Jennings : A Botanical Survey of Presque Isle. 377 



The Rhus-Almis Formation. 



Outside of the Cephalanthus-Cor7uis formation there is present in 

 most of the Chimney Ponds more or less of a zone of the Rhus-Alnus 

 formation. Most of the j)onds are surrounded by forest-trees and, in 

 the more or less shaded conditions, the Rhus has given place to the 

 pure Abuts incana consocies. In the more open places, however, 

 certain other species become more or less prominent and the forma- 

 tion there exhibits the following structure : 



Fades. — 



Rhus iyphiiia, Alnus incana. 



Principal Species. — 



Savilmcus canadensis. 



Secondary Species. — 



Acer rubrum, Acer saccharinum, 



Sassafras sassafras, Salix nigra, 



Cornus stolonifera, Cornus amovium, 



Ceplialanthus occidentalis, Vacciniujn corymbosutn, 



Dryopteris thelypteris. 



The Ulmus-Acer Formation. 



With the advance of the shrub-formations towards the center of the 

 pond, there appears at the rear of the RJius-Alnus formation, or, 

 more generally the Al/ius incana consocies, a semi-hydrophytic forest- 

 formation which includes also most of the area occupied in the earlier 

 stage by the Acer saccharinum consocies of the Prunus-Acer forma- 

 tion. The habitat of this formation consists, of course, of a black 

 muck soil, but little above water-level and imperfectly drained. There 

 is protection from strong air-currents by the surrounding forest and, 

 below the forest-cover afforded by the A/nus, there is deep shade, with 

 freedom from abrupt extremes of heat and cold. 



The ecological conditions are apparently those required by a semi- 

 hydrophytic forest, and, as thus far developed on Presque Isle, the 

 structure of the formation is typically as follow^s : 



Fades. — 



Acer rubrum, Ulmus americana. 



Secondary Species. — 



Fraxinus americana, Liriodendron tulipifera, 



Fraxinus nigra, Nyssa sylvatica, 



