34 ALFRED DACHNOWSKI 



2. Potentilla fruticosa association. This type of bog heath 

 occurs more typically over limestone regions Potentilla pre- 

 dominates, but Aspidium thelypteris often occurs in dense mats 

 in the interspaces. It is accompanied by scattered plants of 

 Andromeda glaucophylla, Betula pumila, Lobelia kalmii, and sev- 

 eral golden rods. 



3. Gaylussaciabaccata-Vaccinium canadense association. This 

 association has few secondary species and varies but little from 

 season to season. It never becomes very abundant and hence 

 represents the least stable type of heath associations. 



The reactions of the heath succession cannot be entirely unsuit- 

 able, for the soil, though sterile to most plants, is undergoing 

 changes which enable the taller shrubs to secure a foothold. 

 The scanty but efficient leaf-fall permits an accumulation of raw 

 humus above the water-table. This brings the humus level 

 within the reach of aeration and weathering. Partial occupation 

 by shrubs ensues and later their complete establishment. 



C. Bog shrub associations 



1. Alnus sp.-Rhus vernix association. This is the most gen- 

 eral and constant component of the bog flora of Ohio at^this stage. 

 Alnus incana, A. rugosa, and Rhus vernix are the facies with 

 Aronia nigra, A. arbutifolia, Vaccinium corymbosum, Ilex ver- 

 ticillata, and Nemopanthes mucronata as principal species. The 

 secondary species are mainly from adjoining associations. There 

 is a tendency toward the segregation of Vaccinium corymbosum. 

 It is more vigorous at the outer border. 



2. Salix sp.-Populus tremuloides association. It is only on 

 shallow peat in which the percentage of mineral matter is high 

 and on shoreward places bordering a grassy marsh that the 

 Decodon-Typha association is followed by willows with button- 

 bush and dogwood without the occurrence of an intervening bog 

 meadow and typical Alnus-Rhus association. Of the willows 

 Salix pedicellaris, S. Candida and S. lucida form an integral but 

 small part of the present bog vegetation. 



3. Cephalanthus occidentalis-Cornus sp. association. The more 



