99° BULLETIN 24, U. S. DEPARTMENT OF AGRICULTURE. 
old. For this reason they are of comparatively little importance 
from the standpoint of management. 
FORM AND GROWTH OF INDIVIDUAL TREES. 
Cottonwood is one of the tallest trees east of the Rocky Mountains. 
Under favorable conditions it attains a height of more than 175 
feet. and mature trees within its optimum range are seldom less than 
125 feet high. The maximum height recorded by the Forest Service 
is 190 feet. Diameters of from 4 to 6 feet are not unusual (PI. 1), 
and trees on well-drained bottomilands in the Mississippi Valley 
have measured nearly 10 feet through at the stump. 
In forest stands of average density, cottonwood prunes itself of 
branches remarkably well, producing a long, straight bole, clear of 
limbs for a distance of from 60 to 80 feet, with comparatively little 
taper. The crown remains narrow and pointed until the trees reach 
25 to 40 years of age. after which it becomes more branchy and 
spreading. In the open or when planted in single rows, cottonwood 
produces a short, stocky stem, which is likely to divide within 20 or 
80 feet of the ground into several large, irregular branches, forming 
a long, open, wide-spreading crown. 
Cottonwood develops a stout taproot from 3 to 6 feet long, re- 
enforced by numerous wide-spreading laterals. Wherever successive 
layers of alluvial soil are deposited by the river about the base of the 
tree. new side rocts appear to develop along the buried part of the 
trunk. 
The bark of mature trees is extremely thick and rough, and firmly 
attached. At 10 years the bark at the base of the tree averages 
about half an inch in thickness; at 35 years, about an inch; and on 
old trees sometimes more than 2 or 3 inches. The bark is character- 
ized by rough, narrow ridges. separated by wide, irregular connecting 
furrows. > (PI. F.) 
Notwithstanding its large dimensions, cottonwood seldom reaches 
great age. It is unusual to find sound trees of this species over 125 
years old. The maximum age recorded by the Forest Service is 166 
years, and it is probable that 200 years is about the maximum longev- 
ity of the species. 
It is a tree of remarkably fast — especially during early life. 
After about 40 years of age, however, the growth rate in height de- 
clines rapidly, although the trees continue to increase in girth at a 
moderate rate. On alluvial bottomland soils which are fairly well 
drained an average annual height growth of from 4 to 5 feet and a 
diameter growth of two-thirds of an inch are not at all unusual for 
the first 25 years. Trees have been measured that were 83 feet tall 
at 12 years and 100 feet at 15 years. Table 3 shows the average 
