FORESTRY SURVEY 221 



B. The Xeeaech Succession. 



The upland, the lake bluff and the ravine, which repre- 

 sent the xerarch sticcession in this region, involve two 

 types of topogi-aphy — the pre-erosion. comprising the up- 

 land, and the erosion, including the lake bluff and the 

 ravine. The xerarch succession exhibited varying types 

 of vegetation from the xerophytic to the mesophytic. A 

 very close relationship was found to exist between the 

 topography, the physical factors, and the vegetation. It 

 was of interest to see how quickly the vegetation re- 

 sponded to modifications of topography or the change in 

 the combination of physical factors. The physical factors 

 which were of greatest importance in either the progres- 

 sion or retrogression of the development of forest associa- 

 tions were light, temperature, soil moisture, wind and 

 waves, gravity and instability of soil. All of these factors 

 were operating in the case of the eroding lake bluft\ The 

 angle and direction of slope, in the case of the ravine and 

 the lake bluff, had an important influence upon the type 

 of vegetation represented. 



1. The ITpland. 



The greater part of the entire area studied is composed 

 of the upland mesophytic climax forest. Excluding the 

 ravines and the depressions in the upland, the forest ex- 

 tends from the lake bltift' on the east, to the hydrarch 

 mesophytic forest of the Skokie on the west ( see map ) . 



The trees most frequently found in this mesophytic for- 

 est association were Qiiercus alha and Qiiercus ruhra. The 

 former species was the most abundant. The other tree 

 species in this flora were: Fra-rinus amcricana, Canja 

 ovafa. Prunus serotina, Ostri/a virginiana, Quercus macro- 

 carpn. Quercus eUipsoidaJis, Acer saccharum. Acer 

 ruhrum, Tilia amcricana. Pi/rus ioensis, Amelanchier can- 

 ruhrum, Tilia amcricana, Pyrus coronaria, Amelanchier 

 canadensis, Crataegus mollis ( ?). Crataegus punctata, Pru- 

 The shrub species in the order of their importance were 

 Hamamelis virginiuna, Corylus amcricana, Cornus pani- 

 culata, Cornus circinata, Thburnum accrifolium. Vi- 

 burnum dcntatuniy Tihurnum lentago, Fuhus occidentalis, 

 Ruhus alleghenicnsis, Samhucus canadensis, and Rosa 

 hlanda ( ?). Titis ( sp. ) was also found, but not in great 



