596 PLANT GROWTH AND PLANT COMMUNITIES 



light, more moisture, etc.). In this particular list there is no acci- 

 dental species. In other stands of the same association some of the 

 following are sometimes found: Impatiens capensis, Galeopsis tet- 

 rahit. An accidental owes its presence to temporary conditions. 



The ecological strategy involved is fairly uniform within each one 

 of these groups in this particular association (the Aceretum saccharo- 

 phori laurentianum ) . The above A-B-C-D-E scale roughly shows five 

 degrees of population-community adjustment. If it is applied to a num- 

 ber of spatially contiguous communities, the inner structure of each 

 one will turn out to be quite different. It is easily seen that the com- 

 panions and even the accidentals are relicts, heralds of change, or mere 

 indicators of a passing disturbance. 



Cohesion of communities. The degree to which two or more 

 species are obhgate associates (fidelity) or statistically frequent as- 

 sociates ( constancy ) can be measured with some degree of objectivity 

 (especially if the fundamental difference between fidelity and con- 

 stancy is kept in sight ) , and this can be used as a proof of the relative 

 cohesion of the community. However, it is in a sense unreal to exclude 

 dominance for this test, inasmuch as the spatially prevalent populations 

 exert major control by their action as the principal agents of the re- 

 source turnover. 



It would seem, therefore, that open vs. closed communities, mono- 

 dominated vs. pluri-dominated communities, floristically pure vs. mixed 

 communities will present various degrees of looseness and tightness. 

 The latter will be due essentially to a fairly full utilization of resources 

 which results from a certain variety of tapping mechanisms in a mutu- 

 ally well-adjusted species population. Such would seem to be not only 

 the climax Aceretum saccharophori laurentianum, but also some of 

 the more stable serai stages, such as the Chamaedaphnetum calycula- 

 tae, the Alnetum rugosae, the Calamagrostetum canadensis. The Betul- 

 etum populifoliae and the Solidaginetum laurentianum are not in this 

 category. A most important feature of these two is their high number of 

 accidental species and the occasionally high development of their 

 populations, in contrast to the power of the other associations to virtu- 

 ally exclude accidental populations. 



Origin and senescence of communities. How, then, are communi- 

 ties formed in the first place? What can we know of their origin and 

 early stages of growth? Having taken them apart in the preceding sec- 

 tions, what key features can we rely upon to trace them to their begin- 

 nings? The juxtaposition and the eventually harmonious partaking of 

 common resources by species populations of different elementary floris- 

 tic groups suggests two possibilities. 



The first hypothesis is the unprecedented coming together of 



