Under present conditions, every one of the larger tree species
dominates at least some small part of the area. For reasons that are
usually obscure, locations that seem otherwise similar differ in the
kinds, numbers, and sizes of trees they support. Probably most of
these differences have arisen in the varying treatments under human
occupation in the last 100 years.
In the two valley areas, presumably heavily wooded under primitive
conditions, the trees growing at present seem to be secondary
invaders. They include groves and isolated trees of elm, honey locust,
walnut, and osage orange, and an occasional red haw, hackberry, or
coffee-tree.
The hilltops likewise are chiefly open, but forest of the hillsides
encroaches onto them for as much as 100 yards in some places. The
slopes between the hilltops and the valleys are almost everywhere
wooded, but the aspect of the woods changes from place to place.
Subdivisions on a vertical scale, might be recognized as follows: the
upper limestone outcrop (Plattsmouth member) at the hilltop; the
usually steep slope strewn with rocks, between the upper and lower
(Toronto) limestone outcrop; the lower limestone outcrop; an almost
level terracelike formation often approximately 50 feet wide a few
feet below the level of the Toronto limestone; the slope below the
terrace, variable in steepness, exposure, and soil type, and usually
several times more extensive than the first four subdivisions
combined. Along both the upper and lower outcrops, elm and hackberry
are especially prominent. Chestnut oak is abundant along the outcrops
and on the rocky slope between them in some situations. Ash grows
abundantly on some upper slopes but there are few growing on the upper
outcrop. On the terrace, elm, ash, hackberry, honey locust,
coffee-tree and black oak are abundant. On the lower slopes grow most
of the blackjack oaks, post oaks, red oaks and mulberries.
Even greater differences in the local aspect of woodland on the
hillsides are caused by slope exposure. On south facing slopes,
especially, the woodland is noticeably different from that in other
situations, and of more xeric aspect. The climax species, _Quercus
Muehlenbergii_, _Q. rubra_, _Q. velutina_ and _Carya ovata_ are almost
totally absent. Such trees as are present are of small to medium size.
They are mostly red elm, American elm, walnut, honey locust,
hackberry, and osage orange, with dogwood (_Cornus Drummondii_) and
plum (_Prunus americanus_) forming dense thickets. Occasional patches
of prairie grasses remain in more exposed situations where they have
not been shaded out. These, together with the small size of most of
the trees, indicate that the south slopes have become wooded rather
recently, and originally were prairie. Nevertheless, the small
remaining groves of blackjack oak and post oak are on slopes that face
south, southeast, or southwest, and probably under original conditions
they occupied these situations, separate from the forests of other
hardwoods. Slopes facing east, west, and north, are more similar in
relative abundance of various kinds of trees, and they do not differ
much from hilltop edges that are wooded. Chestnut oak and hickory are
most abundant on north slopes, and ash occurs mainly on north slopes.
Table 1.--Percentages of Larger Trees (a Foot or More in Trunk
Diameter) on Different Slope Exposures.
-----------------------+----------+------------+---------+----------
| North | | West | South
| slopes | Hilltops | slopes | slopes
-----------------------+----------+------------+---------+----------
Elm | 35.7 | 38.6 | 25.8 | 51.4
Chestnut oak | 22.0 | 18.3 | 17.8 | 2.9
Hickory | 8.8 | 4.0 | 3.6 | 5.0
Walnut | 8.8 | 5.8 | 19.6 | 12.1
Ash | 7.1 | .8 | | .4
Hackberry | 8.2 | 1.6 | 2.4 | 6.9
Black oak | 3.3 | 16.4 | | 1.0
Red oak | 2.2 | | 23.8 |
Locust | 1.8 | 7.5 | 1.9 | 11.6
Osage orange | .5 | 1.5 | .2 | 5.3
Sycamore | .5 | | 2.1 | .1
Coffee-tree | | 1.2 | 2.4 | 1.0
Cherry | | 2.4 | | .1
Red haw | | .4 | | 1.3
Ailanthus | | | | .3
Mulberry | | .5 | | .1
Cottonwood | | | | .1
Redbud | | .8 | .2 | .1
Boxelder | | .1 | | .3
Blackjack oak | | | .2 |
| | | |
Total trees in sample | 182 | 890 | 467 | 898
-----------------------+----------+------------+---------+----------
Table 1 shows the percentages of different kinds of trees a foot or
more in trunk diameter on different slope exposures sampled. Elm is
almost always the dominant tree, making up from one-fourth to one-half
of the total stand. The other species dominate relatively small areas.
Chestnut oak usually makes up a substantial part of the stand on
hilltops and slopes of north, east, or west exposure. Black oak, red
oak, and walnut may be prominent on the east and west slopes. Walnut
and locust are prominent on south slopes.
Hickory usually has a trunk diameter of less than one foot, and,
therefore, it is not prominent anywhere among the larger trees. Table
2, showing ratios of medium-small trees (more than 6 inches and less
than one foot in trunk diameter) demonstrates that hickory is one of
the more prominent trees on hilltops and on slopes other than those of
south exposure.