CLASSIFICATION OF PLANT COMMUNITIES 315 



of a precipitous cliff, in a very general sense, the steepness of the 

 slope may be regarded as the limiting factor. Along a bleak, 

 exposed seacoast, mnd, or perhaps better the absence of quiet 

 air, may similarly represent the limiting factor. Most commonly, 

 unavailability of sufficient water, to whatever causes this may 

 be due, is the direct limiting factor. 



In a measure, of course, the favorable influence of certain 

 habitat factors, may compensate (or integrate) the limiting 

 effect exerted by others. On precipitous slopes in rock ra\'ines, for 

 example, the factor "water runs off quickly" may be offset to 

 such an extent b}^ the factor "atmospheric humidity is great" 

 that a mesophytic bryophyte flora is able to establish itself; 

 but the factor "no foothold for roots," which may be the chief 

 obstacle to the development of a forest cover here, cannot be 

 wholly compensated by any other factor. Most important of 

 all habitat factors in their compensating influence are those 

 due to plant and animal agencies, particularly humus accumu- 

 lation and shade as they affect the water relations of the habitat 

 (see Cowles, 11). The significance of these factors in relation 

 to succession is universally recognized, and in some cases their 

 compensating influence is sufficient to completely offset the 

 effect of the limiting factor: in other words, through the cumula- 

 tive effect of biotic factors the habitat may become so modified 

 that it becomes possible for the chmax association-type of the 

 region to develop. But elsewhere, in varying degree, the in- 

 fluence of the Umiting factor is too pronounced to be completely 

 overcome, and succession becomes permanently arrested at 

 a stage less mesophytic than the regional climax. In the case 

 of a swamp which has originated through the filling in of a 

 lake by vegetable debris, for example, it is quite conceivable 

 that, as a result of the gradual upbuilding of the substratum 

 through the accumulation of humus, the habitat might event- 

 ually come to approximate that of uplands. But here again 

 a limiting factor, which might be designated "decomposition 

 of humus when exposed to air," ordinarily prevents the upbuild- 

 ing process from proceeding beyond a certain point. It should 

 be added, although it is perhaps quite ob\ious, that the effect 



