COTTONWOOD IN THE MISSISSIPPI VALLEY. 19 
almost dependent upon accidents which result in openings in the 
stand and thus provide the light needed for the growth of young 
cottonwoods. The main agencies of this character in the Mississippi 
Valley are the river itself, which is continually building up new 
lands, and destructive winds, which often clear wide swaths through 
the forest. 
PURE STANDS. 
Cottonwood when young normally grows in pure stands. Since 
cottonwood reproduction is so dependent on full’ overhead lght, 
such stands are restricted to sites that at the time of seeding were 
unshaded. Pure cottonwood is therefore most common on the fol- 
lowing situations: (a) Newly formed islands and bars built up by 
deposition; (6) old lake and river bottoms which have been filled 
in by sediment; (¢) old fields which have been abandoned and have 
reverted to natural growth; and (d@) open areas within the forest 
caused by hurricanes or fires. Probably 90 per cent of the pure 
cottonwood stands are on exposed areas outside of the river levees. 
The value of cleared farm land in the Mississippi Valley is so great 
that practically none has been abandoned in recent years, unless 
subject to overflow. In many cases mature stands of cottonwood 
have been cut from such areas in order to use them for farming. 
Pure stands are seldom extensive, although in the southern half 
of the region they are found in more or less solid bodies cver hun- 
dreds of acres adjoining the river. Pure stands are always even- 
aged, or at least consist of even-aged groups. Where several age 
classes are present their arrangement is usually governed by the order 
of succession in the formation of new land by the river. The young- 
est stands lie nearest the river on the ground last built up, and as 
one progresses from the river toward the levees one passes through 
successive belts of even-aged cottonwood, each very similar to the 
preceding, except that the age and consequent size become greater 
and greater. The regularity of this succession, however, is usually 
broken by stands of black willow. 
Pure stands of cottonwood are extremely dense. They undergo 
very rapid~ thinning with age, however, as might be expected 
from fast-growing intolerant trees when starting in dense thickets. 
Tt is not unusual to find two-year-old thickets of this character with 
probably 40,000 living trees to the acre. The seeds, in fact, fre- 
quently germinate as close as 2 or 3 inches apart, but thousands of 
the young plants die from lack of ight or moisture very soon after 
germinating. At the age of 10 years there are seldom more than 700 
to 800 trees left, and at 25 years this number is reduced to about 120 
trees per acre. (See Pl. IIL.) 
