^52 MINNESOTA BOTANICAL STUDIES 



In places where a few individuals of F. divaricata are grouped, 

 the ground may be shaded to such an extent that very few plants 

 are able to grow there. Patches of rock covered by Cladonia rangi- 

 ferina, C. gracilis, other Cladonias, Peltigera and Polytrichum, or 

 rocks with crustose lichens only are commonly to be found in the 

 spaces between the tree growths. 



(6) P'uiiis Association 



The appearance of the Juuipcnts associes and the subsequent 

 Betula-Pinus associes brings about a more rapid accumulation of 

 humus, due to the increase in the number and size of individuals able 

 to inhabit the area. The weathering processes still continue to break 

 down the rocks and increase the amovmt of soil. All changes, ac- 

 cordingly, work to bring the habitat to more nearly mesophytic con- 

 ditions. In the spaces between birch and jack pines and more ot 

 less shaded and protected by them, the invading pines of the Pinus 

 association find suitable conditions for development. 



Consocies : , The consocies of the climax forest here as in the 

 hydrarch succession consists of two species of pines. P. rcsinosa 

 and P. strobus. No difiference is to be found in the climax of the 

 xerarch succession as compared with that of the hydrarch, except 

 that the presence of certain secondary species may indicate along 

 which line the succession has been. 



Secondary species : As undershrubs of this associes are found : 



Coryliis rostrata Rosa acicnlaris 



Dicrvilla lonicera Ruhiis strigosiis 



Loniccra hirsuta Vaccinium canadcnse 



Primus pennsylvanica Vaccinicmn pennsylvanicmn 

 Prunus virginiana 



The presence of patches of bare rock or of lichen-covered rock, 

 which have persisted throughout the stages leading up to the climax, 

 breaks up the climax somewhat, and allows more under-vegetation 

 than might otherwise be possible. The herbaceous layer consists 

 of Aralia, Corvus, Maianthemuni, Chimaphila and Pyrola as char- 

 acteristic species. The fact that in an area of even a few acres, 

 hydrarch and xerarch successions occur side by side and, further, 

 that these have been disturbed during their development, perhaps 

 repeatedly, makes it difficult to state with assurance whether cer- 

 tain species are characteristic of normal primary succession or 



