346 MINNESOTA BOTANICAL STUDIES 



now become completely mesophytic in character. The reduction of 

 water-content and the ability of pine seedlings to grow in a reduced 

 light makes it possible for pines to invade and to become established. 

 The actual invasion of the Abies-Betula zone must occur in the 

 spaces between more widely separated individuals of that zone. The 

 destruction of members of the Abies-Betitla associes by diseases, 

 insects, the breaking and uprooting of trees by wind, all contribute 

 to the formation of openings in which pines may invade and estab- 

 lish themselves. The greater longevity of pines as compared with 

 Abies, Bctnla and Picca enables the pines to become dominant in 

 competition with them. 



Consocies : The dominant species of pines of the climax forest 

 are P. strobus and P. resin osa. The two species often occur in a 

 mixture, or extensive tracts may be occupied by either as a pure 

 dominant. It seems probable that P. strobus would finally replace 

 P. resinosa, but at present this cannot be confirmed. 



Secondary species : Shrubs are usually absent in the well-de- 

 veloped climax. The following low shrubs and herbs are the char- 

 acteristic plants of the usually sparse ground layer : 



Anejnone qninquefolia Maiauiiieinum canadeuse 



ChimapHila nmbellata Pyrolo americana 



Corniis canadensis Pyrola secnnda 



Diervilla lonicera Vacciniuui canadense 



Gaitltheria procnmbens Vaeciniion pennsylvanicmn 



Lycopodiiim obscnrum 

 Antennaria canadensis, Lycopodium annotinuni and Pyrola ellip- 

 tica occur occasionally. Epigaea repens is found in certain areas 

 and often, locally, is very abundant. Apparently it is restricted to 

 sandy soils on which pines have become dominant. 



In many places the growth of pines is so dense that the ground 

 is practically bare of vegetation, only scattering plants being found. 

 In such places the ground is well carpeted by pine needles. This, 

 probably, represents the typical condition of the climax pine forest, 

 the presence of an abundance of herbs being due to a disturbance of 

 typical conditions by passage of fire through the forest. 



2. XERARCH SUCCESSION 



Initial causes : A xerarch succession is one originating in a dry 

 habitat. Such a habitat is to be found in IMinnesota only in areas 



