331 



grostis canadensis, whose stems grow so closely as virtually to pro- 

 hibit the development of secondary species. The association varies 

 in width from a meter or two where the slope is evident to thirty to 

 fifty meters or more where there is no evident slope. In all cases 

 the dense growth of Calaniagrostis completely dominates, and the 

 small number of secondary species, which are usually either relics or 

 invaders, are notably more slender, broader-leaved, and taller than 

 individuals of the same species in their normal associations. This is 

 clearly a response to the diminution of the amount of light which 

 'they receive, as this effect is often observed where these plants per- 

 sist under the shade of trees. The foreground of Figure i, Plate 

 LV, shows a typically developed Calaniagrostis swale. 



When this association obtains dominance successions are very 

 nearly at a standstill, since the seedlings of invaders have consider- 

 able difficulty in obtaining a foothold, and they must also be able to 

 withstand a great deal of shade. Normally the Liatris spicata prairie 

 is the association which should succeed. Near Zion City, however, 

 where the swales are occasionally burned over, the thicket association 

 obtains a foothold and is rapidly followed by aspens and willows. 



LIST OF THE SPECIES OE THE CAEAMAGROSTIS CANADENSIS ASSOCIATION 



Dominant Species 

 Calaniagrostis canadensis 



(All of the following species are very poorly represented in num- 

 ber of individuals) 



Secondary^ Species 

 Spartina niichaii.viana Aster ericoides 



Campanula aparinoides 



Relic Species 



Scirpns validus Polygomnu aniphihiitni hart- 

 Scirpns am eric anus zurightii 



Oxy polls rigidior Dulichinm arnndinacenm (very 

 Asclepias incarnata rare) 



Invading Species 

 Ly thrum alatiim Iris versicolor 



Spiraea salicifolia Mentha arvensis canadensis 



Salix Candida Btipatorium perfoliatim 



Salix longifolia 



