34 THE WHITE PINE. 



(iUOWlII IN TUICKNESS. 



The growth in tliicRuess, or diaiueter accretion, altbongli remarkably regular in this species, is 

 nincli more viiriuble, l)iit it is also nioro in-rsisUMit, tliaii the licij^lit growth, as will ai'])car t'loiii the 

 Ibllowiiig coiiii)arisons: Thus, in live groups of trees IVoin ditlerent sites, ninety-tour to one 

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

 'M.\ I'eet, while the diiiinetersdilleied by almost .")() i)er (tent, varying from IC to U.'J.T inches. Again 

 the persistence is ilhistrated by the compaiison of the height growth of live groups from two 

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

 group. just mentioned of somewliat over 2(1 per cent, while the diameters were by 30 per cent 

 greater; and if the poorest groups of the two sets had been compared the diflerence 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 u]) in the virgin forest, 

 it is very apt to be suppressed for a longer or shortei- |)eriod by the large motlier trees and the 

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

 by this shade also impeded, tliis is not so to the same degree as the diameter, wliich is a ilirect 

 function of the amount of foliage that is at work. 



The sai)ling may thus remain a slcTider ]»(ile for many years, and not until it is able to lilt its 

 head above its crowding nc^ighbors, or until light has been a<lmitted to its branches, does it l)egin 

 to expand its crown and conseiiuently thicsken its stem. 



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

 growth in diameter progresses somewhat nu)re 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 rai)id 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 througlutut 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-fifth 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 G to 9 inches in diameter 

 breast high; at fifty years, from 10 to 12 inches; at eighty years, 15 to 17 iiudies; and they reach 

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



To attain a diametei' of 30 to 10 inches, wliich represents the best merchantable material 

 of days now abnost passed, more than two hundred years have been recjuired, while trees four 

 hundred to four hundred and fifty years old attain diameters of 50 U) GO inches and over. Trees 

 of 40 incites diameter at three hundred years wercs 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 

 iu height growth. 



The juogress of diameter development of dominant, codominant, and oppressed tree classes, 

 and in difterent localities, is exhibited in the tables and diagrams in the Ajjpendix. 



The usual method is to determine tht^ diameters at 4A 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 nuich 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 )>rovision to strengthen the stability of the tree. Unfortunately for the 

 inv<-stigations here recorded, it was not practicable to have the trees cut and uieasured 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 2i feet above ground, and at log lengths. 

 Even at that height (2A feet above ground), a difference in the progress of dianu^ter growth from 

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

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

 heights. 



The diameters here given for the lowest section arc, therefore, somewhat larger ilian those 

 usually employed, namely, breast high, especially in lalci- years. 



