COTTONWOOD IN THE MISSISSIPPI VALLEY. 13 
factory growth, provided the roots can readily penetrate to a moist 
subsoil. Moisture is especially essential for the germination of cot- 
tonwood seed. It is this characteristic which accounts for the ec- 
currence of the species on sandbars and overflow lands along streams 
and the borders of swamps and lakes. 
Cottonwood, however, is in no sense a swamp tree, in spite of a 
rather general impression to the contrary. True, it withstands ex- 
tremely long periods of inundation by the spring floods of the Mis- 
sissippi River and its tributaries. Yet such overflows are not com- 
parable to standing swamp water, which is deficient in the free oxygen 
essential to root function. In the spring floods to which the cotton- 
wood is subject there is a continual current of water about the base 
of the trees. After the recession of the floods the water table descends 
often many feet below the ground surface. The upper soil strata 
are then thoroughly drained either toward the river or toward the 
back sloughs. Cottonwood will not show thrifty development on 
very poorly drained situations. 
The quality of the soil itself affects the local occurrence of cotton- 
wood but little. With abundant moisture the tree appears to thrive 
on poor, sandy sites as well as on the stiffer clay soils. On typical 
overflow lands, however, the rich, alluvial deposits of comparatively 
close, fine texture seem to conserve the moisture better than the 
coarse, deep sands. The best growth of cottonwood therefore occurs 
as a general rule on the former sites. Yet if the water table is near 
the surface during the growing season, very well-developed stands are 
found on rather infertile sandy or gravelly soils. 
LIGHT REQUIREMENTS. 
Cottonwood is extremely exacting in its light requirements. With- 
out doubt it is the most intolerant species in the Mississippi bottom- 
jands. Young cottonwoods are seldom found coming up under a 
normally dense stand of older trees. Even under exceptionally 
favorable soil and moisture conditions the young growth seldom sur- 
vives for longer than one summer. Direct overhead light alone is 
scarcely enough to encourage reproduction. This extreme intoler- 
ance of shade remains with the tree throughout its life and accounts 
for the rapid thinning out of pure cottonwood stands with age. The 
intolerance of cottonwood is also responsible for the ease with which 
the trees clear themselves of side branches in close stands. 
Compared with the tolerant pine or spruce, cottonwood of the 
same age has fewer trees to the acre. Thus at 35 years, when the 
trees average 20 inches diameter breasthigh, an acre will bear only 
50 to 75 trees. The amount of wood produced per acre, however, 
is as great, if not greater, in white pine or spruce stands. 
