Bcryiiiaii and Slallard : climax formations 367 



iiig in of eroded material and by the accumulation of organic mat- 

 ter from decay of plant bodies results in a reduction in the water- 

 content. The develoi)ment of the Larix-Picea associes results in a 

 reduction or in the more or less complete disappearance of grasses 

 and sedges. Where the tamaracks are dying out, or in vacant 

 places resulting from windthrows. Popithis and Bctula are able to 

 invade and in time become established. 



Consocies : The species usually appearing as consocies are : 

 Bctula papxrifera. PopuJus frciiiidoidcs, P. grandidentata, and P. 

 balsamifcra. Fraxinus nigra is often found with these but not in 

 sufficient numbers to rank as a consocies. P. tremuloides and P. 

 grandidentata are usually the most abundant, but in places may not 

 appear at all, the associes being represented in such places by Betula. 



Secondary species : The undergrowth consists of the following 

 shrubs : 



CoDius stolonifcra Rihes a^ncricaniim 



Coryhts aniericana Rosa hlanda 



Partlioiicissits quinque- Riibus strigosus 



folia 



The following are the most abundant plants of the ground layer : 



Aralia nudicaulis Galium triflorum 



Aster macrophyllus Maianthemmn canadense 



Carcx longirostris Ory::;opsis aspcrifolia 



Circaea alpina PJiryrna leptostachya 



Circaea hitetiana Pyrola americana 



Cystopteris fragilis Raiuoiculus ahortivus 



Dryopteris cristata Rulnis triflorus 



Erigcron philadelphicus Siiiilacina stellata 



Festuca nutans Viola coiispcrsa 



Fragaria americana Viola papilionacea 



(7) Acer-TUia Association 



Causes : The further filling of the low area by the washing in 

 of soil and by the accumulation of decayed plant-remains brings 

 about a reduction in the water-content of the soil. The habitat has 

 now become mesophytic in character. The invasion of the Populns- 

 Betula associes by the species of the climax forest occurs when the 

 poplars or birches die out as the result of competition or when 

 windfalls occur. The fact that Acer, Tilia and others of this as- 



