122 



in each area studied, except the Dixon area, which is entirely withoui 

 forest. In the others most of the forested portion is covered with 

 this type. It has also a wide distribution beyond Illinois and is 

 mentioned under the same or different names by several writers. 



Cowles described the association (iSgg: 379-382) at the head 

 of Lake Michigan under the name of oak dunes. Nearly all the 

 herbaceous species mentioned in his short list occur also in Illinois. 

 Jenning's Querciis velutina-iiiibricana Forest Formation of Cedar 

 Point {ipo8: 300) is similar, but includes many plants which rep- 

 resent a somewhat later stage in succession. The oak-pine-sassafras 

 society of Livingston {ipoj: 40-42) in Kent county, Michigan, is 

 also much like the Illinois association, but contains many relic speci- 

 mens of the pine forests which preceded it in order of succession. 

 Britton {ipo^: 578, 579) mentions the occurrence of black oak on the 

 sand-plains of Connecticut. He did not differentiate a particular 

 association, but it is very probable that at least some of the vegeta- 

 tion is of this type. 



In each of the extralimital localities mentioned the dominant tree 

 is the black oak, while the herbaceous vegetation shows a considerable 

 variation. This is because of the differences in the neighboring asso- 

 ciations, from which many species find their way into the black oak 

 forest. Warming (ipop: 146) has tenned such conditions geograph- 

 ical variations of an association. With our present knowledge of 

 plant associations it is not clear how much weight should be given 

 to these variations in floristic composition. 



The association is characterized by the black oak, Querciis velu- 

 tina, and in the Havana and Oquawka areas also by the black-jack 

 oak, Qncrcus luarilandica. Both species have the same general habit 

 and live together in various proportions, but with the black oaks 

 usually more numerous. The trees are sometimes close and crowded, 

 sometimes wide apart. Old forests, whose origin probably dates 

 back to the period of prairie fires, and forests on steep dunes are 

 usually open, with trees 5-20 3^ards (5-20 m.) apart. Young forests 

 of recent development or those protected by swamps are usually dense, 

 with an average distance of 3-15 ft. (1-4 m.) between the trees (PI. 

 XIV, Fig. I ) . Densest of all are the young groves which have recently 

 sprung up in abandoned fields and clearings. In these the trees stand 

 at close intervals and the trunks are covered with stiff, crooked, 

 dead branches down to two or three feet (i m.) from the ground 

 (PI. XIV, Fig. 2), making them almost impassal)le. The older 

 and more open forests are especially characterized by bare crooked 

 trunks with divergent branches at a height of 6-12 ft. (2 to 4 m.). 



