216 THE STUDY OF PLANT COMMUNITIES ■ Chapter IX 



Most of the settled parts of North America have little evidence 

 of primary succession today, and even unsettled areas have largely 

 been disturbed by grazing or lumbering. Thus primary succession 

 must often be interpreted in terms of small and often poor exam- 



FlG. 104. Hydrarch succession as illustrated by girdles of vegetation 

 around a shallow lake in northern Minnesota. In what remains of open water 

 are submerged and floating-leaved aquatics, the pioneer angiosperms. A mar- 

 ginal, floating sedge mat is gradually filling the lake with peat and advancing 

 over the water. On the mat are a few bog shrubs, behind which is a girdle of 

 tamarack forming a closed stand. The oldest part of the bog is marked by 

 the spires of black spruce, which succeed the tamarack. On the upland, be- 

 hind the spruce, is a mixed white pine-hardwood forest. Eventually, the en- 

 tire depression will be a peat-filled bog supporting a forest of black spruce. 



pies of what once occurred. Studies of secondary succession may, 

 however, have the greatest practical value because we are in- 

 volved with secondary successions in any problem of applied 

 ecology; yet their interpretation may be partially dependent upon 

 an understanding of primary successions. 



Representative Successions.— Because water and bare rock rep- 

 resent the extremes in types of habitats upon which succession is 

 initiated, the growth form of early stages of each is remarkably 

 similar everywhere and even genera and some species are often 



