374 MINNESOTA BOTANICAL STUDIES 



Actaca ntbra Smilaciiia raceniosa 



Anemone qninquefoUa Suiilacina stellata 



Aralia niidicaitlis Thalictrnm dioecmn 



Aster dntuniio}idii Ui'idaria yrandiflora 



Caiilophylluui thalictruides Vicia aiuericana 



I'ragaria virginiaua Viola papilionacea 



Ilxdrophylliiiit z'Irgiiiiamuii I'iola piibescens 



Latliynis ochrolencus Viola nignlosa 



Lafhyriis venosus Viola sororia 



Sangninaria canadensis Washiugtonia longistylis 

 Saniciila marilandica 



(4) Accr-Tilia Association 



In the later stages of the Populus-Betula associes, Acer sacchar- 

 uui, Praxiniis^ lanceolata, F. americana, Carya cordiformis, Ostrya 

 virginica, Quercus coccinea, Q. macrocarpa, Q. rubra, Tilia ameri- 

 cana, Ulmits americana and U. fulva enter the associes and replace 

 the existing dominants. This is merely a transitional stage be- 

 tween the associes and the final deciduous climax stage described 

 under the Acer-Tilia association on page 367. In addition to the 

 trees mentioned above, Acer spicatum in the northern and eastern 

 edge of the deciduous climax association enters in the final stages 

 of the Populus-Betula associes. 



b. Secoxdarv Succession in the Larix-Picea Subclimax 



The discussion of secondary succession in the Picea-Larix sub- 

 climax as given for the same subclimax in the pine forest region 

 on page 361, applies here with but few dift'erences. Farther south, 

 Picea disappears, so that the Larix-Picea associes is represented by 

 Larix only. 



The causes work in the same way and may operate singly or to- 

 gether, as indicated on pages 355 and 356. Fire, as the more de- 

 structive agent, affects the habitat more profoundly and causes 

 succession to begin at a lower stage. This is especially true if burn- 

 ing occurs during a dry season when the Sphagnum surface is partly 

 dry. At such times burning may result in killing out the Sphagnum 

 and in the initiation of a secondary succession beginning with Carex- 

 Calamagrostis. The sequence of stages after the appearance of 

 Carex-Calamagrostis up to Larix-Picea is essentially that of primary 

 succession as indicated on page 364. 



