﻿1903] UPLAND PLANT SOCIETIES 5 J 



imperceptible. Here and there in a sterile, perhaps in a well 

 r drained, portion, would be left a detachment of the advance 



guard, like the patches of societies V and IV in the southern 

 townships of the county. And these would be surrounded and 

 left as relics by the later comers as they advanced, occupying^ 

 all soils in which they could come to maturity, and preventing 

 the development of new generations of the forms jireviously hi 

 possession. With these thoughts in mind, a glance at the map 

 will suggest much more than was suggested before. 



The beech and maple societies (considered by Cowles and 

 Whitford" to be the climax society for temperate North Amer- 

 ica) extend northward along the lines of soil richest in water 

 content, and reach farthest north in the western part of the 

 county. This latter fact may be due to the lake influence. 

 Chamberlin states'^ that in Wisconsin the beech is limited to 

 regions near the lake. He believes its distribution to be deter- 

 mined by lacustrian climate. This is very doubtful, how^ever. 

 Also the other societies — II, III, and IV — are each a little in 

 advance of the previous one, and each is apparently advancing 

 into the area occupied by the next hardier one. In the extreme 



north we find almost the entire area occupied by societies IV 

 and V. 



According to this line of thought, the reason for the pre- 

 dominance of the pine groups in the northern part of the county 

 is that sufficient time has not yet elapsed since the glacial period 

 for these areas to be reached by the societies found predomi- 

 nant farther south. Along a wavy east and west line passing 

 through Rockford lies the "zone of tension" between societies 

 I. II, and III on the one hand, and IV and V on the other. 

 This line bends far northward at the west, following the western 

 edge of the Rouge valley as far as Kent City and Casnovia. It 

 also bends northward to Sheffield and Harvard on the other side 

 ol the Rouge valley. It may be that the climate, somewhat 

 "colder as we pass northward, has acted as a retarding factor, 



" Whitford, H. N., loc. cit., p. 302. 



"Chamberlin, T. C, Native vegetation of Wisconsin. Geol. of E. Wisconsin 

 i-ilt. 1873-1877. 



