Bcrgnian and Stallard: climax fokmations v35l 



(5) Pinus-Bctula AssociES 



The intensely xerophytic conditions of the habitat, due to the 

 deficiency in water-content and to the great evaporation to which it 

 IS exposed in the bare rock condition, has been amehorated gradu- 

 ally by continual weathering and by the addition of humus as the 

 result of the decay of the pioneer lichens and plants of the succes- 

 sive developmental stages. Thus by the time that Juniperus and 

 Sabina put in their appearance, a considerable depth of soil has been 

 formed and the water-holding capacity of the soil has been increased 

 markedly by the addition of humus. The habitat starting from in- 

 tense xerophytism has approached to a mesophytic condition suit- 

 able for certain forms, at least, of trees. The presence of shrubs of 

 the Juniperus associes further favors the establishment of trees by 

 protecting the young seedlings from too intense evaporation and also 

 by preventing evaporation from the soil in which they grow. 



Consocies : The consocies are Betnla papyrifera and Pinus 

 divaricata. These may be intermixed, or either may be an exclusive 

 dominant within small areas. Although the growth may appear 

 dense when observed at a distance, a closer inspection reveals the 

 fact that the plants are rather scattered, appearing only along crev- 

 ices and in depressions where soil has accumulated more rapidly. 



Secondary species: Juniperus and Sabina may persist indefi- 

 nitely after the appearance of Betula and Pinus divaricata, occupy- 

 ing places less suitable for birches and pines or where the latter do 

 not shade them too much. Other shrubs which grow better when 

 partly shaded make their appearance with the advent of Betula and 

 Pinus. The shrubs found most abundantly with this associes are: 

 Alnus incana Prunus virginiana 



Corylus rostrata Rhus rydbergii 



Diervilla lonicera Rosa acicularis 



Prunus pennsylvanica Ritbus strigosus 



In addition to plants from the preceding associes, many of the 

 shade-loving plants are to be found growing under the birches and 

 pines. The more characteristic species are the following : 

 Actaea rubra Galium triflorum 



Aralia nudicaulis Lactuca canadensis 



Cornus canadensis Maianthemum canadense 



Fragaria virginiana Pteris aquilma 



Galium boreale 



