The soils (Brown Soils group) in these stands contained a 

 moderate number of gravel particles and rocks both on the sur- 

 face and to a depth of 18 in. Between the surface and 12 in. the 

 sandy clay loam was dark reddish-brown to red; between 12 

 and 18 in. the soil varied from sandy clay loam to clay loam, red- 

 dish-brown to weak red in color; the lime content was usually high 

 throughout and the pH was 8.0 to 8.2. 



Presence and Constancy 



As stated in Chapter 3, frequency refers to the degree of 

 uniformity of distribution of a species within one stand; presence 

 and constancy refer to how uniformly a species occurs in a num- 

 ber of stands of the same type of community, for example, when 

 a species is found in 18 of 20 stands in one community type the 

 presence or constancy is 90 per cent. The term constancy is em- 

 ployed when equal, measured sample areas are used in each 

 stand, presence is used when the area of the sampling unit 

 varies from stand to stand, and especially when it is not meas- 

 ured. It is preferable, when possible, to use measured units 

 so comparisons can be made more readily, and for possible 

 statistical analysis. Often, however, the sampling units can- 

 not have the same area because of the nature of the vegeta- 

 tion, as, for example, small irregular stands in rock crevices or on 

 sand deposits along a stream. When a single sampling unit per 

 stand is employed, it should be large enough to include most of 

 the species in the stand. This is called the minimal area, which 

 is determined by using the species number: area curve, the num- 

 ber being represented on the ordinates axis, the area on the 

 abscissas. The point where the curve begins to flatten denotes 

 approximately the minimal area, which may vary from less than 

 10, to 25 sq m or more in various kinds of vegetation. Whenever 

 possible, at least ten stands should be analyzed for each kind of 

 community. 



Species may be classified into five classes of constancy ac- 

 cording to the percentage of stands in which they occur, as 

 follows: 



124 • THe Commmmnity 



