Class 4: Large patches or broken mats (Figure 1-16) 

 Class 5 : Very large mats or stands of nearly pure populations 

 that almost completely cover a large area, such as a 

 Vaccinium heath, an Andropogon scoparius grassland, 

 or a cattail marsh (Figures 1-6 and 1-13) 



Other scales that use the actual areas occupied by groups have 

 also been proposed. ^^ 



The shoots or plants of some species are able to grow much 

 closer together than those of others, notably those that propagate 

 by rhizomes, runners, or roots, forming very dense stands in which 

 the shoots are separated by short internodes, Kentucky bluegrass 

 for example. Species with longer internodes, such as smooth 

 bromegrass, form more open stands. Also important in the form- 

 ing of dense stands is the ability of shoots to tolerate shading, root 

 competition, or some other adverse factor such as high humidity, 

 which may favor infection by disease-producing organisms. 

 Species that spread only by seed may also show a high degree of 

 sociability, especially in the early stages of succession, as in aban- 

 doned fields where certain annual weeds may become very 

 dense. The ability of these plants to form dense groups is related 

 to the number of seeds produced, the mobility of seeds or fruits, 

 the rate of germination, and the ability of seedlings and growing 

 plants to survive disease and intense competition. Bromus tectorum, 

 an annual, has been highly successful in invading large areas of 

 western grasslands, forming stands with sociability of class 5. 

 Salsola kali and Chenopodium album may also rate in class 5 in 

 early stages of succession on abandoned fields, but in later stages 

 they are usually reduced to class 1. Even annuals with fruits or 

 seeds lacking special structures for dispersal, such as C. album 

 and sunflowers {Helianthus spp.) often form dense aggregations 

 because the seeds fall close to the parent; but species with very 

 effective devices for dispersal, such as Tragopogon pratensis, are 

 more likely to rate in class 1, unless they migrate into particularly 

 favorable habitats. 



Vegetative propagation is conducive to aggregation and often 

 gives invaders pronounced advantages in becoming established 

 (Figure 1-19). The new shoots on the rhizomes or runners are sus- 



Analytic Ckaracteristics of tke Community • 97 



