FORESTRY SURVEY • 237 



The forest types^ named in the table are not different 

 from the well known forest associations of other parts of 

 Illinois and the adjoining states. Most of them are well 

 known and onlr two are worthy of special mention at this 

 point. These are pin oak flats and elm-hackberry forest. 



1. Pin oak fats. The pin oak association is found in 

 small groves, a few acres in area, in extremely shallow 

 depressions of the poorly drained npland. Some of the 

 trees found in it are more abundant in bottomlands, 

 others, as the shingle oak. are frequent in upland xero- 

 phytic forest. This vegetation can be correlated with two 

 essential physical conditions, soil which is frequently 

 "watersoaked and great exposure of the parts above ground 

 to dryness. The leaves of the two dominant trees, pin oak 

 (Quercus palustrisj, locally known as water oak, and 

 shingle oak (Quercus imhricariaj are of xerophytic struc- 

 ture.- that is, with thickened cuticle and pronounced 

 development of palisade tissue, the leaf being also thicker. 

 Other trees are .swamp white oak (Quercus hicolor), 

 green ash (Frarinus pennsylvauica, var. lanceolata), 

 honey locust (Gleditsia triacanfhos) and red maple (Acer 

 rubrumj, the last forming an understory. Lines of these 

 trees are frequently seen along small streams or ditches, 

 and even along fence-rows in poorly drained areas. 



2. Elni-hackherri/ forest. A few examples of small for- 

 ested areas are still to be found, isolated from the forest 

 system of the valleys, which are dominated by elm (Uhnus 

 americana) and hackberry (Celtis occidental is). With 

 these may occur honey locust, black haw (Viburnum pru- 

 nifoJium) of small tree size, and in some groves shingle 

 oak.* The seeds of these trees are dispersed readily to 

 considerable distances and it is probable that these forests 

 are of independent and comparatively recent origin in 

 locally favorable spots in the prairie uplands. Those 

 that have been seen are located in areas slightly raised 

 above the general level of the poorly drained tipland. Most 

 of these .slight elevations are now cultivated. This inter- 

 esting type of forest requires further sttidy. 



1 For those not familiar with ecological terms, a few used in the table and 

 in the map legend are here described : xerophitic. of dry exposed situations ; 

 mesophrtic. of intermediate situations, that is, rather sheltered from wind and 

 sun and with a moderate supply of soil moisture : xerarch. developing from xero- 

 phytic vegetation or in formerly xerophytic habitats ; hydrarch. developing from 

 hydrophytic conditions. 



* See footnote 5 page 000. 



