FORESTRY SURVEY 217 



The hydrarch mesopliYtic forests in X. T. 1, D and E 

 differed in aspect considerably from those in X, T. 1, C. 

 This difference was not due entirely to the presence of a 

 few additional tree species, but to the abundance and 

 more varied representation of growth forms, i. e., trees, 

 shrubs, vines and herbs. Except for a few large trees, 

 about two feet in diameter, and mostly all of which were 

 elms, at the very edge of this type of forest, the trees were 

 practically all less than six inches in diameter. 



Along the edge of the forest bordering the Skokie 

 swamp in X. T. 1, 10 the dominant tree was Populus 

 grandidentata. It and Populus tremuloides were found, 

 although less abundantly, at the edge of the forest where- 

 ever it bordered the Skokie. The shrubs and vines found 

 here were the same as those enumerated above. 



The islands in X. T. 1, D and E of the Skokie were of 

 special interest. The first problem which confronts one 

 is, why. or how can forests exist in these places which are 

 somewhat isolated from the main forest body ; the second, 

 how does the vegetation compare with that of the main 

 forest. First, it was found that these islands were areas 

 that were slightly elevated above the surrounding swamp, 

 and that the density of the vegetation indicated continued 

 rise of the land due to the accumulation of humus. Sec- 

 ond, the upper stratum of trees was the same as that in the 

 main forest, but the dominant members of the lower strat- 

 um of trees in general were of a more xerophytic type 

 than those found in the larger forests. Populus tremu- 

 loides. a northern outlier and xerophyte, was abundant 

 along the more exposed western edge of the islands as well 

 as along this edge of the main forest. 



The island in X. T. 1. 23 and 26 merged gently from the 

 reed swamp of the Skokie at its northern end, and rose 

 to a height of a cotiple of feet above it at the sotithern end. 

 The reed swamp association at the north end of the 

 island was followed by the orthodox shrub association to 

 the sotith, and was sticceeded in that same direction by 

 trees of the hydro-mesophytic association to about the 

 center of the island, and then by those of the hydrarch 

 mesophytic association at the southern part of the island. 

 With the exception of a few large trees of Ulmus a m eri- 

 ca na, about two feet in diameter, at the northern end of 



