-27 - 



THE ORIGIN AND GROWTH 

 OF PLANT COMMUNITIES 



Pierre Dansereau 



UNIVERSITY OF MONTREAL 



The spatial arrangement of plants ki the oceans, rivers, lakes, and 

 streams and on the land forms a mosaic in which many observers have 

 recognized the repeated occurrence of a number of pieces. At a cer- 

 tain order of magnitude, the controlling power of climate is clearly 

 responsible for the outstanding features of distribution. From the equa- 

 torial rain forest to the arctic tundra, a number of transitions ( gradual 

 or abrupt ) induce considerable changes in the physiognomy of regional 

 vegetation. Within each geographic region, however, the general homo- 

 geneity gives way to an often complex variety of communities which at 

 first sight appear to be segregated according to site factors (topog- 

 raphy, drainage, soil) but which turn out also to be influenced by 

 history. 



The plant cover of almost any landscape comprises a number of 

 distinct plant communities. Their discontinuity is very often the re- 

 sult of abrupt changes in the site conditions ( edge of the water, rise of 

 the slope, replacement of clay by sand, increase in the organic content 

 of the soil, improvements of drainage, etc.) or of actual interference, 

 usually recent (by grazing, fire, plowing, etc.). 



Whereas there is no denying the objective existence of an individ- 

 ual plant community as a stand in a given place ( a cattail marsh at He 

 Perrot, Quebec; a white pine forest at Grayling, Michigan; a palm forest 

 at El Yunque, Puerto Rico; a kauri forest at Trounson Woods, New 

 Zealand; a live-oak forest at Montserrat, Spain, etc.), some doubt has 

 been cast on the objectivity of the plant "associations" that have been 

 described and defined by phytosociologists. 



The sampling of stands that leads to these definitions is generally 



567 



