62 THE STUDY OF PLANT COMMUNITIES • Chapter IV 



estimated with some success or may be accurately determined by 

 various devices for measurement and recording. When vegetation 

 is stratified, the cover must be considered in terms of the stratum 

 to which the species belongs. For rapid estimation, as well as for 

 analysis of results, there is a distinct advantage at times in using 

 several cover classes. Braun-Blanquet recommends five : 



1. covering less than 5% of the ground surface 



2. covering 5% to 25% 



3. covering 25% to 50% 



4. covering 50% to 75% 



5. covering 75% to 100% 



In studies of grassland, estimates and measurements of cover are 

 extremely useful because the variations in size and form of grasses 

 make counts difficult 'and of little value. For expressing cover, 

 sometimes as area of coverage, sometimes as basal area of clumps, 

 range ecologists frequently use the term, density . This usage is, 

 of course, at variance with the phytosociological application and, 

 consequently, leads to confusion of interpretation unless it is 

 known, for example, that a "density list" 96 applied to grassland, 

 refers to area or cover for each species, and that "square foot 

 density" 247 also indicates coverage evaluated by a different method. 



Determination of the volume of space occupied by species is 

 difficult and has not been widely done. When all plants are small, 

 cover alone serves very well, especially when strata are distin- 

 guished. With grasses, as in pasture studies, clipping and weighing 

 the tops is sometimes necessary for accurate comparisons. In for- 

 est studies, the estimate of volume of standing timber as used by 

 foresters can be used to advantage, but a more useful value is basal 

 area. Diameters can be determined accurately and quickly with a 

 diameter tape, and basal area, easily obtained from standard tables, 

 can add much to an evaluation in terms of size and bulk that can- 

 not be visualized through the other quantitative characters. This 

 provides a particularly useful means of comparing the relative im- 

 portance of species of trees and, in addition, permits analysis in 

 terms of size or diameter classes among the sapling and understory 

 individuals. Several quantitative characters can be advantageously 

 combined in the form of phytographs (Fig. 26) for evaluation. 



