212 CLIMAX FORMATIONS OF NORTH AMERICA. 



ciations have been shown to have comparative values of 100, 65, 20, 17, and 15 

 respectively, and the differences thus indicated are sufficient to be efficient 

 factors in causing succession. The corresponding value of this ratio in the 

 prairie association is 62. 



"8. The midsummer conditions of the prairie association seem to be decidedly 

 xerophytic." 



SUBSERE. 



Whitford (1901 : 316) has indicated the main details in the subsere of the 

 maple-beech chmax of northern Michigan: 



This is best shown in bums, where the initial stage is formed by Chamaener- 

 ium, Leptilon, Solidago, etc. These are soon overtopped and replaced by 

 PopuLus tremuloides, P. grandidentata, and Betula papyrifera, with which may 

 be associated Ostrya virginica, Prunus pennsilvanica, P- serotina, and Qiiercus 

 rubra. If a pine community is near, the pine stage appears in the series. 

 Otherwise, the poplar-birch associes persists imtil conditions permit the appear- 

 ance of maple and beech and the consequent return to the climax. Where 

 repeated fires occur, the subsere is held in the heath condition characterized 

 ^ by Pteris, Gavltheria, and Cladonia rangiferina. 



Jennings (1909 : 394) has described the secondary seres of Presque Isle, 

 Pennsylvania: 



Clearing and burning on a sandy ridge produced an associes composed of 



Populus tremuloides and Rhus typhina, with many other shrubs and grasses. 



The invasion of Quercus velutina indicates that this burn subsere \^t11 pass 



^ into an oak consocies. The subsere due to pasturing is a short ruderal one, 



dominated by Poa pratensis. 



Chrysler (1910) states that the scrub pine is the first tree to invade abandoned 

 fields, but the oak secures a foothold in openings, and becomes dominant after 

 a period characterized by a mictium of pine and oak. The entrance of 

 other oaks and hickory finally converts the forest into the oak-hickory stage. 

 In valleys, the pine is succeeded by the maple-gum associes, while on the 

 lowlands, pine is replaced by the sweet giun and the xerophytic oaks, which are 

 succeeded by the mesophytic oaks, maple, and black gum. The final climax 

 appears to consist of Quercus, Carya, Ldriodendron, and Castanea. 



Howe (1910) has studied the reforestation of sand-plains in western Vermont 

 and describes the secondary succession in cut-over areas and in abandoned 

 fields: 



White pine was originally the controlUng tree on these sandy delta-plains, 

 though only isolated trees of the original forest now remain. The clearing 

 of the white pine forest favored the invasion of the pitch pine. The latter was 

 not cut, leaving an abundance of seed trees. It produces, moreover, more 

 seed than the white pine and in its demands is better able to act as a pioneer. 

 As a consequence, the pitch pine became dominant in the second-growth 

 forest. When the pitch-pine forest is cut clean, the development of the under- 

 growth is stimulated and Myrica, Pteris, and Vaccinium become dominant. 

 In 15 to 20 years, the Myriea-Pteris association begins to die out, giving open 

 places for the establishment of the pitch pine. Where fire is kept out, how- 

 ever, oak coppice and birch come to dominate, and are succeeded ultimately 

 by the white pine rather than by the pitch pine. In the succession upon 



