Bergman and Stallard: cliiMax formations 359 



socies. Alnus, however, usually replaces Corylits and Rubiis only 

 in areas where the water-content of the soil is greater. 



Secondary species : In many places, Priinits pennsylvanica is 

 less abundant and must rank as a secondary species. Alnus incana 

 occurs in some areas also. In rather moister soil and often with 

 Alnus incana, Corylus rostrata, Salix discolor and sometimes other 

 willows occur abundantly. The usual secondary species of this as- 

 socies are : 



Auaplialis margarifacca Lacinaria scariosa 



Aster laez'is Onagra biennis 



Chamacnerion angusti- Ptcris aquilina 



folium Solidago canadensis 



Dicrvilla lonicera Solidago nemoralis 



Alany other species are to be found, viz., plants of the forest 

 which have persisted after its removal or destruction and patches 

 of grass with their attendant secondary species that have not been 

 replaced by the shrubs of this associes. 



(4) Popuhis-Pinns divaricata Associes 



The presence of shrubs of the Corylus-Rubus associes serves to 

 keep the soil of the habitat moist and protect the seedlings by shad- 

 ing them, and thereby enables the trees of the associes to become 

 established. 



Some notable variations in this sequence may occur. In some 

 cases Pinus divaricata and Populus tremuloides may invade and be- 

 come dominant directly following the removal of the climax forest 

 without the appearance of the usual antecedent stages. This is 

 often found to occur in the area of sand and gravel outwash in 

 southern Beltrami, parts of Cass, Hubbard, Wadena. Becker and 

 Crow \\'ing Counties, particularly with reference to Pinus divari- 

 cata. It may occur also in other districts, especially in places where 

 the water-content of the soil is high. Populus tremuloides is apt to 

 become an earh' dominant in such places. 



It has been observed, in some places, that Andropogon furcatus 

 may become the dominant after burning or clearing, and that Pinus 

 divaricata may invade the grass directly v^ithout a previous develop- 

 ment of shrubs. Sometimes P. divaricata alone invades, at other 



