THE ORIGIN AND GROWTH OF PLANT COMMUNITIES 575 



ceeding a single climax area (see example of sugar maples in Dan- 

 sereau, 1957a, Figures 2-6A, 2-6B ) . 



The data in Table III show how diverse the associations of the 

 Montreal region are in this respect. Thus the Atlantic coastal plain ele- 

 ment is absent from all communities except two. The deciduous forest 

 element, so strongly predominant in the climax and in the warmer 

 phase of the quasiclimax, is cut by two-thirds in the cooler, moister 

 phase of the maple forest, and it is notably lower in all non-forest 

 communities. Six units are entirely lacking in non-indigenous species, 

 whereas the bluegrass meadow has 87.4 per cent, the loosestrife marsh 

 50 per cent, the goldenrod prairie 44.3 per cent. 



It will be noted, as it must be repeatedly with reference to other 

 criteria discussed below, that such gradings of floristic Hsts provide an 

 estimate of the qualities of the flora and only indirectly of the vegeta- 

 tion. Many writers have been at great pains to emphasize this point, 

 especially in connection with Hfe forms (see Dansereau, 1945; Dan- 

 sereau and Gille, 1949; Dansereau, 1957a; Cain and Castro, 1959). 

 Therefore, if one uses a quantitative coefficient, such as coverage, fre- 

 quency, constancy, etc., some of the values will rise and others will fall. 



b. Valence. A species is considered a good indicator when it shows 

 a certain narrowness in its ecological adjustment. Thus truly aquatic 

 plants, such as the Nympheaceae, are never found on dry land because 

 they cannot withstand even brief exposure; extremely salt-tolerant spe- 

 cies, such as most Atiiplex species, reveal the nature of the soil on 

 which they grow; extremely air-moisture-loving plants, such as most 

 Hymenophijllum species indicate that the atmosphere where they grow 

 is nearly saturated most of the time. 



There are therefore indicators of this or that condition. It would 

 be difficult to blueprint ecological valence in a table that would show 

 all possible kinds of indicators, but a few general statements can be 

 made and a few examples can be pointed to. 



Those species that are restricted to a narrow band in the ecological 

 spectrum ( for humidity or acidity of soil, for hght intensity, etc. ) and 

 are therefore unable to tolerate a slight increase or decrease, give a 

 sure sign by their mere presence that the habitat is indeed limited to 

 those precise conditions. Stenovalent species therefore are the best in- 

 dicators. But they may reveal only one of the many conditions present 

 in the habitat (reduced light, neutral soil, or constant humidity), and 

 each stand may contain species reunited in a common association pri- 

 marily because of their requirement of one of these {e.g., the condi- 

 tions of light but not of soil nor humidity). Others, however, are not 

 indicators of one but of a combination of factors associated in a rather 

 unique fashion. 



As an extreme of this kind of limitation to the ecological ampli- 



