364 MINNESOTA BOTANICAL STUDIKS 



papyrifera and Fraxiniis nigra come in also, the invasion proceeding 

 from the edge where filling occurs most rapidly. As a result of 

 drainage not complicated by the introduction of other factors, the 

 normal primary succession from Larix-Picea through the Abies- 

 Befula associes to the climax pine occurs. Thus it appears that 

 drainage alone merely accelerates the normal primary succession. 



ACER-TILIA ASSOCIATION 

 I. Primary Succession 



1. hydrarch succession 



Initial causes: The formation of bodies of water of sufficient 

 size and depth to completely denude affected areas provides condi- 

 tions for the initiation of primary succession. The origin of lakes 

 in Minnesota has been indicated briefly under "Initial Causes" in 

 the discussion of primary succession in the area of the pine forest 

 climax. 



A succession having its origin in water and culminating in the 

 Accr-Tilia climax association shows a series of stages as follows : 



(Ij Chara-Philotria Associes 



(2) Castalia-Nymphae Associes 



(3) Scirpus-Zizania Associes 



(4) Car ex Associes 



(5) Larix-Picea Associes 



(6) Populiis-Bctula Associes 



(7) Accr-Tilia Association 



A variation of this sequence may occur by the failure of the 

 Larix-Picea associes to develop between the Carex associes and the 

 climax association. This is true particularly where the land rises 

 rather abruptly from a swamp or meadow, resulting in a sudden 

 drop in the water-content of the soil. In some instances the Larix- 

 Picea associes is found to be preceded by a Chamaedaphne-Andro- 

 meda associes, while in other instances the latter does not appear. 

 Which of these variations is to be regarded as the rule and which 

 the exception it is not possible to state from evidence now available. 

 When more places in the deciduous belt have been studied a definite 

 answer can be made with reference to this point. 



Since the stages in the development of the deciduous forest cli- 

 max in many respects are the same as the corresponding stages in 

 the development of the pine forest climax, the discussion will be re- 

 stricted to the pointing out of differences, when such exist. 



