QUANTITATIVE CHARACTERISTICS 



Population Density 



Population density, broadly considered, denotes the number 

 of individual plants or stalks in an area, but in a strict 

 sense it refers to the number of individuals or stalks in a 

 unit of space. When the measured unit area is divided by 

 the number of individuals, the average area occupied by 

 each individual is obtained. The number of individuals of a 

 species varies from place to place in a stand, often considerably, 

 so numerous sample areas are needed for reliable results. In 

 range-management literature, "density" is often used incorrectly 

 for "cover." 



The term population density, when referring to animals, 

 often conveys not merely the idea of the number of animals in a 

 unit of space, but also their number in relation to the available 

 quantity of a limiting requisite, ^^° and this thought is generally 

 implied when the term density-dependent factor or density- 

 governing factor is used. An example of this factor in vegetation 

 is a pine stand where the density of the trees reduces the light in- 

 tensity so much that the number of trees reaching maturity is 

 restricted (Figure 2-3). A density-independent factor or non- 

 reactive factor^ ^° is one that has little or no effect on density (for 

 example, the CO2 content of the atmosphere), because it rarely 

 if ever influences the number of plants in an area. 



Density values are significant because they show the relative 

 importance of each species in a stand when they are similar in 

 life-form and size (Figure 2-5). However, where the plants are dif- 

 ferent, such as grasses, forbs, and dwarf-shrubs (Figure 3-1), 

 density alone is insufficient for comparison, and data on other 

 characteristics, especially cover (see p. 102), should then be con- 

 sidered. For instance, in studies on forest vegetation, the trunk 

 diameter and height and spread of the crown are frequently 

 measured in addition to numerical abundance. Moreover, it is 

 often difficult, as in the case of mat plants, to determine density 

 because the stalks grow very close together; therefore it is often 



100 • THe Conmznunity 



