THE ORIGIN AND GROWTH OF PLANT COMMUNITIES 571 



stable— floristic assemblage and spatial distribution. Thus we may ar- 

 rive at an ordination of plant communities within a given region, by 

 applying a shifting scale of population-community adjustments. 



I shall attempt to implement this approach to plant communities, 

 at least in part. My demonstration will be centered upon a sampling of 

 vegetation in the Saint Lawrence \''alley. However, because I believe 

 that the same criteria and principles apply very widely, I shall also 

 draw from evidence in other areas where I have had field experience 

 and have gathered data exactly in the same way as I have in Quebec. 

 If this does not guarantee objectivity, it probably affords consistency 

 through uniform application of an identical subjectivity. The criteria I 

 propose to use will be applied primarily to 14 stands of vegetation upon 

 which I have made a previous report (1957a, Appendix). The report 

 contains complete species lists taken in the spring and in the summer 

 and a number of specific coefficients, some of them relative to the 

 species wherever it is found, others applicable only to a particular 

 stand. My tables therefore show a rather extensive grading by species, 

 emphasizing the diverse modalities of their place in the community. 

 Each community', in turn, can be evaluated in terms of the relative 

 weight of each of these characters. 



Anatomy of the plant community 



A stand of vegetation can be taken apart in two different ways: 

 by considering its species and the various qualities of each one, or by 

 analyzing the mass as a whole and distinguishing its parts without 

 primarily referring to composition. In my opinion, no interpretation of 

 the community is possible without the application of both of these tests. 

 In fact, no description is complete if both criteria are not used. 



Floristic composition. There are three principal considerations con- 

 cerning flora: its relative richness, the geographic affinities it shows, 

 and the indicator-value of all or some of its members. The first is a 

 quantitative assessment, whereas the latter two require qualitative dis- 

 crimination. Both are strongly affected by historical circumstances, al- 

 though immediately governed by present conditions. 



1. Richness. Some stands harbor a large number of species, others 

 very few (Table I). In the comparable data of our 14 Laurentian 

 tables, the mature undisturbed forests have the highest numbers of 

 species, whereas most of the secondary associations have very few ( see 

 Table II). 



This is generally considered a significant aspect of community 

 anatomy. It can be envisaged in a number of ways. One can seek a 

 series of floristic ratios between (a) stand and area, (b) association 

 and stand, and ( c ) region and association. 



a. Species-area curve. A standard procedure in phytosociology 



