26 IOWA STUDIES IN NATURAL HISTORY 
bushes which form the undergrowth in not only the oak but also 
the other types of forest formation. The density of this forma- 
tion affords the seclusion preferred by the grey squirrel, while 
its relative the fox squirrel favors the more scattered forest. 
There is, however, considerable overlapping of their distribution. 
(2) Oak-hickory. Oak-hickory formations differ from the 
oak formations only in the presence of the hickory. The pro- 
portion of hickory and oak trees is quite variable, ranging from 
a few scattered hickory to approximately fifty per cent. Very 
much the same classification of oak-hickory that refers to the 
density of the timber will apply, as this combination is found in 
conditions varying from much scattered groves to solid timber 
with profuse underbrush. The same analyses for strata or sub- 
habitats will apply to this as to the oak proper. 
(3) Hlm. Elm is rarely found unmixed with other trees, 
except in a few groves, the most notable of which are in Sharon 
township. It has been chosen, however, to represent that timber 
of which it forms the predominating species. Except for an 
occasional grove in the upland, where it is not infrequently 
mixed with oak, it lies uniformly along the flood plains of the 
rivers and larger creeks, and frequently extends in a thin line 
along the courses of the smaller streams. It is indicated on all 
the township maps by X-shaped characters. Along the smaller 
water courses it is regularly mixed with soft maple, cottonwood, 
box elder, red hawthorne, river birch, and hackberry. On the 
banks of the streams is usually a generous mixture of willow. 
This so-called elm forest is essentially alluvial, and, as such, 
presents physical characters which differ uniformly from those 
found in the oak formations. The ground is regularly level, the 
drainage rather poor, and the soil black, loose, and rich. Hu- 
midity is relatively high, as is the water content of the soil, 
owing to the poor drainage. As has already been suggested, 
these formations vary in extent from a thin line of scattered 
trees along the smaller creeks to forested areas many rods wide 
along the Iowa River. 
Open elm groves. The few upland groves of American elm, 
the thin lines of timber along the small creeks, and certain elms 
along the larger water courses fall under this classification. The 
