﻿COTTONWOOD IN THE MISSISSIPPI VALLEi'. 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 light, 

 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; (b) old lake and river bottoms which have been filled 

 in by sediment; (c) 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 over 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. 

 It 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 light or moisture very soon after 

 germinating. At the age of 10 years there are seldom more than TOO 

 to 800 trees left, and at 25 years this number is reduced to about 120 

 trees per acre. (See PI. III.) 



