Ecology of the Cincinnati Region 
205 
On this level is the fourth zone of the flood plain formation (Hg. 
It contains the same hydrophytic trees as the previous associations. 
Those commonly occurring here are the willows (Salix nigra and S', alba 
vitellina), cotton-wood (Populus deltoides), white elm (Ulmiis ameri- 
cana), silver maple (Acer saccharinnm), box elder (Acer Negundo), 
and sycamore (Platanus occidentalis). These vary in abundance in dif¬ 
ferent parts of the association. The fourth zone is in striking contrast 
to the previous association with its scant undergrowth. The ground is 
covered with herbaceous plants, growing in dense patches of single 
species. The most abundant of these are the wood nettle (Laportea 
canadensis), touch-me-not (Inipatiens pallida), skullcap (Scutellaria la- 
terifolia) and in sunny spots, the giant ragweed (Ambrosia tridda). 
Saplings are not uncommon in this association, but because of the dense 
shade, willow, poplar, and sycamore are almost excluded. White elm 
and box elder are frequent and sugar maple (Aeer saccharum) occa¬ 
sionally occurs. This last is the first indication of an advance toward 
mesophytic conditions. 
This association, in various aspects, is the most wide-spread of any 
of the flood plain formation. It often occupies a number of successive 
levels, very slowly changing toward a more mesophytic forest, as the 
flood plain is built up. On some of the broadest flood plains, it is capable 
of further subdivision on a basis of age, and of the character of its 
herbaceous growth. 
The next zone of the flood plain formation is not of the same char¬ 
acter along all the streams. Much depends on the history of the valley 
and on its topography. 
In the area shown in dg. 5y, the fourth zone is the last that is typical 
of flood plains (cf., dg. 55j. The next zone occupies the lower slopes 
of the hills. It is a zone only reached by exceptional floods, as that of 
March, 1913 . Its trees are a medley of flood plain types, slope xero- 
phytes, and mesophytes. Here are found hackberry (Celtis occidentalis), 
walnut (Juglans nigra), honey locust (Gleditsia triacanthos), red elm 
(Ulmns fulva), red-fruited thorn (Crataegus mollis), and buckeye 
(Aesculus octandra), with the last remaining trees of the flood plain, 
white elm and box elder. Among the saplings, sugar maple (Acer sac¬ 
charum) and wild cherry (Primus serotina) are abundant, with a few 
of the other trees. Herbaceous plants are very scarce in this zone, pos¬ 
sibly because of the erosive work of the floods. Succeeding associations 
do not belong to the flood plain formation. This mixed zone is the last 
