334: Annals of the Carnegie Museum. 



Secondary Species. — 



Rhus typhina, Rubus occidentalis, 



Alnus incana, Juniperus vh'giniana, 



Vitis vulpina, Andropogofifurcatus, 



Poa compressa. 



Quite conspicuous in the Myrica thicket-formation in autumn is 

 the Solidago canadetisis society, an autumnal aspect thus being charac- 

 terized. At this time the tall golden -rods considerably overtop the 

 shrubby J/vr/Vrt, and being quite abundant, the effect is often to almost 

 hide the shrubs from view. The difference in the choice of habitats 

 of Solidago nemoralis and S. canadensis is indeed very striking. 

 Neither species is found to occur in the habitat of the other, although 

 there is often but a very few years intervening between the disappear- 

 ance of the former and the occupation of the same spot by the latter 

 species. The effect of the thicket-formation upon its habitat is rapid, 

 as is evidenced by the quick change in the species of Solidago. The 

 bushy, compact clumps of Myrica catch and securely retain vegetable 

 litter, leaves, etc., blown about by the wind, and in a short time a 

 considerable layer of humus is formed. In this manner the ecological 

 conditions soon become favorable to species other than those of the 

 Myrica thicket- formation, and in the course of but a iew years the 

 thicket will be displaced by a forest-formation. 



Ganong notes "^ that on sheltered slopes of the low dune-beaches on 

 the sand-plain of the island of Miscou, in situations similar to those 

 occupied by the low shrubby mats of Juniperus nana and Hudsonia 

 tomentosa, are patches of " A bright green, leathery-leaved, tufted 

 shrub, the wax-berry, Myrica ca?-olinensis, which comes to form dis- 

 coid (sometimes almost fairy-ring-like) masses on the crests and 

 inner slopes," and he further remarks that the shelter of the shrubby 

 mats formed by this species " Affords in reality the principal starting 

 point for the development of other plants, which lead gradually to the 

 development of the forest." 



Kearney, in his description of the "Myrica association" of the 

 "Middle (Open) Dunes" near the Dismal Swamp, says:"* "In 

 sheltered flat places Ainmophila sometimes makes a comparatively 

 dense, almost meadow-like growth, often associated with scattered 



** Ganong, W. F. /. c, pp. 92-93. 



8* Kearney, T. H. " Report on Dismal Swamp." /. c, pp. 370-395. 



