falling to one twenty-fifth of an inch. 



The average annual accretion reaches its maximum about the fiftieth to the sixtieth year 

 with somewhat over one-fiftli of an inch on the diameter of dominant trees, which rate is nearly 

 maintained to the one hundred and fiftieth year. 



Thrifty trees at forty years of age grown in the forest, measure from 6 to 9 inches in diameter 

 breast high; at fifty years, from TO to 12 inches; at eighty years, 15 to 17 inches; and they reach 

 a diameter of 18 to 20 inches by the time they are a hundred years old. 



To attain a diameter of 30 to -10 inches, which represents the best merchantable material 

 of days now almost passed, more than two hundred years have been required, while trees four 

 hundred to four hundred and fifty years old attain diameters of 50 to 60 inches and over. Trees 

 of 40 inches diameter at three hundred years were by no means rare. 



To be sure, there are exceptional individuals which exceed these dimensions, and variation in 

 the rate of growth, due to soil, climate, and surrounding conditions, are naturally as frequent as 

 in height growth. 



The progress of diameter development of dominant, codominaut, and oppressed tree classes, 

 and in different localities, is exhibited in the tables and diagrams in the Appendix. 



The usual method is to determine the diameters at 4i feet from the ground (breast high), not 

 only because when measuring standing trees the measurement is most conveniently made at this 

 height, but because the lower diameters show- much more irregularity. There is also more wood 

 deposited near the base at and above the root collar, giving rise to the so-called root swelling (butt 

 swelling), undoubtedly a provision to strengthen the stability of the tree. Unfortunately for the 

 investigations here recorded, it was not practicable to have the trees cut and measured at breast' 

 height, since the measurements were made on trees felled in regular lumbering operations, exposing 

 only the cross sections at the height of the stump, mostly 2 feet above ground, and at log lengths. 

 Even at that height (2.i feet above ground), a difference in the progress of diameter growth from 

 that on higher cross sections is noticeable and becomes especially pronounced in later life, as 

 is shown in the curves representing the progress of diameter growth on cross sections at various 

 heights. 



The diameters here given for the lowest section are, therefore, somewhat larger than those 

 usually employed, namely, breast high, especially in later years. 



