FORESTRY SURVEY 231 



the presence or absence of vegetation, and the grouping of 

 plants into associations are soil moisture, most conspicu- 

 ous in the hydiarch succession, and wind, waves, light, 

 alternating temperatures, and soil moisture accompanied 

 by soil movement in the xerarch succession. 



The hydrarch succession is comfjosed of the hydro- 

 mesophytic association and the hydrarch mesophytic as- 

 sociation. Fraj-inus nigra. Fra.iinus pennsylranica and 

 Quercus hkolor are the trees defining the hydro-meso- 

 phytic forest; Quercus clUpsoidalis is the tree determin- 

 ing the hydrarch mesophytic forest. There were many 

 varieties of the latter species in this region. 



The xerarch succession has the largest representation of 

 plant associations. Practically every gradation in the 

 development of a xerarch succession in clay soil, from the 

 most xerophytic to the most mesophytic. was found in 

 this region. The former was represented by the Rhus-, the 

 Populus-shrub. and the Juniperus-shrub associations of 

 the lake bluff: the latter by the Quercus alha, Fracinus 

 americana. Carya ovata association of the upland, and the 

 Acer saccharum-Tilki americami association of the 

 ravines. 



The old ravine, in its representation of vegetation at the 

 mouth, head and intermediate area recapittdates the his- 

 tory of the xerarch succession for this region. 



Both hydrarch and xerarch successions are comprised 

 primarily of secondary associations. Where this was true 

 in the upland and lowland areas, it was attributed to the 

 influence of man through cutting, grazing and fires. Nat- 

 ural delay in the development of associations toward the 

 mesophytic climax of the region, as illustrated by the lake 

 bluff, was due to the severity of conditions brought about 

 by a combination of physical factors. 



On the whole, shrubs were found to be more widespread 

 in their distribution than trees. Ostrya virgin iana proved 

 to be an exception to this, in that it was found equally 

 abundant in all associations of the hydrarch and xerarch 

 succession. This may be due to the fact that it is growing 

 on clay soil, and that it is a soil hydrohypte. and either an 

 air mesophyte or an air xerophyte. HarnameJis virginiuna 



