WHITE PINE UNDER FOREST MANAGEMENT. 19 
Height growth is also influenced by the amount of light which the 
tree receives. It is most rapid when the upper part of the crown alone 
receives a full supply. In open-grown trees, which receive light from 
all sides, the growth energy is distributed over many large branches, 
and height growth culminates early, so that the tree remains rela- 
tively short, stout, and branchy. On the other hand, if the trees 
grow so closely together that the crowns are small and crowded, the 
rate of growth will again be slow. 
It is thus evident that trees of the same age and growing in the same 
situations may vary somewhat in the rate of growth, according to 
their nearness to one another. This difference, however, is much less 
than that caused by a marked difference in the quality of the soil. 
Height growth is, in fact, commonly considered the most reliable 
single indication of site quality. 
Since ordinarily only one whorl of branches is produced in a single 
year, the age of young to moderately old trees can usually be de- 
termined roughly by counting the number of whorls or the scars left 
by them. When these are absent close to the ground it is necessary to 
estimate the period which the tree required to grow to the height of 
the first visible whorl. For rough age determination it will usually do 
to assume that it took 10 years to reach breast height. 
Growth in Diameter. 
Growth in diameter takes place through the formation of the thin, 
concentric layers of wood, commonly called annual rings. With very 
rare exceptions only a single ring is formed each year, so that the 
number and width of the rings are a safe guide in determining the 
age and rate of growth of the tree. To determine the precise age it 
would be necessary to cut the tree at or within an inch of the surface 
of the ground, since only in this way can the first year's growth be 
included. Naturally, the further up the tree the section is cut, the 
less will be the number of rings, and the amount of this difference is 
the number of years it has taken the tree to reach the height of the 
cross section. It is customary to express growth in terms of the 
diameter breasthigh, since on standing trees this is the point at which 
the diameter is usually measured. 
Like growth in height, the diameter growth of white pine reaches 
its maximum early in life, and then slowly decreases. It is more per- 
sistent than the height growth, however, and continues at a moderate 
rate long after the former has practically ceased. Trees 60 or 70 
years old may still be growing fairly rapidly in diameter. 
Diameter growth is slowest when the light supply is barely suf- 
ficient to keep the tree alive, and most rapid when the tree grows 
with its crown in full light and produces a heavy foliage. On dry 
soils or where there is deficient air moisture diameter growth, like 
height growth, is slow. This is true also of swampy soils. 
