32 MICHIGAN SURVEY, 1905. 



largely to the exposure of this habitat to wind, Fig. 12. The aspen zone 

 is in turn succeeded by the biota of the mesophytic forest type. This suc- 

 cession prevails on the north, east and west sides of the crest. On the south 

 side, owing to the presence of the precipice, the soil accumulates only to a 

 slight extent and the succession only progresses as far as the heath or pine 

 stage. The biota of the cliff and upper part of the talus slopes are not in 

 the succession, for owing to the fact that the material is removed nearly as 

 fast as it is formed, the conditions remain practically the same, and the forms 

 only gain a temporary foothold. It is true that on the ledges there is a more 

 or less definite succession of societies leading up to the pine stage, but it 

 rarely proceeds beyond this stage, while it is liable to be destroyed before 

 this stage as is shown by the debris that accumulates on the talus below. 

 The biota of these habitats is evidently derived from the mountain top. 

 A large percentage of the seeds of the vegetation on the south side of the 

 crest is washed over the cliff with the soil. Many of the ants and snails 

 of this area probably have a similar fate, while it is a common sight to see 

 grasshoppers on the mountain top caught by the wind, when on the wing, 

 and carried over the precipice to light on the talus slope below. The biota 

 of the narrow strip of finer material at the top of the talus slope, owing to 

 the more favorable soil conditions and the shade furnished by the cliff, is 

 composed of forms that occur in the pioneer societies both on rock and soil 

 habitats, and it is thus a complex pioneer society. The strip of finer material 

 is only formed at the foot of the cliff and migrates with it, leaving its lower edge 

 to be covered by the larger talus blocks as the cliff retreats. The biota thus be- 

 comes practically destroyed, and the conditions change toward those of the 

 talus slope. At the foot of the talus slope the conditions are nearly the same as 

 on the mountain top. The pioneer forms are the lichens that cover the rocks 

 and hold the material as the rocks disintegrates. The soil that is formed 

 accumulates rapidly among the talus blocks and becomes in time sufficient 

 to support the reindeer lichen society. As the soil continues to increase, 

 the forms of the heath society push onto this mat, followed in turn by the 

 climax forest society, the pioneer forms of which generally extend well up 

 the slopes. The most prominent form among the pioneers of the climax 

 forest on the talus slopes is the Paper Birch, broken and twisted individuals 

 of which are often found well within the range of falling rock fragments. 

 As Harvey ('03, p. 37) has suggested for Mt. Ktaadn, it seems to be adapted 

 to this habitat by its flexibility. 



There is thus a series of lowland societies steadily encroaching on the cliff 

 habitats from all sides, as the physical processes reduce the ridge toward 

 sea level. The order of succeeding societies is generally the same in a parti- 

 cular region. Cowles ('01) and Whitford ('01) both give the pine stage as 

 generally following the heath society in northern Michigan, but on the cliff 

 habitats in the Porcupine Mountains, owing to the effect of the wind and 

 the shallow soil, the coniferous society may be nearly, if not entirely, left 

 put of the succession. In this case, an entire stage in the order of succession 

 is made impossible by a particular combination of the environmental condi- 

 tions, and the fact is enforced that habitats are composed of a complex of 

 physical conditions. This is further shown by the dwarfed nature of the 

 aspens and oaks that border the "bald" areas. These trees often grow as 

 shrubby mats, owing to the breaking off of their tops by the wind. This 

 is undoubtedly due indirectly to the presence of the escarpment, for on higher 

 ranges to the south, which possess no escarpment, the mesophytic forest 

 covers the highest peaks with no noticeable decrease in the size of the trees. 



